Fail gracefully if mutating on a use closure and the closure it not declared mut

Author: spastorino

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -823,7 +823,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                             ) => {
                                 capture_reason = format!("mutable borrow of `{upvar}`");
                             }
-                            ty::UpvarCapture::ByValue => {
+                            ty::UpvarCapture::ByValue | ty::UpvarCapture::ByUse => {
                                 capture_reason = format!("possible mutation of `{upvar}`");
                             }
                             _ => bug!("upvar `{upvar}` borrowed, but not mutably"),

tests/ui/ergonomic-clones/closure/mutation2.rs

@@ -0,0 +1,10 @@
+#![feature(ergonomic_clones)]
+
+fn main() {
+    let mut my_var = false;
+    let callback = use || {
+        my_var = true;
+    };
+    callback();
+    //~^ ERROR cannot borrow `callback` as mutable, as it is not declared as mutable [E0596]
+}

tests/ui/ergonomic-clones/closure/mutation2.stderr

@@ -0,0 +1,17 @@
+error[E0596]: cannot borrow `callback` as mutable, as it is not declared as mutable
+  --> $DIR/mutation2.rs:8:5
+   |
+LL |         my_var = true;
+   |         ------ calling `callback` requires mutable binding due to possible mutation of `my_var`
+LL |     };
+LL |     callback();
+   |     ^^^^^^^^ cannot borrow as mutable
+   |
+help: consider changing this to be mutable
+   |
+LL |     let mut callback = use || {
+   |         +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0596`.