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Rollup merge of #78865 - Aaron1011:fix/const-item-mut-reborrow, r=varkor

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Don't fire `CONST_ITEM_MUTATION` lint when borrowing a deref

Fixes #78819

This extends the check for dereferences added in PR #77324
to cover mutable borrows, as well as direct writes. If we're operating
on a dereference of a `const` item, we shouldn't be firing the lint.
This commit is contained in:
Mara Bos 2020-11-08 13:36:33 +01:00 • committed by GitHub
commit 91759b2de5
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3 changed files with 47 additions and 33 deletions
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51
compiler/rustc_mir/src/transform/check_const_item_mutation.rs
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@ -61,22 +61,35 @@ impl<'a, 'tcx> ConstMutationChecker<'a, 'tcx> {
fn lint_const_item_usage(
&self,
place: &Place<'tcx>,
const_item: DefId,
location: Location,
decorate: impl for<'b> FnOnce(LintDiagnosticBuilder<'b>) -> DiagnosticBuilder<'b>,
) {
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;
// Don't lint on borrowing/assigning to a dereference
// e.g:
//
// `unsafe { *FOO = 0; *BAR.field = 1; }`
// `unsafe { &mut *FOO }`
if !matches!(place.projection.last(), Some(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;
self.tcx.struct_span_lint_hir(CONST_ITEM_MUTATION, lint_root, source_info.span, |lint| {
decorate(lint)
.span_note(self.tcx.def_span(const_item), "`const` item defined here")
.emit()
});
self.tcx.struct_span_lint_hir(
CONST_ITEM_MUTATION,
lint_root,
source_info.span,
|lint| {
decorate(lint)
.span_note(self.tcx.def_span(const_item), "`const` item defined here")
.emit()
},
);
}
}
}
@ -88,15 +101,11 @@ impl<'a, 'tcx> Visitor<'tcx> for ConstMutationChecker<'a, 'tcx> {
// 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) {
// Don't lint on writes through a pointer
// (e.g. `unsafe { *FOO = 0; *BAR.field = 1; }`)
if !matches!(lhs.projection.last(), Some(PlaceElem::Deref)) {
self.lint_const_item_usage(def_id, loc, |lint| {
let mut lint = lint.build("attempting to modify a `const` item");
lint.note("each usage of a `const` item creates a new temporary - the original `const` item will not be modified");
lint
})
}
self.lint_const_item_usage(&lhs, def_id, loc, |lint| {
let mut lint = lint.build("attempting to modify a `const` item");
lint.note("each usage of a `const` item creates a new temporary; the original `const` item will not be modified");
lint
})
}
}
// We are looking for MIR of the form:
@ -127,7 +136,7 @@ impl<'a, 'tcx> Visitor<'tcx> for ConstMutationChecker<'a, 'tcx> {
});
let lint_loc =
if method_did.is_some() { self.body.terminator_loc(loc.block) } else { loc };
self.lint_const_item_usage(def_id, lint_loc, |lint| {
self.lint_const_item_usage(place, def_id, lint_loc, |lint| {
let mut lint = lint.build("taking a mutable reference to a `const` item");
lint
.note("each usage of a `const` item creates a new temporary")