Fix ICE with let...else and ref mut

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -45,12 +45,12 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         let item_msg;
         let reason;
         let mut opt_source = None;
-        let access_place_desc = self.describe_place(access_place.as_ref());
+        let access_place_desc = self.describe_any_place(access_place.as_ref());
         debug!("report_mutability_error: access_place_desc={:?}", access_place_desc);
 
         match the_place_err {
             PlaceRef { local, projection: [] } => {
-                item_msg = format!("`{}`", access_place_desc.unwrap());
+                item_msg = access_place_desc;
                 if access_place.as_local().is_some() {
                     reason = ", as it is not declared as mutable".to_string();
                 } else {
@@ -83,7 +83,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     // If we deref an immutable ref then the suggestion here doesn't help.
                     return;
                 } else {
-                    item_msg = format!("`{}`", access_place_desc.unwrap());
+                    item_msg = access_place_desc;
                     if self.is_upvar_field_projection(access_place.as_ref()).is_some() {
                         reason = ", as it is not declared as mutable".to_string();
                     } else {
@@ -96,17 +96,17 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             PlaceRef { local, projection: [ProjectionElem::Deref] }
                 if self.body.local_decls[local].is_ref_for_guard() =>
             {
-                item_msg = format!("`{}`", access_place_desc.unwrap());
+                item_msg = access_place_desc;
                 reason = ", as it is immutable for the pattern guard".to_string();
             }
             PlaceRef { local, projection: [ProjectionElem::Deref] }
                 if self.body.local_decls[local].is_ref_to_static() =>
             {
                 if access_place.projection.len() == 1 {
-                    item_msg = format!("immutable static item `{}`", access_place_desc.unwrap());
+                    item_msg = format!("immutable static item {}", access_place_desc);
                     reason = String::new();
                 } else {
-                    item_msg = format!("`{}`", access_place_desc.unwrap());
+                    item_msg = access_place_desc;
                     let local_info = &self.body.local_decls[local].local_info;
                     if let Some(box LocalInfo::StaticRef { def_id, .. }) = *local_info {
                         let static_name = &self.infcx.tcx.item_name(def_id);
@@ -121,7 +121,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     && proj_base.is_empty()
                     && !self.upvars.is_empty()
                 {
-                    item_msg = format!("`{}`", access_place_desc.unwrap());
+                    item_msg = access_place_desc;
                     debug_assert!(
                         self.body.local_decls[ty::CAPTURE_STRUCT_LOCAL].ty.is_region_ptr()
                     );
@@ -147,7 +147,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     });
                     let pointer_type = source.describe_for_immutable_place(self.infcx.tcx);
                     opt_source = Some(source);
-                    if let Some(desc) = access_place_desc {
+                    if let Some(desc) = self.describe_place(access_place.as_ref()) {
                         item_msg = format!("`{}`", desc);
                         reason = match error_access {
                             AccessKind::Mutate => format!(", which is behind {}", pointer_type),

src/test/ui/let-else/issue-89960.rs

@@ -0,0 +1,7 @@
+#![feature(let_else)]
+
+fn main() {
+    // FIXME: more precise diagnostics
+    let Some(ref mut meow) = Some(()) else { return };
+    //~^ ERROR: cannot borrow value as mutable, as `val` is not declared as mutable
+}

src/test/ui/let-else/issue-89960.stderr

@@ -0,0 +1,12 @@
+error[E0596]: cannot borrow value as mutable, as `val` is not declared as mutable
+  --> $DIR/issue-89960.rs:5:14
+   |
+LL |     let Some(ref mut meow) = Some(()) else { return };
+   |     ---------^^^^^^^^^^^^-----------------------------
+   |     |        |
+   |     |        cannot borrow as mutable
+   |     help: consider changing this to be mutable: `mut val`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.