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//! # Type Coercion
//!
//! Under certain circumstances we will coerce from one type to another,
//! for example by auto-borrowing. This occurs in situations where the
//! compiler has a firm 'expected type' that was supplied from the user,
//! and where the actual type is similar to that expected type in purpose
//! but not in representation (so actual subtyping is inappropriate).
//!
//! ## Reborrowing
//!
//! Note that if we are expecting a reference, we will *reborrow*
//! even if the argument provided was already a reference. This is
//! useful for freezing mut things (that is, when the expected type is &T
//! but you have &mut T) and also for avoiding the linearity
//! of mut things (when the expected is &mut T and you have &mut T). See
//! the various `tests/ui/coerce/*.rs` tests for
//! examples of where this is useful.
//!
//! ## Subtle note
//!
//! When inferring the generic arguments of functions, the argument
//! order is relevant, which can lead to the following edge case:
//!
//! ```ignore (illustrative)
//! fn foo<T>(a: T, b: T) {
//! // ...
//! }
//!
//! foo(&7i32, &mut 7i32);
//! // This compiles, as we first infer `T` to be `&i32`,
//! // and then coerce `&mut 7i32` to `&7i32`.
//!
//! foo(&mut 7i32, &7i32);
//! // This does not compile, as we first infer `T` to be `&mut i32`
//! // and are then unable to coerce `&7i32` to `&mut i32`.
//! ```
use crate::FnCtxt;
use rustc_errors::{struct_span_err, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan};
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_hir::intravisit::{self, Visitor};
use rustc_hir::Expr;
use rustc_hir_analysis::astconv::AstConv;
use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use rustc_infer::infer::{Coercion, DefineOpaqueTypes, InferOk, InferResult};
use rustc_infer::traits::{Obligation, PredicateObligation};
use rustc_middle::lint::in_external_macro;
use rustc_middle::traits::BuiltinImplSource;
use rustc_middle::ty::adjustment::{
Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCoercion,
};
use rustc_middle::ty::error::TypeError;
use rustc_middle::ty::relate::RelateResult;
use rustc_middle::ty::visit::TypeVisitableExt;
use rustc_middle::ty::GenericArgsRef;
use rustc_middle::ty::{self, Ty, TypeAndMut};
use rustc_session::parse::feature_err;
use rustc_span::symbol::sym;
use rustc_span::{self, DesugaringKind};
use rustc_target::spec::abi::Abi;
use rustc_trait_selection::infer::InferCtxtExt as _;
use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
use rustc_trait_selection::traits::{
self, NormalizeExt, ObligationCause, ObligationCauseCode, ObligationCtxt,
};
use smallvec::{smallvec, SmallVec};
use std::ops::Deref;
struct Coerce<'a, 'tcx> {
fcx: &'a FnCtxt<'a, 'tcx>,
cause: ObligationCause<'tcx>,
use_lub: bool,
/// Determines whether or not allow_two_phase_borrow is set on any
/// autoref adjustments we create while coercing. We don't want to
/// allow deref coercions to create two-phase borrows, at least initially,
/// but we do need two-phase borrows for function argument reborrows.
/// See #47489 and #48598
/// See docs on the "AllowTwoPhase" type for a more detailed discussion
allow_two_phase: AllowTwoPhase,
}
impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
type Target = FnCtxt<'a, 'tcx>;
fn deref(&self) -> &Self::Target {
&self.fcx
}
}
type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
struct CollectRetsVisitor<'tcx> {
ret_exprs: Vec<&'tcx hir::Expr<'tcx>>,
}
impl<'tcx> Visitor<'tcx> for CollectRetsVisitor<'tcx> {
fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
if let hir::ExprKind::Ret(_) = expr.kind {
self.ret_exprs.push(expr);
}
intravisit::walk_expr(self, expr);
}
}
/// Coercing a mutable reference to an immutable works, while
/// coercing `&T` to `&mut T` should be forbidden.
fn coerce_mutbls<'tcx>(
from_mutbl: hir::Mutability,
to_mutbl: hir::Mutability,
) -> RelateResult<'tcx, ()> {
if from_mutbl >= to_mutbl { Ok(()) } else { Err(TypeError::Mutability) }
}
/// Do not require any adjustments, i.e. coerce `x -> x`.
fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> {
vec![]
}
fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'_>> {
move |target| vec![Adjustment { kind, target }]
}
/// This always returns `Ok(...)`.
fn success<'tcx>(
adj: Vec<Adjustment<'tcx>>,
target: Ty<'tcx>,
obligations: traits::PredicateObligations<'tcx>,
) -> CoerceResult<'tcx> {
Ok(InferOk { value: (adj, target), obligations })
}
impl<'f, 'tcx> Coerce<'f, 'tcx> {
fn new(
fcx: &'f FnCtxt<'f, 'tcx>,
cause: ObligationCause<'tcx>,
allow_two_phase: AllowTwoPhase,
) -> Self {
Coerce { fcx, cause, allow_two_phase, use_lub: false }
}
fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
self.commit_if_ok(|_| {
let at = self.at(&self.cause, self.fcx.param_env);
let res = if self.use_lub {
at.lub(DefineOpaqueTypes::Yes, b, a)
} else {
at.sup(DefineOpaqueTypes::Yes, b, a)
.map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
};
// In the new solver, lazy norm may allow us to shallowly equate
// more types, but we emit possibly impossible-to-satisfy obligations.
// Filter these cases out to make sure our coercion is more accurate.
if self.next_trait_solver() {
if let Ok(res) = &res {
for obligation in &res.obligations {
if !self.predicate_may_hold(&obligation) {
return Err(TypeError::Mismatch);
}
}
}
}
res
})
}
/// Unify two types (using sub or lub) and produce a specific coercion.
fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
where
F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
{
self.unify(a, b)
.and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
}
#[instrument(skip(self))]
fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
// First, remove any resolved type variables (at the top level, at least):
let a = self.shallow_resolve(a);
let b = self.shallow_resolve(b);
debug!("Coerce.tys({:?} => {:?})", a, b);
// Just ignore error types.
if let Err(guar) = (a, b).error_reported() {
// Best-effort try to unify these types -- we're already on the error path,
// so this will have the side-effect of making sure we have no ambiguities
// due to `[type error]` and `_` not coercing together.
let _ = self.commit_if_ok(|_| {
self.at(&self.cause, self.param_env).eq(DefineOpaqueTypes::Yes, a, b)
});
return success(vec![], Ty::new_error(self.fcx.tcx, guar), vec![]);
}
// Coercing from `!` to any type is allowed:
if a.is_never() {
return success(simple(Adjust::NeverToAny)(b), b, vec![]);
}
// Coercing *from* an unresolved inference variable means that
// we have no information about the source type. This will always
// ultimately fall back to some form of subtyping.
if a.is_ty_var() {
return self.coerce_from_inference_variable(a, b, identity);
}
// Consider coercing the subtype to a DST
//
// NOTE: this is wrapped in a `commit_if_ok` because it creates
// a "spurious" type variable, and we don't want to have that
// type variable in memory if the coercion fails.
let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
match unsize {
Ok(_) => {
debug!("coerce: unsize successful");
return unsize;
}
Err(error) => {
debug!(?error, "coerce: unsize failed");
}
}
// Examine the supertype and consider auto-borrowing.
match *b.kind() {
ty::RawPtr(mt_b) => {
return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
}
ty::Ref(r_b, _, mutbl_b) => {
return self.coerce_borrowed_pointer(a, b, r_b, mutbl_b);
}
ty::Dynamic(predicates, region, ty::DynStar) if self.tcx.features().dyn_star => {
return self.coerce_dyn_star(a, b, predicates, region);
}
_ => {}
}
match *a.kind() {
ty::FnDef(..) => {
// Function items are coercible to any closure
// type; function pointers are not (that would
// require double indirection).
