Do not suggest adding move to closure when move is already used

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -902,7 +902,11 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             hir::ExprKind::MethodCall(.., args, _) => {
                 // only the first closre parameter of the method. args[0] is MethodCall PathSegment
                 for i in 1..args.len() {
-                    if let hir::ExprKind::Closure(..) = args[i].kind {
+                    if let hir::ExprKind::Closure(hir::Closure {
+                        capture_clause: hir::CaptureBy::Ref,
+                        ..
+                    }) = args[i].kind
+                    {
                         closure_span = Some(args[i].span.shrink_to_lo());
                         break;
                     }
@@ -911,7 +915,11 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             hir::ExprKind::Block(blk, _) => {
                 if let Some(ref expr) = blk.expr {
                     // only when the block is a closure
-                    if let hir::ExprKind::Closure(..) = expr.kind {
+                    if let hir::ExprKind::Closure(hir::Closure {
+                        capture_clause: hir::CaptureBy::Ref,
+                        ..
+                    }) = expr.kind
+                    {
                         closure_span = Some(expr.span.shrink_to_lo());
                     }
                 }
@@ -921,7 +929,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         if let Some(closure_span) = closure_span {
             diag.span_suggestion_verbose(
                 closure_span,
-                format!("consider adding 'move' keyword before the nested closure"),
+                "consider adding 'move' keyword before the nested closure",
                 "move ",
                 Applicability::MaybeIncorrect,
             );

src/test/ui/borrowck/do-not-suggest-adding-move-when-closure-is-already-marked-as-move.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let mut vec: Vec<i32> = Vec::new();
+    let closure = move || {
+        vec.clear();
+        let mut iter = vec.iter();
+        move || { iter.next() } //~ ERROR captured variable cannot escape `FnMut` closure bod
+    };
+}

src/test/ui/borrowck/do-not-suggest-adding-move-when-closure-is-already-marked-as-move.stderr

@@ -0,0 +1,18 @@
+error: captured variable cannot escape `FnMut` closure body
+  --> $DIR/do-not-suggest-adding-move-when-closure-is-already-marked-as-move.rs:6:9
+   |
+LL |     let mut vec: Vec<i32> = Vec::new();
+   |         ------- variable defined here
+LL |     let closure = move || {
+   |                         - inferred to be a `FnMut` closure
+LL |         vec.clear();
+   |         --- variable captured here
+LL |         let mut iter = vec.iter();
+LL |         move || { iter.next() }
+   |         ^^^^^^^^^^^^^^^^^^^^^^^ returns a closure that contains a reference to a captured variable, which then escapes the closure body
+   |
+   = note: `FnMut` closures only have access to their captured variables while they are executing...
+   = note: ...therefore, they cannot allow references to captured variables to escape
+
+error: aborting due to previous error
+