Fix ICE of for-loop mut borrowck where no suggestions are available

Author: fee1-dead

compiler/rustc_mir/src/borrow_check/diagnostics/mutability_errors.rs

@@ -642,15 +642,18 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                                         .starts_with(&original_method_ident.name.to_string())
                             })
                             .map(|ident| format!("{}()", ident))
+                            .peekable()
                     });
 
-                if let Some(suggestions) = opt_suggestions {
-                    err.span_suggestions(
-                        path_segment.ident.span,
-                        &format!("use mutable method"),
-                        suggestions,
-                        Applicability::MaybeIncorrect,
-                    );
+                if let Some(mut suggestions) = opt_suggestions {
+                    if suggestions.peek().is_some() {
+                        err.span_suggestions(
+                            path_segment.ident.span,
+                            &format!("use mutable method"),
+                            suggestions,
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
                 }
             }
         };

src/test/ui/borrowck/issue-83309-ice-immut-in-for-loop.rs

@@ -0,0 +1,32 @@
+// rust-lang/rust#83309: The compiler tries to suggest potential
+// methods that return `&mut` items. However, when it doesn't
+// find such methods, it still tries to add suggestions
+// which then fails an assertion later because there was
+// no suggestions to make.
+
+
+fn main() {
+    for v in Query.iter_mut() {
+        //~^ NOTE this iterator yields `&` references
+        *v -= 1;
+        //~^ ERROR cannot assign to `*v` which is behind a `&` reference
+        //~| NOTE `v` is a `&` reference, so the data it refers to cannot be written
+    }
+}
+
+pub struct Query;
+pub struct QueryIter<'a>(&'a i32);
+
+impl Query {
+    pub fn iter_mut<'a>(&'a mut self) -> QueryIter<'a> {
+        todo!();
+    }
+}
+
+impl<'a> Iterator for QueryIter<'a> {
+    type Item = &'a i32;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        todo!();
+    }
+}

src/test/ui/borrowck/issue-83309-ice-immut-in-for-loop.stderr

@@ -0,0 +1,12 @@
+error[E0594]: cannot assign to `*v` which is behind a `&` reference
+  --> $DIR/issue-83309-ice-immut-in-for-loop.rs:11:9
+   |
+LL |     for v in Query.iter_mut() {
+   |              ---------------- this iterator yields `&` references
+LL |
+LL |         *v -= 1;
+   |         ^^^^^^^ `v` is a `&` reference, so the data it refers to cannot be written
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.