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use rustc_hir as hir;
use rustc_infer::infer::TyCtxtInferExt;
use rustc_macros::{LintDiagnostic, Subdiagnostic};
use rustc_middle::ty::{
self, fold::BottomUpFolder, print::TraitPredPrintModifiersAndPath, Ty, TypeFoldable,
};
use rustc_span::Span;
use rustc_trait_selection::traits;
use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
use crate::{LateContext, LateLintPass, LintContext};
declare_lint! {
/// The `opaque_hidden_inferred_bound` lint detects cases in which nested
/// `impl Trait` in associated type bounds are not written generally enough
/// to satisfy the bounds of the associated type.
///
/// ### Explanation
///
/// This functionality was removed in #97346, but then rolled back in #99860
/// because it caused regressions.
///
/// We plan on reintroducing this as a hard error, but in the mean time,
/// this lint serves to warn and suggest fixes for any use-cases which rely
/// on this behavior.
///
/// ### Example
///
/// ```rust
/// trait Duh {}
///
/// impl Duh for i32 {}
///
/// trait Trait {
/// type Assoc: Duh;
/// }
///
/// struct Struct;
///
/// impl<F: Duh> Trait for F {
/// type Assoc = F;
/// }
///
/// fn test() -> impl Trait<Assoc = impl Sized> {
/// 42
/// }
/// ```
///
/// {{produces}}
///
/// In this example, `test` declares that the associated type `Assoc` for
/// `impl Trait` is `impl Sized`, which does not satisfy the `Send` bound
/// on the associated type.
///
/// Although the hidden type, `i32` does satisfy this bound, we do not
/// consider the return type to be well-formed with this lint. It can be
/// fixed by changing `impl Sized` into `impl Sized + Send`.
pub OPAQUE_HIDDEN_INFERRED_BOUND,
Warn,
"detects the use of nested `impl Trait` types in associated type bounds that are not general enough"
}
declare_lint_pass!(OpaqueHiddenInferredBound => [OPAQUE_HIDDEN_INFERRED_BOUND]);
impl<'tcx> LateLintPass<'tcx> for OpaqueHiddenInferredBound {
fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
let hir::ItemKind::OpaqueTy(_) = &item.kind else { return; };
let def_id = item.owner_id.def_id.to_def_id();
let infcx = &cx.tcx.infer_ctxt().build();
// For every projection predicate in the opaque type's explicit bounds,
// check that the type that we're assigning actually satisfies the bounds
// of the associated type.
for &(pred, pred_span) in cx.tcx.explicit_item_bounds(def_id) {
// Liberate bound regions in the predicate since we
// don't actually care about lifetimes in this check.
let predicate = cx.tcx.liberate_late_bound_regions(def_id, pred.kind());
let ty::PredicateKind::Clause(ty::Clause::Projection(proj)) = predicate else {
continue;
};
// Only check types, since those are the only things that may
// have opaques in them anyways.
let Some(proj_term) = proj.term.ty() else { continue };
let proj_ty =
cx.tcx.mk_projection(proj.projection_ty.def_id, proj.projection_ty.substs);
// For every instance of the projection type in the bounds,
// replace them with the term we're assigning to the associated
// type in our opaque type.
let proj_replacer = &mut BottomUpFolder {
tcx: cx.tcx,
ty_op: |ty| if ty == proj_ty { proj_term } else { ty },
lt_op: |lt| lt,
ct_op: |ct| ct,
};
// For example, in `impl Trait<Assoc = impl Send>`, for all of the bounds on `Assoc`,
// e.g. `type Assoc: OtherTrait`, replace `<impl Trait as Trait>::Assoc: OtherTrait`
// with `impl Send: OtherTrait`.
for (assoc_pred, assoc_pred_span) in cx
.tcx
.bound_explicit_item_bounds(proj.projection_ty.def_id)
.subst_iter_copied(cx.tcx, &proj.projection_ty.substs)
{
let assoc_pred = assoc_pred.fold_with(proj_replacer);
let Ok(assoc_pred) = traits::fully_normalize(infcx, traits::ObligationCause::dummy(), cx.param_env, assoc_pred) else {
continue;
};
// If that predicate doesn't hold modulo regions (but passed during type-check),
// then we must've taken advantage of the hack in `project_and_unify_types` where
// we replace opaques with inference vars. Emit a warning!
if !infcx.predicate_must_hold_modulo_regions(&traits::Obligation::new(
cx.tcx,
traits::ObligationCause::dummy(),
cx.param_env,
assoc_pred,
)) {
// If it's a trait bound and an opaque that doesn't satisfy it,
// then we can emit a suggestion to add the bound.
let add_bound = match (proj_term.kind(), assoc_pred.kind().skip_binder()) {
(
ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }),
ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)),
) => Some(AddBound {
suggest_span: cx.tcx.def_span(*def_id).shrink_to_hi(),
trait_ref: trait_pred.print_modifiers_and_trait_path(),
}),
_ => None,
};
cx.emit_spanned_lint(
OPAQUE_HIDDEN_INFERRED_BOUND,
pred_span,
OpaqueHiddenInferredBoundLint {
ty: cx.tcx.mk_opaque(
def_id,
ty::InternalSubsts::identity_for_item(cx.tcx, def_id),
),
proj_ty: proj_term,
assoc_pred_span,
add_bound,
},
);
}
}
}
}
}
#[derive(LintDiagnostic)]
#[diag(lint_opaque_hidden_inferred_bound)]
struct OpaqueHiddenInferredBoundLint<'tcx> {
ty: Ty<'tcx>,
proj_ty: Ty<'tcx>,
#[label(specifically)]
assoc_pred_span: Span,
#[subdiagnostic]
add_bound: Option<AddBound<'tcx>>,
}
#[derive(Subdiagnostic)]
#[suggestion(
lint_opaque_hidden_inferred_bound_sugg,
style = "verbose",
applicability = "machine-applicable",
code = " + {trait_ref}"
)]
struct AddBound<'tcx> {
#[primary_span]
suggest_span: Span,
#[skip_arg]
trait_ref: TraitPredPrintModifiersAndPath<'tcx>,
}