Suppress `CONST_ITEM_MUTATION` lint if a dereference occurs anywhere

Fixes #79971

3 files changed, 21 insertions, 11 deletions

diff --git a/compiler/rustc_mir/src/transform/check_const_item_mutation.rs b/compiler/rustc_mir/src/transform/check_const_item_mutation.rs
index a8457043278..e2d50ba034a 100644
--- a/compiler/rustc_mir/src/transform/check_const_item_mutation.rs
+++ b/compiler/rustc_mir/src/transform/check_const_item_mutation.rs
@@ -66,12 +66,14 @@ impl<'a, 'tcx> ConstMutationChecker<'a, 'tcx> {
         location: Location,
         decorate: impl for<'b> FnOnce(LintDiagnosticBuilder<'b>) -> DiagnosticBuilder<'b>,
     ) {
-        // Don't lint on borrowing/assigning to a dereference
-        // e.g:
+        // 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 }`
-        if !matches!(place.projection.last(), Some(PlaceElem::Deref)) {
+        // `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
diff --git a/src/test/ui/lint/lint-const-item-mutation.rs b/src/test/ui/lint/lint-const-item-mutation.rs
index ef55f31593b..4bf5e0a9e21 100644
--- a/src/test/ui/lint/lint-const-item-mutation.rs
+++ b/src/test/ui/lint/lint-const-item-mutation.rs
@@ -30,6 +30,8 @@ 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 _;
+const PTR_TO_ARRAY: *mut [u32; 4] = 0x12345678 as _;
+const ARRAY_OF_PTR: [*mut u32; 1] = [1 as *mut _];
 
 fn main() {
     ARRAY[0] = 5; //~ WARN attempting to modify
@@ -55,4 +57,10 @@ fn main() {
     // Test that we don't warn when converting a raw pointer
     // into a mutable reference
     unsafe { &mut *PTR };
+
+    // Test that we don't warn when there's a dereference involved.
+    // If we ever 'leave' the const via a deference, we're going
+    // to end up modifying something other than the temporary
+    unsafe { (*PTR_TO_ARRAY)[0] = 1 };
+    unsafe { *ARRAY_OF_PTR[0] = 25; }
 }
diff --git a/src/test/ui/lint/lint-const-item-mutation.stderr b/src/test/ui/lint/lint-const-item-mutation.stderr
index ae95abc72f3..74505eeb987 100644
--- a/src/test/ui/lint/lint-const-item-mutation.stderr
+++ b/src/test/ui/lint/lint-const-item-mutation.stderr
@@ -1,5 +1,5 @@
 warning: attempting to modify a `const` item
-  --> $DIR/lint-const-item-mutation.rs:35:5
+  --> $DIR/lint-const-item-mutation.rs:37:5
    |
 LL |     ARRAY[0] = 5;
    |     ^^^^^^^^^^^^
@@ -13,7 +13,7 @@ LL | const ARRAY: [u8; 1] = [25];
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: attempting to modify a `const` item
-  --> $DIR/lint-const-item-mutation.rs:36:5
+  --> $DIR/lint-const-item-mutation.rs:38:5
    |
 LL |     MY_STRUCT.field = false;
    |     ^^^^^^^^^^^^^^^^^^^^^^^
@@ -26,7 +26,7 @@ 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:37:5
+  --> $DIR/lint-const-item-mutation.rs:39:5
    |
 LL |     MY_STRUCT.inner_array[0] = 'b';
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -39,7 +39,7 @@ 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:40:5
    |
 LL |     MY_STRUCT.use_mut();
    |     ^^^^^^^^^^^^^^^^^^^
@@ -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:39:5
+  --> $DIR/lint-const-item-mutation.rs:41:5
    |
 LL |     &mut MY_STRUCT;
    |     ^^^^^^^^^^^^^^
@@ -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:40:5
+  --> $DIR/lint-const-item-mutation.rs:42:5
    |
 LL |     (&mut MY_STRUCT).use_mut();
    |     ^^^^^^^^^^^^^^^^
@@ -86,7 +86,7 @@ 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:52:5
+  --> $DIR/lint-const-item-mutation.rs:54:5
    |
 LL |     MUTABLE2.msg = "wow";
    |     ^^^^^^^^^^^^^^^^^^^^
@@ -99,7 +99,7 @@ LL | const MUTABLE2: Mutable2 = Mutable2 { msg: "", other: String::new() };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 warning: taking a mutable reference to a `const` item
-  --> $DIR/lint-const-item-mutation.rs:53:5
+  --> $DIR/lint-const-item-mutation.rs:55:5
    |
 LL |     VEC.push(0);
    |     ^^^^^^^^^^^