// Additionally, we permit coercion of function
// items to drop the unsafe qualifier.
self.coerce_from_fn_item(a, b)
}
ty::FnPtr(a_f) => {
// We permit coercion of fn pointers to drop the
// unsafe qualifier.
self.coerce_from_fn_pointer(a, a_f, b)
}
ty::Closure(closure_def_id_a, args_a) => {
// Non-capturing closures are coercible to
// function pointers or unsafe function pointers.
// It cannot convert closures that require unsafe.
self.coerce_closure_to_fn(a, closure_def_id_a, args_a, b)
}
_ => {
// Otherwise, just use unification rules.
self.unify_and(a, b, identity)
}
}
}
/// Coercing *from* an inference variable. In this case, we have no information
/// about the source type, so we can't really do a true coercion and we always
/// fall back to subtyping (`unify_and`).
fn coerce_from_inference_variable(
&self,
a: Ty<'tcx>,
b: Ty<'tcx>,
make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
) -> CoerceResult<'tcx> {
debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
assert!(self.shallow_resolve(b) == b);
if b.is_ty_var() {
// Two unresolved type variables: create a `Coerce` predicate.
let target_ty = if self.use_lub {
self.next_ty_var(TypeVariableOrigin {
kind: TypeVariableOriginKind::LatticeVariable,
span: self.cause.span,
})
} else {
b
};
let mut obligations = Vec::with_capacity(2);
for &source_ty in &[a, b] {
if source_ty != target_ty {
obligations.push(Obligation::new(
self.tcx(),
self.cause.clone(),
self.param_env,
ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate {
a: source_ty,
b: target_ty,
})),
));
}
}
debug!(
"coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
target_ty, obligations
);
let adjustments = make_adjustments(target_ty);
InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
} else {
// One unresolved type variable: just apply subtyping, we may be able
// to do something useful.
self.unify_and(a, b, make_adjustments)
}
}
/// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
/// To match `A` with `B`, autoderef will be performed,
/// calling `deref`/`deref_mut` where necessary.
fn coerce_borrowed_pointer(
&self,
a: Ty<'tcx>,
b: Ty<'tcx>,
r_b: ty::Region<'tcx>,
mutbl_b: hir::Mutability,
) -> CoerceResult<'tcx> {
debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
// If we have a parameter of type `&M T_a` and the value
// provided is `expr`, we will be adding an implicit borrow,
// meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
// to type check, we will construct the type that `&M*expr` would
// yield.
let (r_a, mt_a) = match *a.kind() {
ty::Ref(r_a, ty, mutbl) => {
let mt_a = ty::TypeAndMut { ty, mutbl };
coerce_mutbls(mt_a.mutbl, mutbl_b)?;
(r_a, mt_a)
}
_ => return self.unify_and(a, b, identity),
};
let span = self.cause.span;
let mut first_error = None;
let mut r_borrow_var = None;
let mut autoderef = self.autoderef(span, a);
let mut found = None;
for (referent_ty, autoderefs) in autoderef.by_ref() {
if autoderefs == 0 {
// Don't let this pass, otherwise it would cause
// &T to autoref to &&T.
continue;
}
// At this point, we have deref'd `a` to `referent_ty`. So
// imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
// In the autoderef loop for `&'a mut Vec<T>`, we would get
// three callbacks:
//
// - `&'a mut Vec<T>` -- 0 derefs, just ignore it
// - `Vec<T>` -- 1 deref
// - `[T]` -- 2 deref
//
// At each point after the first callback, we want to
// check to see whether this would match out target type
// (`&'b mut [T]`) if we autoref'd it. We can't just
// compare the referent types, though, because we still
// have to consider the mutability. E.g., in the case
// we've been considering, we have an `&mut` reference, so
// the `T` in `[T]` needs to be unified with equality.
//
// Therefore, we construct reference types reflecting what
// the types will be after we do the final auto-ref and
// compare those. Note that this means we use the target
// mutability [1], since it may be that we are coercing
// from `&mut T` to `&U`.
//
// One fine point concerns the region that we use. We
// choose the region such that the region of the final
// type that results from `unify` will be the region we
// want for the autoref:
//
// - if in sub mode, that means we want to use `'b` (the
// region from the target reference) for both
// pointers [2]. This is because sub mode (somewhat
// arbitrarily) returns the subtype region. In the case
// where we are coercing to a target type, we know we
// want to use that target type region (`'b`) because --
// for the program to type-check -- it must be the
// smaller of the two.
// - One fine point. It may be surprising that we can
// use `'b` without relating `'a` and `'b`. The reason
// that this is ok is that what we produce is
// effectively a `&'b *x` expression (if you could
// annotate the region of a borrow), and regionck has
// code that adds edges from the region of a borrow
// (`'b`, here) into the regions in the borrowed
// expression (`*x`, here). (Search for "link".)
// - if in lub mode, things can get fairly complicated. The
// easiest thing is just to make a fresh
// region variable [4], which effectively means we defer
// the decision to region inference (and regionck, which will add
// some more edges to this variable). However, this can wind up
// creating a crippling number of variables in some cases --
// e.g., #32278 -- so we optimize one particular case [3].
// Let me try to explain with some examples:
// - The "running example" above represents the simple case,
// where we have one `&` reference at the outer level and
// ownership all the rest of the way down. In this case,
// we want `LUB('a, 'b)` as the resulting region.
// - However, if there are nested borrows, that region is
// too strong. Consider a coercion from `&'a &'x Rc<T>` to
// `&'b T`. In this case, `'a` is actually irrelevant.
// The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
// we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
// (The errors actually show up in borrowck, typically, because
// this extra edge causes the region `'a` to be inferred to something
// too big, which then results in borrowck errors.)
