don't mangle duplicate #[used] warnings

Author: iximeow

compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs

@@ -424,13 +424,29 @@ impl<S: Stage> AttributeParser<S> for UsedParser {
                 ArgParser::NameValue(_) => return,
             };
 
+            let attr_span = cx.attr_span;
+
+            // `#[used]` is interpreted as `#[used(linker)]` (though depending on target OS the
+            // circumstances are more complicated). While we're checking `used_by`, also report
+            // these cross-`UsedBy` duplicates to warn.
             let target = match used_by {
                 UsedBy::Compiler => &mut group.first_compiler,
-                UsedBy::Linker => &mut group.first_linker,
-                UsedBy::Any => &mut group.first_generic,
+                UsedBy::Linker => {
+                    if let Some(prev) = group.first_generic {
+                        cx.warn_unused_duplicate(prev, attr_span);
+                        return;
+                    }
+                    &mut group.first_linker
+                }
+                UsedBy::Any => {
+                    if let Some(prev) = group.first_linker {
+                        cx.warn_unused_duplicate(prev, attr_span);
+                        return;
+                    }
+                    &mut group.first_generic
+                }
             };
 
-            let attr_span = cx.attr_span;
             if let Some(prev) = *target {
                 cx.warn_unused_duplicate(prev, attr_span);
             } else {