On unused assign lint, detect `mut arg: &Ty` meant to be `arg: &mut Ty`

```
error: value assigned to `object` is never read
  --> $DIR/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs:11:5
   |
LL |     object = &object2;
   |     ^^^^^^
   |
note: the lint level is defined here
  --> $DIR/mut-arg-of-borrowed-type-meant-to-be-arg-of-mut-borrow.rs:1:9
   |
LL | #![deny(unused_assignments, unused_variables)]
   |         ^^^^^^^^^^^^^^^^^^
help: you might have meant to mutate the pointed at value being passed in, instead of changing the reference in the local binding
   |
LL ~ fn change_object2(object: &mut Object) {
LL |     let object2 = Object;
LL ~     *object = object2;
   |
```

This might be the first thing someone tries to write to mutate the value *behind* an argument, trying to avoid an E0308.

1 files changed, 18 insertions, 1 deletions

diff --git a/compiler/rustc_passes/src/errors.rs b/compiler/rustc_passes/src/errors.rs
index 13da021c614..c3043ac60aa 100644
--- a/compiler/rustc_passes/src/errors.rs
+++ b/compiler/rustc_passes/src/errors.rs
@@ -1787,9 +1787,26 @@ pub(crate) struct IneffectiveUnstableImpl;
 
 #[derive(LintDiagnostic)]
 #[diag(passes_unused_assign)]
-#[help]
 pub(crate) struct UnusedAssign {
     pub name: String,
+    #[subdiagnostic]
+    pub suggestion: Option<UnusedAssignSuggestion>,
+    #[help]
+    pub help: bool,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(passes_unused_assign_suggestion, applicability = "maybe-incorrect")]
+pub(crate) struct UnusedAssignSuggestion {
+    pub pre: &'static str,
+    #[suggestion_part(code = "{pre}mut ")]
+    pub ty_span: Span,
+    #[suggestion_part(code = "")]
+    pub ty_ref_span: Span,
+    #[suggestion_part(code = "*")]
+    pub ident_span: Span,
+    #[suggestion_part(code = "")]
+    pub expr_ref_span: Span,
 }
 
 #[derive(LintDiagnostic)]