// - We could track the innermost shared reference, but there is already
// code in regionck that has the job of creating links between
// the region of a borrow and the regions in the thing being
// borrowed (here, `'a` and `'x`), and it knows how to handle
// all the various cases. So instead we just make a region variable
// and let regionck figure it out.
let r = if !self.use_lub {
r_b // [2] above
} else if autoderefs == 1 {
r_a // [3] above
} else {
if r_borrow_var.is_none() {
// create var lazily, at most once
let coercion = Coercion(span);
let r = self.next_region_var(coercion);
r_borrow_var = Some(r); // [4] above
}
r_borrow_var.unwrap()
};
let derefd_ty_a = Ty::new_ref(
self.tcx,
r,
TypeAndMut {
ty: referent_ty,
mutbl: mutbl_b, // [1] above
},
);
match self.unify(derefd_ty_a, b) {
Ok(ok) => {
found = Some(ok);
break;
}
Err(err) => {
if first_error.is_none() {
first_error = Some(err);
}
}
}
}
// Extract type or return an error. We return the first error
// we got, which should be from relating the "base" type
// (e.g., in example above, the failure from relating `Vec<T>`
// to the target type), since that should be the least
// confusing.
let Some(InferOk { value: ty, mut obligations }) = found else {
let err = first_error.expect("coerce_borrowed_pointer had no error");
debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
return Err(err);
};
if ty == a && mt_a.mutbl.is_not() && autoderef.step_count() == 1 {
// As a special case, if we would produce `&'a *x`, that's
// a total no-op. We end up with the type `&'a T` just as
// we started with. In that case, just skip it
// altogether. This is just an optimization.
//
// Note that for `&mut`, we DO want to reborrow --
// otherwise, this would be a move, which might be an
// error. For example `foo(self.x)` where `self` and
// `self.x` both have `&mut `type would be a move of
// `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
// which is a borrow.
assert!(mutbl_b.is_not()); // can only coerce &T -> &U
return success(vec![], ty, obligations);
}
let InferOk { value: mut adjustments, obligations: o } =
self.adjust_steps_as_infer_ok(&autoderef);
obligations.extend(o);
obligations.extend(autoderef.into_obligations());
// Now apply the autoref. We have to extract the region out of
// the final ref type we got.
let ty::Ref(r_borrow, _, _) = ty.kind() else {
span_bug!(span, "expected a ref type, got {:?}", ty);
};
let mutbl = AutoBorrowMutability::new(mutbl_b, self.allow_two_phase);
adjustments.push(Adjustment {
kind: Adjust::Borrow(AutoBorrow::Ref(*r_borrow, mutbl)),
target: ty,
});
debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
success(adjustments, ty, obligations)
}
/// Performs [unsized coercion] by emulating a fulfillment loop on a
/// `CoerceUnsized` goal until all `CoerceUnsized` and `Unsize` goals
/// are successfully selected.
///
/// [unsized coercion](https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions)
#[instrument(skip(self), level = "debug")]
fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
source = self.shallow_resolve(source);
target = self.shallow_resolve(target);
debug!(?source, ?target);
// We don't apply any coercions incase either the source or target
// aren't sufficiently well known but tend to instead just equate
// them both.
if source.is_ty_var() {
debug!("coerce_unsized: source is a TyVar, bailing out");
return Err(TypeError::Mismatch);
}
if target.is_ty_var() {
debug!("coerce_unsized: target is a TyVar, bailing out");
return Err(TypeError::Mismatch);
}
let traits =
(self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
debug!("missing Unsize or CoerceUnsized traits");
return Err(TypeError::Mismatch);
};
// Note, we want to avoid unnecessary unsizing. We don't want to coerce to
// a DST unless we have to. This currently comes out in the wash since
// we can't unify [T] with U. But to properly support DST, we need to allow
// that, at which point we will need extra checks on the target here.
// Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
let reborrow = match (source.kind(), target.kind()) {
(&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
coerce_mutbls(mutbl_a, mutbl_b)?;
let coercion = Coercion(self.cause.span);
let r_borrow = self.next_region_var(coercion);
// We don't allow two-phase borrows here, at least for initial
// implementation. If it happens that this coercion is a function argument,
// the reborrow in coerce_borrowed_ptr will pick it up.
let mutbl = AutoBorrowMutability::new(mutbl_b, AllowTwoPhase::No);
Some((
Adjustment { kind: Adjust::Deref(None), target: ty_a },
Adjustment {
kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
target: Ty::new_ref(
self.tcx,
r_borrow,
ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a },
),
},
))
}
(&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
coerce_mutbls(mt_a, mt_b)?;
Some((
Adjustment { kind: Adjust::Deref(None), target: ty_a },
Adjustment {
kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
target: Ty::new_ptr(self.tcx, ty::TypeAndMut { mutbl: mt_b, ty: ty_a }),
},
))
}
_ => None,
};
let coerce_source = reborrow.as_ref().map_or(source, |(_, r)| r.target);
// Setup either a subtyping or a LUB relationship between
// the `CoerceUnsized` target type and the expected type.
// We only have the latter, so we use an inference variable
// for the former and let type inference do the rest.
let origin = TypeVariableOrigin {
kind: TypeVariableOriginKind::MiscVariable,
span: self.cause.span,
};
let coerce_target = self.next_ty_var(origin);
let mut coercion = self.unify_and(coerce_target, target, |target| {
let unsize = Adjustment { kind: Adjust::Pointer(PointerCoercion::Unsize), target };
match reborrow {
None => vec![unsize],
Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
}
})?;
let mut selcx = traits::SelectionContext::new(self);
// Create an obligation for `Source: CoerceUnsized<Target>`.
let cause = ObligationCause::new(
self.cause.span,
self.body_id,
ObligationCauseCode::Coercion { source, target },
);
// Use a FIFO queue for this custom fulfillment procedure.
//
// A Vec (or SmallVec) is not a natural choice for a queue. However,
// this code path is hot, and this queue usually has a max length of 1
// and almost never more than 3. By using a SmallVec we avoid an
// allocation, at the (very small) cost of (occasionally) having to
// shift subsequent elements down when removing the front element.
let mut queue: SmallVec<[PredicateObligation<'tcx>; 4]> = smallvec![Obligation::new(
self.tcx,
cause,
self.fcx.param_env,
ty::TraitRef::new(self.tcx, coerce_unsized_did, [coerce_source, coerce_target])
)];
let mut has_unsized_tuple_coercion = false;
let mut has_trait_upcasting_coercion = None;
// Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
// emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
// inference might unify those two inner type variables later.
let traits = [coerce_unsized_did, unsize_did];
while !queue.is_empty() {
let obligation = queue.remove(0);
debug!("coerce_unsized resolve step: {:?}", obligation);
let trait_pred = match obligation.predicate.kind().no_bound_vars() {
Some(ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)))
if traits.contains(&trait_pred.def_id()) =>
{
trait_pred
}
_ => {
coercion.obligations.push(obligation);
continue;
}
};
let trait_pred = self.resolve_vars_if_possible(trait_pred);
match selcx.select(&obligation.with(selcx.tcx(), trait_pred)) {
// Uncertain or unimplemented.
Ok(None) => {
if trait_pred.def_id() == unsize_did {
let self_ty = trait_pred.self_ty();
let unsize_ty = trait_pred.trait_ref.args[1].expect_ty();
debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
match (self_ty.kind(), unsize_ty.kind()) {
(&ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
if self.type_var_is_sized(v) =>
{
debug!("coerce_unsized: have sized infer {:?}", v);
coercion.obligations.push(obligation);
// `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
// for unsizing.
