From 23c3a307300827dbd53637742905293b9240506a Mon Sep 17 00:00:00 2001
From: clubby789 <jamie@hill-daniel.co.uk>
Date: Tue, 3 Jan 2023 17:02:25 +0000
Subject: [PATCH] Explain error with `&mut self` for unsized trait impls

---
 .../src/diagnostics/mutability_errors.rs      | 27 +++++++++++--------
 src/test/ui/borrowck/issue-93078.rs           | 15 +++++++++++
 src/test/ui/borrowck/issue-93078.stderr       | 12 +++++++++
 3 files changed, 43 insertions(+), 11 deletions(-)
 create mode 100644 src/test/ui/borrowck/issue-93078.rs
 create mode 100644 src/test/ui/borrowck/issue-93078.stderr

diff --git a/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs b/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
index 6f6d1b01bd429..bf830e276999c 100644
--- a/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
@@ -344,20 +344,25 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     } else {
                         err.span_help(source_info.span, "try removing `&mut` here");
                     }
-                } else if decl.mutability == Mutability::Not
-                    && !matches!(
+                } else if decl.mutability == Mutability::Not {
+                    if matches!(
                         decl.local_info,
                         Some(box LocalInfo::User(ClearCrossCrate::Set(BindingForm::ImplicitSelf(
                             hir::ImplicitSelfKind::MutRef
-                        ))))
-                    )
-                {
-                    err.span_suggestion_verbose(
-                        decl.source_info.span.shrink_to_lo(),
-                        "consider making the binding mutable",
-                        "mut ",
-                        Applicability::MachineApplicable,
-                    );
+                        ),)))
+                    ) {
+                        err.note(
+                            "as `Self` may be unsized, this call attempts to take `&mut &mut self`",
+                        );
+                        err.note("however, `&mut self` expands to `self: &mut Self`, therefore `self` cannot be borrowed mutably");
+                    } else {
+                        err.span_suggestion_verbose(
+                            decl.source_info.span.shrink_to_lo(),
+                            "consider making the binding mutable",
+                            "mut ",
+                            Applicability::MachineApplicable,
+                        );
+                    };
                 }
             }
 
diff --git a/src/test/ui/borrowck/issue-93078.rs b/src/test/ui/borrowck/issue-93078.rs
new file mode 100644
index 0000000000000..2e608c5db3e1d
--- /dev/null
+++ b/src/test/ui/borrowck/issue-93078.rs
@@ -0,0 +1,15 @@
+trait Modify {
+    fn modify(&mut self) ;
+}
+
+impl<T> Modify for T  {
+    fn modify(&mut self)  {}
+}
+
+trait Foo {
+    fn mute(&mut self) {
+        self.modify(); //~ ERROR cannot borrow `self` as mutable
+    }
+}
+
+fn main() {}
diff --git a/src/test/ui/borrowck/issue-93078.stderr b/src/test/ui/borrowck/issue-93078.stderr
new file mode 100644
index 0000000000000..771a652a1739c
--- /dev/null
+++ b/src/test/ui/borrowck/issue-93078.stderr
@@ -0,0 +1,12 @@
+error[E0596]: cannot borrow `self` as mutable, as it is not declared as mutable
+  --> $DIR/issue-93078.rs:11:9
+   |
+LL |         self.modify();
+   |         ^^^^^^^^^^^^^ cannot borrow as mutable
+   |
+   = note: as `Self` may be unsized, this call attempts to take `&mut &mut self`
+   = note: however, `&mut self` expands to `self: &mut Self`, therefore `self` cannot be borrowed mutably
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.