emit specific warning to clarify that foreign items can't have no_mangle

remove extra commented code
Deduplicate some diagnostics code
add code symbols, machine applicable suggestion
clarify error message

2 files changed, 72 insertions, 0 deletions

diff --git a/src/test/ui/extern/extern-no-mangle.rs b/src/test/ui/extern/extern-no-mangle.rs
new file mode 100644
index 00000000000..ab7c9824af0
--- /dev/null
+++ b/src/test/ui/extern/extern-no-mangle.rs
@@ -0,0 +1,30 @@
+#![warn(unused_attributes)]
+
+// Tests that placing the #[no_mangle] attribute on a foreign fn or static emits
+// a specialized warning.
+// The previous warning only talks about a "function or static" but foreign fns/statics
+// are also not allowed to have #[no_mangle]
+
+// build-pass
+
+extern "C" {
+    #[no_mangle]
+    //~^ WARNING `#[no_mangle]` has no effect on a foreign static
+    //~^^ WARNING this was previously accepted by the compiler
+    pub static FOO: u8;
+
+    #[no_mangle]
+    //~^ WARNING `#[no_mangle]` has no effect on a foreign function
+    //~^^ WARNING this was previously accepted by the compiler
+    pub fn bar();
+}
+
+fn no_new_warn() {
+    // Should emit the generic "not a function or static" warning
+    #[no_mangle]
+    //~^ WARNING attribute should be applied to a free function, impl method or static
+    //~^^ WARNING this was previously accepted by the compiler
+    let x = 0_u8;
+}
+
+fn main() {}
diff --git a/src/test/ui/extern/extern-no-mangle.stderr b/src/test/ui/extern/extern-no-mangle.stderr
new file mode 100644
index 00000000000..b5642814114
--- /dev/null
+++ b/src/test/ui/extern/extern-no-mangle.stderr
@@ -0,0 +1,42 @@
+warning: attribute should be applied to a free function, impl method or static
+  --> $DIR/extern-no-mangle.rs:24:5
+   |
+LL |     #[no_mangle]
+   |     ^^^^^^^^^^^^
+...
+LL |     let x = 0_u8;
+   |     ------------- not a free function, impl method or static
+   |
+note: the lint level is defined here
+  --> $DIR/extern-no-mangle.rs:1:9
+   |
+LL | #![warn(unused_attributes)]
+   |         ^^^^^^^^^^^^^^^^^
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+
+warning: `#[no_mangle]` has no effect on a foreign static
+  --> $DIR/extern-no-mangle.rs:11:5
+   |
+LL |     #[no_mangle]
+   |     ^^^^^^^^^^^^ help: remove this attribute
+...
+LL |     pub static FOO: u8;
+   |     ------------------- foreign static
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: symbol names in extern blocks are not mangled
+
+warning: `#[no_mangle]` has no effect on a foreign function
+  --> $DIR/extern-no-mangle.rs:16:5
+   |
+LL |     #[no_mangle]
+   |     ^^^^^^^^^^^^ help: remove this attribute
+...
+LL |     pub fn bar();
+   |     ------------- foreign function
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: symbol names in extern blocks are not mangled
+
+warning: 3 warnings emitted
+