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use rustc_hir::HirId;
use rustc_middle::mir::visit::Visitor;
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
use rustc_session::lint::builtin::CONST_ITEM_MUTATION;
use rustc_span::def_id::DefId;
use rustc_span::Span;
use crate::{errors, MirLint};
pub struct CheckConstItemMutation;
impl<'tcx> MirLint<'tcx> for CheckConstItemMutation {
fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) {
let mut checker = ConstMutationChecker { body, tcx, target_local: None };
checker.visit_body(&body);
}
}
struct ConstMutationChecker<'a, 'tcx> {
body: &'a Body<'tcx>,
tcx: TyCtxt<'tcx>,
target_local: Option<Local>,
}
impl<'tcx> ConstMutationChecker<'_, 'tcx> {
fn is_const_item(&self, local: Local) -> Option<DefId> {
if let LocalInfo::ConstRef { def_id } = *self.body.local_decls[local].local_info() {
Some(def_id)
} else {
None
}
}
fn is_const_item_without_destructor(&self, local: Local) -> Option<DefId> {
let def_id = self.is_const_item(local)?;
// We avoid linting mutation of a const item if the const's type has a
// Drop impl. The Drop logic observes the mutation which was performed.
//
// pub struct Log { msg: &'static str }
// pub const LOG: Log = Log { msg: "" };
// impl Drop for Log {
// fn drop(&mut self) { println!("{}", self.msg); }
// }
//
// LOG.msg = "wow"; // prints "wow"
//
// FIXME(https://github.com/rust-lang/rust/issues/77425):
// Drop this exception once there is a stable attribute to suppress the
// const item mutation lint for a single specific const only. Something
// equivalent to:
//
// #[const_mutation_allowed]
// pub const LOG: Log = Log { msg: "" };
match self.tcx.calculate_dtor(def_id, |_, _| Ok(())) {
Some(_) => None,
None => Some(def_id),
}
}
/// If we should lint on this usage, return the [`HirId`], source [`Span`]
/// and [`Span`] of the const item to use in the lint.
fn should_lint_const_item_usage(
&self,
place: &Place<'tcx>,
const_item: DefId,
location: Location,
) -> Option<(HirId, Span, Span)> {
// Don't lint on borrowing/assigning when a dereference is involved.
// If we 'leave' the temporary via a dereference, we must
// be modifying something else
//
// `unsafe { *FOO = 0; *BAR.field = 1; }`
// `unsafe { &mut *FOO }`
// `unsafe { (*ARRAY)[0] = val; }`
if !place.projection.iter().any(|p| matches!(p, PlaceElem::Deref)) {
let source_info = self.body.source_info(location);
let lint_root = self.body.source_scopes[source_info.scope]
.local_data
.as_ref()
.assert_crate_local()
.lint_root;
Some((lint_root, source_info.span, self.tcx.def_span(const_item)))
} else {
None
}
}
}
impl<'tcx> Visitor<'tcx> for ConstMutationChecker<'_, 'tcx> {
fn visit_statement(&mut self, stmt: &Statement<'tcx>, loc: Location) {
if let StatementKind::Assign(box (lhs, _)) = &stmt.kind {
// Check for assignment to fields of a constant
// Assigning directly to a constant (e.g. `FOO = true;`) is a hard error,
// so emitting a lint would be redundant.
if !lhs.projection.is_empty() {
if let Some(def_id) = self.is_const_item_without_destructor(lhs.local)
&& let Some((lint_root, span, item)) = self.should_lint_const_item_usage(&lhs, def_id, loc) {
self.tcx.emit_spanned_lint(
CONST_ITEM_MUTATION,
lint_root,
span,
errors::ConstMutate::Modify { konst: item }
);
}
}
// We are looking for MIR of the form:
//
// ```
// _1 = const FOO;
// _2 = &mut _1;
// method_call(_2, ..)
// ```
//
// Record our current LHS, so that we can detect this
// pattern in `visit_rvalue`
self.target_local = lhs.as_local();
}
self.super_statement(stmt, loc);
self.target_local = None;
}
fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, loc: Location) {
if let Rvalue::Ref(_, BorrowKind::Mut { .. }, place) = rvalue {
let local = place.local;
if let Some(def_id) = self.is_const_item(local) {
// If this Rvalue is being used as the right-hand side of a
// `StatementKind::Assign`, see if it ends up getting used as
// the `self` parameter of a method call (as the terminator of our current
// BasicBlock). If so, we emit a more specific lint.
let method_did = self.target_local.and_then(|target_local| {
rustc_middle::util::find_self_call(
self.tcx,
&self.body,
target_local,
loc.block,
)
});
let lint_loc =
if method_did.is_some() { self.body.terminator_loc(loc.block) } else { loc };
let method_call = if let Some((method_did, _)) = method_did {
Some(self.tcx.def_span(method_did))
} else {
None
};
if let Some((lint_root, span, item)) =
self.should_lint_const_item_usage(place, def_id, lint_loc)
{
self.tcx.emit_spanned_lint(
CONST_ITEM_MUTATION,
lint_root,
span,
errors::ConstMutate::MutBorrow { method_call, konst: item },
);
}
}
}
self.super_rvalue(rvalue, loc);
}
}