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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.
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@m-ou-se
m-ou-se authored Nov 8, 2020
2 parents c4e262e + bd3f3fa commit 91759b2
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Showing 3 changed files with 47 additions and 33 deletions.
51 changes: 30 additions & 21 deletions compiler/rustc_mir/src/transform/check_const_item_mutation.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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()
},
);
}
}
}

Expand All @@ -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:
Expand Down Expand Up @@ -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")
Expand Down
5 changes: 5 additions & 0 deletions src/test/ui/lint/lint-const-item-mutation.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,7 @@ const RAW_PTR: *mut u8 = 1 as *mut u8;
const MUTABLE: Mutable = Mutable { msg: "" };
const MUTABLE2: Mutable2 = Mutable2 { msg: "", other: String::new() };
const VEC: Vec<i32> = Vec::new();
const PTR: *mut () = 1 as *mut _;

fn main() {
ARRAY[0] = 5; //~ WARN attempting to modify
Expand All @@ -50,4 +51,8 @@ fn main() {
MUTABLE.msg = "wow"; // no warning, because Drop observes the mutation
MUTABLE2.msg = "wow"; //~ WARN attempting to modify
VEC.push(0); //~ WARN taking a mutable reference to a `const` item

// Test that we don't warn when converting a raw pointer
// into a mutable reference
unsafe { &mut *PTR };
}
24 changes: 12 additions & 12 deletions src/test/ui/lint/lint-const-item-mutation.stderr
View file
Original file line number Diff line number Diff line change
@@ -1,45 +1,45 @@
warning: attempting to modify a `const` item
--> $DIR/lint-const-item-mutation.rs:34:5
--> $DIR/lint-const-item-mutation.rs:35:5
|
LL | ARRAY[0] = 5;
| ^^^^^^^^^^^^
|
= note: `#[warn(const_item_mutation)]` on by default
= note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified
= note: each usage of a `const` item creates a new temporary; the original `const` item will not be modified
note: `const` item defined here
--> $DIR/lint-const-item-mutation.rs:26:1
|
LL | const ARRAY: [u8; 1] = [25];
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: attempting to modify a `const` item
--> $DIR/lint-const-item-mutation.rs:35:5
--> $DIR/lint-const-item-mutation.rs:36:5
|
LL | MY_STRUCT.field = false;
| ^^^^^^^^^^^^^^^^^^^^^^^
|
= note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified
= note: each usage of a `const` item creates a new temporary; the original `const` item will not be modified
note: `const` item defined here
--> $DIR/lint-const-item-mutation.rs:27:1
|
LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: attempting to modify a `const` item
--> $DIR/lint-const-item-mutation.rs:36:5
--> $DIR/lint-const-item-mutation.rs:37:5
|
LL | MY_STRUCT.inner_array[0] = 'b';
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified
= note: each usage of a `const` item creates a new temporary; the original `const` item will not be modified
note: `const` item defined here
--> $DIR/lint-const-item-mutation.rs:27:1
|
LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw_ptr: 2 as *mut u8 };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: taking a mutable reference to a `const` item
--> $DIR/lint-const-item-mutation.rs:37:5
--> $DIR/lint-const-item-mutation.rs:38:5
|
LL | MY_STRUCT.use_mut();
| ^^^^^^^^^^^^^^^^^^^
Expand All @@ -58,7 +58,7 @@ LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: taking a mutable reference to a `const` item
--> $DIR/lint-const-item-mutation.rs:38:5
--> $DIR/lint-const-item-mutation.rs:39:5
|
LL | &mut MY_STRUCT;
| ^^^^^^^^^^^^^^
Expand All @@ -72,7 +72,7 @@ LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: taking a mutable reference to a `const` item
--> $DIR/lint-const-item-mutation.rs:39:5
--> $DIR/lint-const-item-mutation.rs:40:5
|
LL | (&mut MY_STRUCT).use_mut();
| ^^^^^^^^^^^^^^^^
Expand All @@ -86,20 +86,20 @@ LL | const MY_STRUCT: MyStruct = MyStruct { field: true, inner_array: ['a'], raw
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: attempting to modify a `const` item
--> $DIR/lint-const-item-mutation.rs:51:5
--> $DIR/lint-const-item-mutation.rs:52:5
|
LL | MUTABLE2.msg = "wow";
| ^^^^^^^^^^^^^^^^^^^^
|
= note: each usage of a `const` item creates a new temporary - the original `const` item will not be modified
= note: each usage of a `const` item creates a new temporary; the original `const` item will not be modified
note: `const` item defined here
--> $DIR/lint-const-item-mutation.rs:30:1
|
LL | const MUTABLE2: Mutable2 = Mutable2 { msg: "", other: String::new() };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: taking a mutable reference to a `const` item
--> $DIR/lint-const-item-mutation.rs:52:5
--> $DIR/lint-const-item-mutation.rs:53:5
|
LL | VEC.push(0);
| ^^^^^^^^^^^
Expand Down

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