}
_ => {
// Some other case for `$0: Unsize<Something>`. Note that we
// hit this case even if `Something` is a sized type, so just
// don't do the coercion.
debug!("coerce_unsized: ambiguous unsize");
return Err(TypeError::Mismatch);
}
}
} else {
debug!("coerce_unsized: early return - ambiguous");
return Err(TypeError::Mismatch);
}
}
Err(traits::Unimplemented) => {
debug!("coerce_unsized: early return - can't prove obligation");
return Err(TypeError::Mismatch);
}
// Object safety violations or miscellaneous.
Err(err) => {
self.err_ctxt().report_selection_error(obligation.clone(), &obligation, &err);
// Treat this like an obligation and follow through
// with the unsizing - the lack of a coercion should
// be silent, as it causes a type mismatch later.
}
Ok(Some(impl_source)) => {
// Some builtin coercions are still unstable so we detect
// these here and emit a feature error if coercion doesn't fail
// due to another reason.
match impl_source {
traits::ImplSource::Builtin(
BuiltinImplSource::TraitUpcasting { .. },
_,
) => {
has_trait_upcasting_coercion =
Some((trait_pred.self_ty(), trait_pred.trait_ref.args.type_at(1)));
}
traits::ImplSource::Builtin(BuiltinImplSource::TupleUnsizing, _) => {
has_unsized_tuple_coercion = true;
}
_ => {}
}
queue.extend(impl_source.nested_obligations())
}
}
}
if let Some((sub, sup)) = has_trait_upcasting_coercion
&& !self.tcx().features().trait_upcasting
{
// Renders better when we erase regions, since they're not really the point here.
let (sub, sup) = self.tcx.erase_regions((sub, sup));
let mut err = feature_err(
&self.tcx.sess.parse_sess,
sym::trait_upcasting,
self.cause.span,
format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
);
err.note(format!("required when coercing `{source}` into `{target}`"));
err.emit();
}
if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
feature_err(
&self.tcx.sess.parse_sess,
sym::unsized_tuple_coercion,
self.cause.span,
"unsized tuple coercion is not stable enough for use and is subject to change",
)
.emit();
}
Ok(coercion)
}
fn coerce_dyn_star(
&self,
a: Ty<'tcx>,
b: Ty<'tcx>,
predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
b_region: ty::Region<'tcx>,
) -> CoerceResult<'tcx> {
if !self.tcx.features().dyn_star {
return Err(TypeError::Mismatch);
}
if let ty::Dynamic(a_data, _, _) = a.kind()
&& let ty::Dynamic(b_data, _, _) = b.kind()
&& a_data.principal_def_id() == b_data.principal_def_id()
{
return self.unify_and(a, b, |_| vec![]);
}
// Check the obligations of the cast -- for example, when casting
// `usize` to `dyn* Clone + 'static`:
let mut obligations: Vec<_> = predicates
.iter()
.map(|predicate| {
// For each existential predicate (e.g., `?Self: Clone`) substitute
// the type of the expression (e.g., `usize` in our example above)
// and then require that the resulting predicate (e.g., `usize: Clone`)
// holds (it does).
let predicate = predicate.with_self_ty(self.tcx, a);
Obligation::new(self.tcx, self.cause.clone(), self.param_env, predicate)
})
.chain([
// Enforce the region bound (e.g., `usize: 'static`, in our example).
Obligation::new(
self.tcx,
self.cause.clone(),
self.param_env,
ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(
ty::OutlivesPredicate(a, b_region),
))),
),
])
.collect();
// Enforce that the type is `usize`/pointer-sized.
obligations.push(Obligation::new(
self.tcx,
self.cause.clone(),
self.param_env,
ty::TraitRef::from_lang_item(
self.tcx,
hir::LangItem::PointerLike,
self.cause.span,
[a],
),
));
Ok(InferOk {
value: (vec![Adjustment { kind: Adjust::DynStar, target: b }], b),
obligations,
})
}
fn coerce_from_safe_fn<F, G>(
&self,
a: Ty<'tcx>,
fn_ty_a: ty::PolyFnSig<'tcx>,
b: Ty<'tcx>,
to_unsafe: F,
normal: G,
) -> CoerceResult<'tcx>
where
F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
{
self.commit_if_ok(|snapshot| {
let outer_universe = self.infcx.universe();
let result = if let ty::FnPtr(fn_ty_b) = b.kind()
&& let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) =
(fn_ty_a.unsafety(), fn_ty_b.unsafety())
{
let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
self.unify_and(unsafe_a, b, to_unsafe)
} else {
self.unify_and(a, b, normal)
};
// FIXME(#73154): This is a hack. Currently LUB can generate
// unsolvable constraints. Additionally, it returns `a`
// unconditionally, even when the "LUB" is `b`. In the future, we
// want the coerced type to be the actual supertype of these two,
// but for now, we want to just error to ensure we don't lock
// ourselves into a specific behavior with NLL.
self.leak_check(outer_universe, Some(snapshot))?;
result
})
}
fn coerce_from_fn_pointer(
&self,
a: Ty<'tcx>,
fn_ty_a: ty::PolyFnSig<'tcx>,
b: Ty<'tcx>,
) -> CoerceResult<'tcx> {
//! Attempts to coerce from the type of a Rust function item
//! into a closure or a `proc`.
//!
let b = self.shallow_resolve(b);
debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
self.coerce_from_safe_fn(
a,
fn_ty_a,
b,
simple(Adjust::Pointer(PointerCoercion::UnsafeFnPointer)),
identity,
)
}
fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
//! Attempts to coerce from the type of a Rust function item
//! into a closure or a `proc`.
let b = self.shallow_resolve(b);
let InferOk { value: b, mut obligations } =
self.at(&self.cause, self.param_env).normalize(b);
debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
match b.kind() {
ty::FnPtr(b_sig) => {
let a_sig = a.fn_sig(self.tcx);
if let ty::FnDef(def_id, _) = *a.kind() {
// Intrinsics are not coercible to function pointers
if self.tcx.is_intrinsic(def_id) {
return Err(TypeError::IntrinsicCast);
}
// Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).
if b_sig.unsafety() == hir::Unsafety::Normal
&& !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
{
return Err(TypeError::TargetFeatureCast(def_id));
}
}
let InferOk { value: a_sig, obligations: o1 } =
self.at(&self.cause, self.param_env).normalize(a_sig);
obligations.extend(o1);
let a_fn_pointer = Ty::new_fn_ptr(self.tcx, a_sig);
let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
a_fn_pointer,
a_sig,
b,
|unsafe_ty| {
vec![
Adjustment {
kind: Adjust::Pointer(PointerCoercion::ReifyFnPointer),
target: a_fn_pointer,
},
Adjustment {
kind: Adjust::Pointer(PointerCoercion::UnsafeFnPointer),
target: unsafe_ty,
},
]
},
simple(Adjust::Pointer(PointerCoercion::ReifyFnPointer)),
)?;
obligations.extend(o2);
Ok(InferOk { value, obligations })
}
_ => self.unify_and(a, b, identity),
}
}
fn coerce_closure_to_fn(
&self,
a: Ty<'tcx>,
closure_def_id_a: DefId,
args_a: GenericArgsRef<'tcx>,
b: Ty<'tcx>,
) -> CoerceResult<'tcx> {
//! Attempts to coerce from the type of a non-capturing closure
//! into a function pointer.
