add fn visit_capture_by to MutVisitor and fix pprust-expr-roundtrip.rs

compiler/rustc_ast/src/mut_visit.rs

@@ -302,6 +302,10 @@ pub trait MutVisitor: Sized {
     fn visit_format_args(&mut self, fmt: &mut FormatArgs) {
         noop_visit_format_args(fmt, self)
     }
+
+    fn visit_capture_by(&mut self, capture_by: &mut CaptureBy) {
+        noop_visit_capture_by(capture_by, self)
+    }
 }
 
 /// Use a map-style function (`FnOnce(T) -> T`) to overwrite a `&mut T`. Useful
@@ -1562,6 +1566,15 @@ pub fn noop_visit_vis<T: MutVisitor>(visibility: &mut Visibility, vis: &mut T) {
     vis.visit_span(&mut visibility.span);
 }
 
+pub fn noop_visit_capture_by<T: MutVisitor>(capture_by: &mut CaptureBy, vis: &mut T) {
+    match capture_by {
+        CaptureBy::Ref => {}
+        CaptureBy::Value { move_kw } => {
+            vis.visit_span(move_kw);
+        }
+    }
+}
+
 /// Some value for the AST node that is valid but possibly meaningless.
 pub trait DummyAstNode {
     fn dummy() -> Self;

tests/ui-fulldeps/pprust-expr-roundtrip.rs

@@ -130,7 +130,7 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
                 iter_exprs(depth - 1, &mut |e| {
                     g(ExprKind::Closure(Box::new(Closure {
                         binder: ClosureBinder::NotPresent,
-                        capture_clause: CaptureBy::Value,
+                        capture_clause: CaptureBy::Value { move_kw: DUMMY_SP },
                         constness: Const::No,
                         asyncness: Async::No,
                         movability: Movability::Movable,