Emit warnings on misplaced #[no_mangle]

compiler/rustc_passes/src/check_attr.rs

@@ -83,6 +83,8 @@ impl CheckAttrVisitor<'tcx> {
                     self.check_link_name(hir_id, attr, span, target);
                 } else if self.tcx.sess.check_name(attr, sym::link_section) {
                     self.check_link_section(hir_id, attr, span, target);
+                } else if self.tcx.sess.check_name(attr, sym::no_mangle) {
+                    self.check_no_mangle(hir_id, attr, span, target);
                 }
                 true
             };
@@ -419,6 +421,27 @@ impl CheckAttrVisitor<'tcx> {
         }
     }
 
+    /// Checks if `#[no_mangle]` is applied to a function or static.
+    fn check_no_mangle(&self, hir_id: HirId, attr: &Attribute, span: &Span, target: Target) {
+        match target {
+            Target::Static | Target::Fn | Target::Method(..) => {}
+            _ => {
+                // FIXME: #[no_mangle] was previously allowed on non-functions/statics and some
+                // crates used this, so only emit a warning.
+                self.tcx.struct_span_lint_hir(UNUSED_ATTRIBUTES, hir_id, attr.span, |lint| {
+                    lint.build("attribute should be applied to a function or static")
+                        .warn(
+                            "this was previously accepted by the compiler but is \
+                             being phased out; it will become a hard error in \
+                             a future release!",
+                        )
+                        .span_label(*span, "not a function or static")
+                        .emit();
+                });
+            }
+        }
+    }
+
     /// Checks if the `#[repr]` attributes on `item` are valid.
     fn check_repr(
         &self,

src/test/ui/feature-gate/issue-43106-gating-of-builtin-attrs.rs

@@ -372,16 +372,31 @@ mod automatically_derived {
 }
 
 #[no_mangle]
+//~^ WARN attribute should be applied to a function or static [unused_attributes]
+//~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
 mod no_mangle {
+    //~^ NOTE not a function or static
     mod inner { #![no_mangle] }
+    //~^ WARN attribute should be applied to a function or static [unused_attributes]
+    //~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    //~| NOTE not a function or static
 
     #[no_mangle] fn f() { }
 
     #[no_mangle] struct S;
+    //~^ WARN attribute should be applied to a function or static [unused_attributes]
+    //~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    //~| NOTE not a function or static
 
     #[no_mangle] type T = S;
+    //~^ WARN attribute should be applied to a function or static [unused_attributes]
+    //~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    //~| NOTE not a function or static
 
     #[no_mangle] impl S { }
+    //~^ WARN attribute should be applied to a function or static [unused_attributes]
+    //~| WARN this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    //~| NOTE not a function or static
 }
 
 #[should_panic]