//!
let b = self.shallow_resolve(b);
match b.kind() {
// At this point we haven't done capture analysis, which means
// that the ClosureArgs just contains an inference variable instead
// of tuple of captured types.
//
// All we care here is if any variable is being captured and not the exact paths,
// so we check `upvars_mentioned` for root variables being captured.
ty::FnPtr(fn_ty)
if self
.tcx
.upvars_mentioned(closure_def_id_a.expect_local())
.map_or(true, |u| u.is_empty()) =>
{
// We coerce the closure, which has fn type
// `extern "rust-call" fn((arg0,arg1,...)) -> _`
// to
// `fn(arg0,arg1,...) -> _`
// or
// `unsafe fn(arg0,arg1,...) -> _`
let closure_sig = args_a.as_closure().sig();
let unsafety = fn_ty.unsafety();
let pointer_ty =
Ty::new_fn_ptr(self.tcx, self.tcx.signature_unclosure(closure_sig, unsafety));
debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
self.unify_and(
pointer_ty,
b,
simple(Adjust::Pointer(PointerCoercion::ClosureFnPointer(unsafety))),
)
}
_ => self.unify_and(a, b, identity),
}
}
fn coerce_unsafe_ptr(
&self,
a: Ty<'tcx>,
b: Ty<'tcx>,
mutbl_b: hir::Mutability,
) -> CoerceResult<'tcx> {
debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
let (is_ref, mt_a) = match *a.kind() {
ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
ty::RawPtr(mt) => (false, mt),
_ => return self.unify_and(a, b, identity),
};
coerce_mutbls(mt_a.mutbl, mutbl_b)?;
// Check that the types which they point at are compatible.
let a_unsafe = Ty::new_ptr(self.tcx, ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty });
// Although references and unsafe ptrs have the same
// representation, we still register an Adjust::DerefRef so that
// regionck knows that the region for `a` must be valid here.
if is_ref {
self.unify_and(a_unsafe, b, |target| {
vec![
Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
]
})
} else if mt_a.mutbl != mutbl_b {
self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCoercion::MutToConstPointer)))
} else {
self.unify_and(a_unsafe, b, identity)
}
}
}
impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
/// Attempt to coerce an expression to a type, and return the
/// adjusted type of the expression, if successful.
/// Adjustments are only recorded if the coercion succeeded.
/// The expressions *must not* have any preexisting adjustments.
pub fn coerce(
&self,
expr: &hir::Expr<'_>,
expr_ty: Ty<'tcx>,
mut target: Ty<'tcx>,
allow_two_phase: AllowTwoPhase,
cause: Option<ObligationCause<'tcx>>,
) -> RelateResult<'tcx, Ty<'tcx>> {
let source = self.try_structurally_resolve_type(expr.span, expr_ty);
if self.next_trait_solver() {
target = self.try_structurally_resolve_type(
cause.as_ref().map_or(expr.span, |cause| cause.span),
target,
);
}
debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
let cause =
cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
let coerce = Coerce::new(self, cause, allow_two_phase);
let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
let (adjustments, _) = self.register_infer_ok_obligations(ok);
self.apply_adjustments(expr, adjustments);
Ok(if let Err(guar) = expr_ty.error_reported() {
Ty::new_error(self.tcx, guar)
} else {
target
})
}
/// Same as `coerce()`, but without side-effects.
///
/// Returns false if the coercion creates any obligations that result in
/// errors.
pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
// FIXME(-Ztrait-solver=next): We need to structurally resolve both types here.
let source = self.resolve_vars_with_obligations(expr_ty);
debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);
let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
// We don't ever need two-phase here since we throw out the result of the coercion
let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
self.probe(|_| {
let Ok(ok) = coerce.coerce(source, target) else {
return false;
};
let ocx = ObligationCtxt::new(self);
ocx.register_obligations(ok.obligations);
ocx.select_where_possible().is_empty()
})
}
/// Given a type and a target type, this function will calculate and return
/// how many dereference steps needed to achieve `expr_ty <: target`. If
/// it's not possible, return `None`.
pub fn deref_steps(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> Option<usize> {
let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
// We don't ever need two-phase here since we throw out the result of the coercion
let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
coerce
.autoderef(rustc_span::DUMMY_SP, expr_ty)
.find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
}
/// Given a type, this function will calculate and return the type given
/// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
///
/// This function is for diagnostics only, since it does not register
/// trait or region sub-obligations. (presumably we could, but it's not
/// particularly important for diagnostics...)
pub fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
self.autoderef(rustc_span::DUMMY_SP, expr_ty).nth(1).and_then(|(deref_ty, _)| {
self.infcx
.type_implements_trait(
self.tcx.lang_items().deref_mut_trait()?,
[expr_ty],
self.param_env,
)
.may_apply()
.then_some(deref_ty)
})
}
/// Given some expressions, their known unified type and another expression,
/// tries to unify the types, potentially inserting coercions on any of the
/// provided expressions and returns their LUB (aka "common supertype").
///
/// This is really an internal helper. From outside the coercion
/// module, you should instantiate a `CoerceMany` instance.
fn try_find_coercion_lub<E>(
&self,
cause: &ObligationCause<'tcx>,
exprs: &[E],
prev_ty: Ty<'tcx>,
new: &hir::Expr<'_>,
new_ty: Ty<'tcx>,
) -> RelateResult<'tcx, Ty<'tcx>>
where
E: AsCoercionSite,
{
let prev_ty = self.try_structurally_resolve_type(cause.span, prev_ty);
let new_ty = self.try_structurally_resolve_type(new.span, new_ty);
debug!(
"coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
prev_ty,
new_ty,
exprs.len()
);
// The following check fixes #88097, where the compiler erroneously
// attempted to coerce a closure type to itself via a function pointer.
if prev_ty == new_ty {
return Ok(prev_ty);
}
// Special-case that coercion alone cannot handle:
// Function items or non-capturing closures of differing IDs or GenericArgs.
let (a_sig, b_sig) = {
let is_capturing_closure = |ty: Ty<'tcx>| {
if let &ty::Closure(closure_def_id, _args) = ty.kind() {
self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some()
} else {
false
}
};
if is_capturing_closure(prev_ty) || is_capturing_closure(new_ty) {
(None, None)
} else {
match (prev_ty.kind(), new_ty.kind()) {
(ty::FnDef(..), ty::FnDef(..)) => {
// Don't reify if the function types have a LUB, i.e., they
// are the same function and their parameters have a LUB.
match self.commit_if_ok(|_| {
self.at(cause, self.param_env).lub(
DefineOpaqueTypes::No,
prev_ty,
new_ty,
)
}) {
// We have a LUB of prev_ty and new_ty, just return it.
Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
Err(_) => {
(Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx)))
}
}
}
(ty::Closure(_, args), ty::FnDef(..)) => {
let b_sig = new_ty.fn_sig(self.tcx);
let a_sig =
self.tcx.signature_unclosure(args.as_closure().sig(), b_sig.unsafety());
(Some(a_sig), Some(b_sig))
}
(ty::FnDef(..), ty::Closure(_, args)) => {
let a_sig = prev_ty.fn_sig(self.tcx);
let b_sig =
self.tcx.signature_unclosure(args.as_closure().sig(), a_sig.unsafety());
(Some(a_sig), Some(b_sig))
}
(ty::Closure(_, args_a), ty::Closure(_, args_b)) => {
(
Some(self.tcx.signature_unclosure(
args_a.as_closure().sig(),
hir::Unsafety::Normal,
)),
Some(self.tcx.signature_unclosure(
args_b.as_closure().sig(),
hir::Unsafety::Normal,
)),
)
}
_ => (None, None),
}
}
};
if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) {
// Intrinsics are not coercible to function pointers.
if a_sig.abi() == Abi::RustIntrinsic
|| a_sig.abi() == Abi::PlatformIntrinsic
|| b_sig.abi() == Abi::RustIntrinsic
|| b_sig.abi() == Abi::PlatformIntrinsic
{
return Err(TypeError::IntrinsicCast);
}
// The signature must match.
let (a_sig, b_sig) = self.normalize(new.span, (a_sig, b_sig));
let sig = self
.at(cause, self.param_env)
.trace(prev_ty, new_ty)
.lub(DefineOpaqueTypes::No, a_sig, b_sig)
.map(|ok| self.register_infer_ok_obligations(ok))?;
// Reify both sides and return the reified fn pointer type.
let fn_ptr = Ty::new_fn_ptr(self.tcx, sig);
let prev_adjustment = match prev_ty.kind() {
ty::Closure(..) => {
Adjust::Pointer(PointerCoercion::ClosureFnPointer(a_sig.unsafety()))
}
ty::FnDef(..) => Adjust::Pointer(PointerCoercion::ReifyFnPointer),
_ => unreachable!(),
};
let next_adjustment = match new_ty.kind() {
ty::Closure(..) => {
Adjust::Pointer(PointerCoercion::ClosureFnPointer(b_sig.unsafety()))
}
ty::FnDef(..) => Adjust::Pointer(PointerCoercion::ReifyFnPointer),
_ => unreachable!(),
};
for expr in exprs.iter().map(|e| e.as_coercion_site()) {
self.apply_adjustments(
expr,
vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
);
}
self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
return Ok(fn_ptr);
}
// Configure a Coerce instance to compute the LUB.
// We don't allow two-phase borrows on any autorefs this creates since we
// probably aren't processing function arguments here and even if we were,
// they're going to get autorefed again anyway and we can apply 2-phase borrows
// at that time.
let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
coerce.use_lub = true;
// First try to coerce the new expression to the type of the previous ones,
// but only if the new expression has no coercion already applied to it.
let mut first_error = None;
if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
match result {
Ok(ok) => {
let (adjustments, target) = self.register_infer_ok_obligations(ok);
self.apply_adjustments(new, adjustments);
debug!(
"coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
new_ty, prev_ty, target
);
return Ok(target);
}
Err(e) => first_error = Some(e),
}
}
// Then try to coerce the previous expressions to the type of the new one.
// This requires ensuring there are no coercions applied to *any* of the
// previous expressions, other than noop reborrows (ignoring lifetimes).
for expr in exprs {
let expr = expr.as_coercion_site();
let noop = match self.typeck_results.borrow().expr_adjustments(expr) {
&[
Adjustment { kind: Adjust::Deref(_), .. },
Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. },
] => {
match *self.node_ty(expr.hir_id).kind() {
ty::Ref(_, _, mt_orig) => {
let mutbl_adj: hir::Mutability = mutbl_adj.into();
// Reborrow that we can safely ignore, because
// the next adjustment can only be a Deref
// which will be merged into it.
mutbl_adj == mt_orig
}
_ => false,
}
}
&[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
_ => false,
};
if !noop {
debug!(
"coercion::try_find_coercion_lub: older expression {:?} had adjustments, requiring LUB",
expr,
);
return self
.commit_if_ok(|_| {
self.at(cause, self.param_env).lub(DefineOpaqueTypes::No, prev_ty, new_ty)
})
.map(|ok| self.register_infer_ok_obligations(ok));
}
}
match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
Err(_) => {
// Avoid giving strange errors on failed attempts.
if let Some(e) = first_error {
Err(e)
} else {
self.commit_if_ok(|_| {
self.at(cause, self.param_env).lub(DefineOpaqueTypes::No, prev_ty, new_ty)
})
.map(|ok| self.register_infer_ok_obligations(ok))
}
}
Ok(ok) => {
let (adjustments, target) = self.register_infer_ok_obligations(ok);
for expr in exprs {
let expr = expr.as_coercion_site();
self.apply_adjustments(expr, adjustments.clone());
}
debug!(
"coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
prev_ty, new_ty, target
);
Ok(target)
}
}
}
}
/// CoerceMany encapsulates the pattern you should use when you have
/// many expressions that are all getting coerced to a common
/// type. This arises, for example, when you have a match (the result
/// of each arm is coerced to a common type). It also arises in less
/// obvious places, such as when you have many `break foo` expressions
/// that target the same loop, or the various `return` expressions in
/// a function.
///
/// The basic protocol is as follows:
///
/// - Instantiate the `CoerceMany` with an initial `expected_ty`.
/// This will also serve as the "starting LUB". The expectation is
/// that this type is something which all of the expressions *must*
/// be coercible to. Use a fresh type variable if needed.
/// - For each expression whose result is to be coerced, invoke `coerce()` with.
/// - In some cases we wish to coerce "non-expressions" whose types are implicitly
/// unit. This happens for example if you have a `break` with no expression,
/// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
/// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
/// from you so that you don't have to worry your pretty head about it.
/// But if an error is reported, the final type will be `err`.
/// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
/// previously coerced expressions.
/// - When all done, invoke `complete()`. This will return the LUB of
/// all your expressions.
/// - WARNING: I don't believe this final type is guaranteed to be
/// related to your initial `expected_ty` in any particular way,
/// although it will typically be a subtype, so you should check it.
/// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
/// previously coerced expressions.
///
/// Example:
///
/// ```ignore (illustrative)
/// let mut coerce = CoerceMany::new(expected_ty);
/// for expr in exprs {
/// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
/// coerce.coerce(fcx, &cause, expr, expr_ty);
/// }
/// let final_ty = coerce.complete(fcx);
/// ```
pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
expected_ty: Ty<'tcx>,
final_ty: Option<Ty<'tcx>>,
expressions: Expressions<'tcx, 'exprs, E>,
pushed: usize,
}
/// The type of a `CoerceMany` that is storing up the expressions into
/// a buffer. We use this in `check/mod.rs` for things like `break`.
pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;
enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
UpFront(&'exprs [E]),
}
impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
/// The usual case; collect the set of expressions dynamically.
/// If the full set of coercion sites is known before hand,
/// consider `with_coercion_sites()` instead to avoid allocation.
pub fn new(expected_ty: Ty<'tcx>) -> Self {
Self::make(expected_ty, Expressions::Dynamic(vec![]))
}
/// As an optimization, you can create a `CoerceMany` with a
/// preexisting slice of expressions. In this case, you are
/// expected to pass each element in the slice to `coerce(...)` in
/// order. This is used with arrays in particular to avoid
/// needlessly cloning the slice.
pub fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
Self::make(expected_ty, Expressions::UpFront(coercion_sites))
}
fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
}
/// Returns the "expected type" with which this coercion was
/// constructed. This represents the "downward propagated" type
/// that was given to us at the start of typing whatever construct
/// we are typing (e.g., the match expression).
