Do not ICE on missing access place description during mutability error reporting

src/librustc_mir/borrow_check/mutability_errors.rs

@@ -7,7 +7,7 @@ use rustc::mir::{
 use rustc::mir::{Terminator, TerminatorKind};
 use rustc::ty::{self, Const, DefIdTree, Ty, TyS, TyCtxt};
 use rustc_data_structures::indexed_vec::Idx;
-use syntax_pos::Span;
+use syntax_pos::{Span, CompilerDesugaringKind};
 use syntax_pos::symbol::kw;
 
 use crate::dataflow::move_paths::InitLocation;
@@ -41,14 +41,19 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
         );
 
         let mut err;
-        let item_msg;
         let reason;
         let access_place_desc = self.describe_place(access_place);
         debug!("report_mutability_error: access_place_desc={:?}", access_place_desc);
 
+        let mut item_msg = match (&access_place_desc, &the_place_err) {
+            (Some(desc), _) => format!("`{}`", desc),
+            (None, Place::Base(PlaceBase::Local(local))) if self.mir.local_decls[*local]
+                .source_info.span.is_compiler_desugaring(CompilerDesugaringKind::Async)
+            => "`async fn` parameter".to_string(),
+            (None, _) => "temporary place".to_string(),
+        };
         match the_place_err {
             Place::Base(PlaceBase::Local(local)) => {
-                item_msg = format!("`{}`", access_place_desc.unwrap());
                 if let Place::Base(PlaceBase::Local(_)) = access_place {
                     reason = ", as it is not declared as mutable".to_string();
                 } else {
@@ -67,7 +72,6 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
                     base.ty(self.mir, self.infcx.tcx).ty
                 ));
 
-                item_msg = format!("`{}`", access_place_desc.unwrap());
                 if self.is_upvar_field_projection(access_place).is_some() {
                     reason = ", as it is not declared as mutable".to_string();
                 } else {
@@ -82,7 +86,6 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
             }) => {
                 if *base == Place::Base(PlaceBase::Local(Local::new(1))) &&
                     !self.upvars.is_empty() {
-                    item_msg = format!("`{}`", access_place_desc.unwrap());
                     debug_assert!(self.mir.local_decls[Local::new(1)].ty.is_region_ptr());
                     debug_assert!(is_closure_or_generator(
                         the_place_err.ty(self.mir, self.infcx.tcx).ty
@@ -105,7 +108,6 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
                         false
                     }
                 } {
-                    item_msg = format!("`{}`", access_place_desc.unwrap());
                     reason = ", as it is immutable for the pattern guard".to_string();
                 } else {
                     let pointer_type =
@@ -114,8 +116,7 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
                         } else {
                             "`*const` pointer"
                         };
-                    if let Some(desc) = access_place_desc {
-                        item_msg = format!("`{}`", desc);
+                    if access_place_desc.is_some() {
                         reason = match error_access {
                             AccessKind::Move |
                             AccessKind::Mutate => format!(" which is behind a {}", pointer_type),
@@ -135,10 +136,9 @@ impl<'a, 'gcx, 'tcx> MirBorrowckCtxt<'a, 'gcx, 'tcx> {
 
             Place::Base(PlaceBase::Static(box Static { kind: StaticKind::Static(def_id), .. })) => {
                 if let Place::Base(PlaceBase::Static(_)) = access_place {
-                    item_msg = format!("immutable static item `{}`", access_place_desc.unwrap());
+                    item_msg = format!("immutable static item {}", item_msg);
                     reason = String::new();
                 } else {
-                    item_msg = format!("`{}`", access_place_desc.unwrap());
                     let static_name = &self.infcx.tcx.item_name(*def_id);
                     reason = format!(", as `{}` is an immutable static item", static_name);
                 }

src/test/ui/async-await/issues/issue-61187.rs

@@ -0,0 +1,9 @@
+// edition:2018
+#![feature(async_await)]
+
+fn main() {
+}
+
+async fn response(data: Vec<u8>) {
+    data.reverse(); //~ ERROR E0596
+}

src/test/ui/async-await/issues/issue-61187.stderr

@@ -0,0 +1,11 @@
+error[E0596]: cannot borrow `async fn` parameter as mutable, as it is not declared as mutable
+  --> $DIR/issue-61187.rs:8:5
+   |
+LL | async fn response(data: Vec<u8>) {
+   |                   ---- help: consider changing this to be mutable: `mut data`
+LL |     data.reverse();
+   |     ^^^^ cannot borrow as mutable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.