Helpful suggestions for incorrect address-of mutability

compiler/rustc_hir_typeck/src/diagnostics.rs

@@ -679,6 +679,30 @@ pub(crate) struct SuggestPtrNullMut {
     pub span: Span,
 }
 
+#[derive(Subdiagnostic)]
+#[suggestion(
+    "consider using `&mut` instead",
+    applicability = "maybe-incorrect",
+    style = "verbose",
+    code = "&mut "
+)]
+pub(crate) struct SuggestRefMut {
+    #[primary_span]
+    pub ref_span: Span,
+}
+
+#[derive(Subdiagnostic)]
+#[suggestion(
+    "consider using `&raw mut` instead",
+    applicability = "maybe-incorrect",
+    style = "verbose",
+    code = "&raw mut "
+)]
+pub(crate) struct SuggestRawMut {
+    #[primary_span]
+    pub raw_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(
     "trivial {$numeric ->

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -37,7 +37,7 @@ use crate::Expectation::*;
 use crate::TupleArgumentsFlag::*;
 use crate::callee::SplatLoweringInfo;
 use crate::coercion::CoerceMany;
-use crate::diagnostics::{ExprParenthesesNeeded, SuggestPtrNullMut};
+use crate::diagnostics::{ExprParenthesesNeeded, SuggestPtrNullMut, SuggestRawMut, SuggestRefMut};
 use crate::fn_ctxt::arg_matrix::{ArgMatrix, Compatibility, Error, ExpectedIdx, ProvidedIdx};
 use crate::gather_locals::Declaration;
 use crate::inline_asm::InlineAsmCtxt;
@@ -982,7 +982,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         err.emit()
     }
 
-    fn suggest_ptr_null_mut(
+    fn suggest_mut_addr(
         &self,
         expected_ty: Ty<'tcx>,
         provided_ty: Ty<'tcx>,
@@ -1000,6 +1000,39 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // `ptr::null_mut()`.
             err.subdiagnostic(SuggestPtrNullMut { span: arg.span });
         }
+
+        if let ty::RawPtr(expected_ty, hir::Mutability::Mut) = expected_ty.kind()
+            && let ty::RawPtr(provided_ty, hir::Mutability::Not) = provided_ty.kind()
+            && let hir::ExprKind::AddrOf(hir::BorrowKind::Raw, hir::Mutability::Not, expr) =
+                arg.kind
+            && expected_ty == provided_ty
+        {
+            // The user provided `&raw const T`, but the function expects `&raw mut T`.
+            let raw_span = arg.span.until(expr.span);
+            err.subdiagnostic(SuggestRawMut { raw_span });
+        }
+
+        if let ty::Ref(_, expected_ty, hir::Mutability::Mut) = expected_ty.kind()
+            && let ty::Ref(_, provided_ty, hir::Mutability::Not) = provided_ty.kind()
+            && let hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, expr) =
+                arg.kind
+            && expected_ty == provided_ty
+        {
+            // The user provided `&T`, but the function expects `&mut T`.
+            let ref_span = arg.span.until(expr.span);
+            err.subdiagnostic(SuggestRefMut { ref_span });
+        }
+
+        if let ty::RawPtr(expected_ty, hir::Mutability::Mut) = expected_ty.kind()
+            && let ty::Ref(_, provided_ty, hir::Mutability::Not) = provided_ty.kind()
+            && let hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, expr) =
+                arg.kind
+            && expected_ty == provided_ty
+        {
+            // The user provided `&T`, but the function expects `&raw mut T`
+            let raw_span = arg.span.until(expr.span);
+            err.subdiagnostic(SuggestRawMut { raw_span });
+        }
     }
 
     // AST fragment checking
@@ -2491,7 +2524,7 @@ impl<'a, 'tcx> FnCallDiagCtxt<'a, 'tcx> {
                 );
             }
 
-            self.suggest_ptr_null_mut(
+            self.suggest_mut_addr(
                 expected_ty,
                 provided_ty,
                 self.provided_args[provided_idx],

tests/ui/span/coerce-suggestions.stderr

@@ -34,6 +34,10 @@ note: function defined here
    |
 LL | fn test(_x: &mut String) {}
    |    ^^^^ ---------------
+help: consider using `&mut` instead
+   |
+LL |     test(&mut y);
+   |           +++
 
 error[E0308]: mismatched types
   --> $DIR/coerce-suggestions.rs:14:11