///
/// Typically, this is used as the expected type when
/// type-checking each of the alternative expressions whose types
/// we are trying to merge.
pub fn expected_ty(&self) -> Ty<'tcx> {
self.expected_ty
}
/// Returns the current "merged type", representing our best-guess
/// at the LUB of the expressions we've seen so far (if any). This
/// isn't *final* until you call `self.complete()`, which will return
/// the merged type.
pub fn merged_ty(&self) -> Ty<'tcx> {
self.final_ty.unwrap_or(self.expected_ty)
}
/// Indicates that the value generated by `expression`, which is
/// of type `expression_ty`, is one of the possibilities that we
/// could coerce from. This will record `expression`, and later
/// calls to `coerce` may come back and add adjustments and things
/// if necessary.
pub fn coerce<'a>(
&mut self,
fcx: &FnCtxt<'a, 'tcx>,
cause: &ObligationCause<'tcx>,
expression: &'tcx hir::Expr<'tcx>,
expression_ty: Ty<'tcx>,
) {
self.coerce_inner(fcx, cause, Some(expression), expression_ty, |_| {}, false)
}
/// Indicates that one of the inputs is a "forced unit". This
/// occurs in a case like `if foo { ... };`, where the missing else
/// generates a "forced unit". Another example is a `loop { break;
/// }`, where the `break` has no argument expression. We treat
/// these cases slightly differently for error-reporting
/// purposes. Note that these tend to correspond to cases where
/// the `()` expression is implicit in the source, and hence we do
/// not take an expression argument.
///
/// The `augment_error` gives you a chance to extend the error
/// message, in case any results (e.g., we use this to suggest
/// removing a `;`).
pub fn coerce_forced_unit<'a>(
&mut self,
fcx: &FnCtxt<'a, 'tcx>,
cause: &ObligationCause<'tcx>,
augment_error: impl FnOnce(&mut Diagnostic),
label_unit_as_expected: bool,
) {
self.coerce_inner(
fcx,
cause,
None,
Ty::new_unit(fcx.tcx),
augment_error,
label_unit_as_expected,
)
}
/// The inner coercion "engine". If `expression` is `None`, this
/// is a forced-unit case, and hence `expression_ty` must be
/// `Nil`.
#[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
pub(crate) fn coerce_inner<'a>(
&mut self,
fcx: &FnCtxt<'a, 'tcx>,
cause: &ObligationCause<'tcx>,
expression: Option<&'tcx hir::Expr<'tcx>>,
mut expression_ty: Ty<'tcx>,
augment_error: impl FnOnce(&mut Diagnostic),
label_expression_as_expected: bool,
) {
// Incorporate whatever type inference information we have
// until now; in principle we might also want to process
// pending obligations, but doing so should only improve
// compatibility (hopefully that is true) by helping us
// uncover never types better.
if expression_ty.is_ty_var() {
expression_ty = fcx.infcx.shallow_resolve(expression_ty);
}
// If we see any error types, just propagate that error
// upwards.
if let Err(guar) = (expression_ty, self.merged_ty()).error_reported() {
self.final_ty = Some(Ty::new_error(fcx.tcx, guar));
return;
}
// Handle the actual type unification etc.
let result = if let Some(expression) = expression {
if self.pushed == 0 {
// Special-case the first expression we are coercing.
// To be honest, I'm not entirely sure why we do this.
// We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
fcx.coerce(
expression,
expression_ty,
self.expected_ty,
AllowTwoPhase::No,
Some(cause.clone()),
)
} else {
match self.expressions {
Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
cause,
exprs,
self.merged_ty(),
expression,
expression_ty,
),
Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub(
cause,
&coercion_sites[0..self.pushed],
self.merged_ty(),
expression,
expression_ty,
),
}
}
} else {
// this is a hack for cases where we default to `()` because
// the expression etc has been omitted from the source. An
// example is an `if let` without an else:
//
// if let Some(x) = ... { }
//
// we wind up with a second match arm that is like `_ =>
// ()`. That is the case we are considering here. We take
// a different path to get the right "expected, found"
// message and so forth (and because we know that
// `expression_ty` will be unit).
//
// Another example is `break` with no argument expression.
assert!(expression_ty.is_unit(), "if let hack without unit type");
fcx.at(cause, fcx.param_env)
// needed for tests/ui/type-alias-impl-trait/issue-65679-inst-opaque-ty-from-val-twice.rs
.eq_exp(
DefineOpaqueTypes::Yes,
label_expression_as_expected,
expression_ty,
self.merged_ty(),
)
.map(|infer_ok| {
fcx.register_infer_ok_obligations(infer_ok);
expression_ty
})
};
debug!(?result);
match result {
Ok(v) => {
self.final_ty = Some(v);
if let Some(e) = expression {
match self.expressions {
Expressions::Dynamic(ref mut buffer) => buffer.push(e),
Expressions::UpFront(coercion_sites) => {
// if the user gave us an array to validate, check that we got
// the next expression in the list, as expected
assert_eq!(
coercion_sites[self.pushed].as_coercion_site().hir_id,
e.hir_id
);
}
}
self.pushed += 1;
}
}
Err(coercion_error) => {
// Mark that we've failed to coerce the types here to suppress
// any superfluous errors we might encounter while trying to
// emit or provide suggestions on how to fix the initial error.
fcx.set_tainted_by_errors(
fcx.tcx.sess.delay_span_bug(cause.span, "coercion error but no error emitted"),
);
let (expected, found) = if label_expression_as_expected {
// In the case where this is a "forced unit", like
// `break`, we want to call the `()` "expected"
// since it is implied by the syntax.
// (Note: not all force-units work this way.)"
