macros: support adding warnings to diags

Both diagnostic and subdiagnostic derives were missing the ability to
add warnings to diagnostics - this is made more difficult by the `warn`
attribute already existing, so this name being unavailable for the
derives to use. `#[warn_]` is used instead, which requires
special-casing so that `{span_,}warn` is called instead of
`{span_,}warn_`.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_macros/src/lib.rs

@@ -130,8 +130,9 @@ decl_derive!(
         warning,
         error,
         lint,
-        note,
         help,
+        note,
+        warn_,
         // field attributes
         skip_arg,
         primary_span,
@@ -148,8 +149,9 @@ decl_derive!(
         warning,
         error,
         lint,
-        note,
         help,
+        note,
+        warn_,
         // field attributes
         skip_arg,
         primary_span,
@@ -166,6 +168,7 @@ decl_derive!(
         label,
         help,
         note,
+        warn_,
         suggestion,
         suggestion_short,
         suggestion_hidden,