fn_must_use soft feature-gate warning on methods too, not only functions

This continues to be in the matter of #43302.

Author: zackmdavis

src/libsyntax/feature_gate.rs

@@ -1318,7 +1318,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                                     and possibly buggy");
             }
 
-            ast::ItemKind::Impl(_, polarity, defaultness, _, _, _, _) => {
+            ast::ItemKind::Impl(_, polarity, defaultness, _, _, _, ref impl_items) => {
                 if polarity == ast::ImplPolarity::Negative {
                     gate_feature_post!(&self, optin_builtin_traits,
                                        i.span,
@@ -1331,6 +1331,16 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                                        i.span,
                                        "specialization is unstable");
                 }
+
+                for impl_item in impl_items {
+                    if let ast::ImplItemKind::Method(..) = impl_item.node {
+                        if attr::contains_name(&impl_item.attrs[..], "must_use") {
+                            gate_feature_post!(&self, fn_must_use, impl_item.span,
+                                               "`#[must_use]` on methods is experimental",
+                                               GateStrength::Soft);
+                        }
+                    }
+                }
             }
 
             ast::ItemKind::MacroDef(ast::MacroDef { legacy: false, .. }) => {

src/test/compile-fail/feature-gate-fn_must_use.rs

@@ -10,6 +10,13 @@
 
 #![feature(rustc_attrs)]
 
+struct MyStruct;
+
+impl MyStruct {
+    #[must_use]
+    fn need_to_use_method() -> bool { true } //~ WARN `#[must_use]` on methods is experimental
+}
+
 #[must_use]
 fn need_to_use_it() -> bool { true } //~ WARN `#[must_use]` on functions is experimental