(expression_ty, self.merged_ty())
} else {
// Otherwise, the "expected" type for error
// reporting is the current unification type,
// which is basically the LUB of the expressions
// we've seen so far (combined with the expected
// type)
(self.merged_ty(), expression_ty)
};
let (expected, found) = fcx.resolve_vars_if_possible((expected, found));
let mut err;
let mut unsized_return = false;
let mut visitor = CollectRetsVisitor { ret_exprs: vec![] };
match *cause.code() {
ObligationCauseCode::ReturnNoExpression => {
err = struct_span_err!(
fcx.tcx.sess,
cause.span,
E0069,
"`return;` in a function whose return type is not `()`"
);
err.span_label(cause.span, "return type is not `()`");
}
ObligationCauseCode::BlockTailExpression(blk_id, ..) => {
let parent_id = fcx.tcx.hir().parent_id(blk_id);
err = self.report_return_mismatched_types(
cause,
expected,
found,
coercion_error,
fcx,
parent_id,
expression,
Some(blk_id),
);
if !fcx.tcx.features().unsized_locals {
unsized_return = self.is_return_ty_definitely_unsized(fcx);
}
if let Some(expression) = expression
&& let hir::ExprKind::Loop(loop_blk, ..) = expression.kind {
intravisit::walk_block(& mut visitor, loop_blk);
}
}
ObligationCauseCode::ReturnValue(id) => {
err = self.report_return_mismatched_types(
cause,
expected,
found,
coercion_error,
fcx,
id,
expression,
None,
);
if !fcx.tcx.features().unsized_locals {
unsized_return = self.is_return_ty_definitely_unsized(fcx);
}
}
_ => {
err = fcx.err_ctxt().report_mismatched_types(
cause,
expected,
found,
coercion_error,
);
}
}
augment_error(&mut err);
if let Some(expr) = expression {
fcx.emit_coerce_suggestions(
&mut err,
expr,
found,
expected,
None,
Some(coercion_error),
);
}
if visitor.ret_exprs.len() > 0 && let Some(expr) = expression {
self.note_unreachable_loop_return(&mut err, &expr, &visitor.ret_exprs);
}
let reported = err.emit_unless(unsized_return);
self.final_ty = Some(Ty::new_error(fcx.tcx, reported));
}
}
}
fn note_unreachable_loop_return(
&self,
err: &mut Diagnostic,
expr: &hir::Expr<'tcx>,
ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>,
) {
let hir::ExprKind::Loop(_, _, _, loop_span) = expr.kind else {
return;
};
let mut span: MultiSpan = vec![loop_span].into();
span.push_span_label(loop_span, "this might have zero elements to iterate on");
const MAXITER: usize = 3;
let iter = ret_exprs.iter().take(MAXITER);
for ret_expr in iter {
span.push_span_label(
ret_expr.span,
"if the loop doesn't execute, this value would never get returned",
);
}
err.span_note(
span,
"the function expects a value to always be returned, but loops might run zero times",
);
if MAXITER < ret_exprs.len() {
err.note(format!(
"if the loop doesn't execute, {} other values would never get returned",
ret_exprs.len() - MAXITER
));
}
err.help(
"return a value for the case when the loop has zero elements to iterate on, or \
consider changing the return type to account for that possibility",
);
}
fn report_return_mismatched_types<'a>(
&self,
cause: &ObligationCause<'tcx>,
expected: Ty<'tcx>,
found: Ty<'tcx>,
ty_err: TypeError<'tcx>,
fcx: &FnCtxt<'a, 'tcx>,
id: hir::HirId,
expression: Option<&'tcx hir::Expr<'tcx>>,
blk_id: Option<hir::HirId>,
) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
let mut err = fcx.err_ctxt().report_mismatched_types(cause, expected, found, ty_err);
let parent_id = fcx.tcx.hir().parent_id(id);
let parent = fcx.tcx.hir().get(parent_id);
if let Some(expr) = expression
&& let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(&hir::Closure { body, .. }), .. }) = parent
&& !matches!(fcx.tcx.hir().body(body).value.kind, hir::ExprKind::Block(..))
{
fcx.suggest_missing_semicolon(&mut err, expr, expected, true);
}
// Verify that this is a tail expression of a function, otherwise the
// label pointing out the cause for the type coercion will be wrong
// as prior return coercions would not be relevant (#57664).
let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) {
let pointing_at_return_type =
fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id);
if let (Some(cond_expr), true, false) = (
fcx.tcx.hir().get_if_cause(expr.hir_id),
expected.is_unit(),
pointing_at_return_type,
)
// If the block is from an external macro or try (`?`) desugaring, then
// do not suggest adding a semicolon, because there's nowhere to put it.
// See issues #81943 and #87051.
&& matches!(
cond_expr.span.desugaring_kind(),
None | Some(DesugaringKind::WhileLoop)
) && !in_external_macro(fcx.tcx.sess, cond_expr.span)
&& !matches!(
cond_expr.kind,
hir::ExprKind::Match(.., hir::MatchSource::TryDesugar(_))
)
{
err.span_label(cond_expr.span, "expected this to be `()`");
if expr.can_have_side_effects() {
fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
}
}
fcx.get_node_fn_decl(parent)
.map(|(fn_id, fn_decl, _, is_main)| (fn_id, fn_decl, is_main))
} else {
fcx.get_fn_decl(parent_id)
};
if let Some((fn_id, fn_decl, can_suggest)) = fn_decl {
if blk_id.is_none() {
fcx.suggest_missing_return_type(
&mut err,
&fn_decl,
expected,
found,
can_suggest,
fn_id,
);
}
}
let parent_id = fcx.tcx.hir().get_parent_item(id);
let parent_item = fcx.tcx.hir().get_by_def_id(parent_id.def_id);
if let (Some(expr), Some(_), Some((fn_id, fn_decl, _, _))) =
(expression, blk_id, fcx.get_node_fn_decl(parent_item))
{
fcx.suggest_missing_break_or_return_expr(
&mut err, expr, fn_decl, expected, found, id, fn_id,
);
}
let ret_coercion_span = fcx.ret_coercion_span.get();
if let Some(sp) = ret_coercion_span
// If the closure has an explicit return type annotation, or if
// the closure's return type has been inferred from outside
// requirements (such as an Fn* trait bound), then a type error
// may occur at the first return expression we see in the closure
// (if it conflicts with the declared return type). Skip adding a
// note in this case, since it would be incorrect.
&& let Some(fn_sig) = fcx.body_fn_sig()
&& fn_sig.output().is_ty_var()
{
err.span_note(
sp,
format!(
"return type inferred to be `{expected}` here"
),
);
}
err
}
/// Checks whether the return type is unsized via an obligation, which makes
/// sure we consider `dyn Trait: Sized` where clauses, which are trivially
/// false but technically valid for typeck.
fn is_return_ty_definitely_unsized(&self, fcx: &FnCtxt<'_, 'tcx>) -> bool {
if let Some(sig) = fcx.body_fn_sig() {
!fcx.predicate_may_hold(&Obligation::new(
fcx.tcx,
ObligationCause::dummy(),
fcx.param_env,
ty::TraitRef::new(
fcx.tcx,
fcx.tcx.require_lang_item(hir::LangItem::Sized, None),
[sig.output()],
),
))
} else {
false
}
}
pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
if let Some(final_ty) = self.final_ty {
final_ty
} else {
// If we only had inputs that were of type `!` (or no
// inputs at all), then the final type is `!`.
assert_eq!(self.pushed, 0);
fcx.tcx.types.never
}
}
}
/// Something that can be converted into an expression to which we can
/// apply a coercion.
pub trait AsCoercionSite {
fn as_coercion_site(&self) -> &hir::Expr<'_>;
}
impl AsCoercionSite for hir::Expr<'_> {
fn as_coercion_site(&self) -> &hir::Expr<'_> {
self
}
}
impl<'a, T> AsCoercionSite for &'a T
where
T: AsCoercionSite,
{
fn as_coercion_site(&self) -> &hir::Expr<'_> {
(**self).as_coercion_site()
}
}
impl AsCoercionSite for ! {
fn as_coercion_site(&self) -> &hir::Expr<'_> {
unreachable!()
}
}
impl AsCoercionSite for hir::Arm<'_> {
fn as_coercion_site(&self) -> &hir::Expr<'_> {
&self.body
}
}