Add a lint for duplicated attributes.

compiler/rustc_builtin_macros/src/cfg_eval.rs

@@ -1,4 +1,4 @@
-use crate::util::check_builtin_macro_attribute;
+use crate::util::{check_builtin_macro_attribute, warn_on_duplicate_attribute};
 
 use rustc_ast as ast;
 use rustc_ast::mut_visit::MutVisitor;
@@ -25,6 +25,7 @@ crate fn expand(
     annotatable: Annotatable,
 ) -> Vec<Annotatable> {
     check_builtin_macro_attribute(ecx, meta_item, sym::cfg_eval);
+    warn_on_duplicate_attribute(&ecx, &annotatable, sym::cfg_eval);
     vec![cfg_eval(ecx.sess, ecx.ecfg.features, annotatable)]
 }
 

compiler/rustc_builtin_macros/src/test.rs

@@ -1,6 +1,6 @@
 /// The expansion from a test function to the appropriate test struct for libtest
 /// Ideally, this code would be in libtest but for efficiency and error messages it lives here.
-use crate::util::check_builtin_macro_attribute;
+use crate::util::{check_builtin_macro_attribute, warn_on_duplicate_attribute};
 
 use rustc_ast as ast;
 use rustc_ast::attr;
@@ -27,6 +27,7 @@ pub fn expand_test_case(
     anno_item: Annotatable,
 ) -> Vec<Annotatable> {
     check_builtin_macro_attribute(ecx, meta_item, sym::test_case);
+    warn_on_duplicate_attribute(&ecx, &anno_item, sym::test_case);
 
     if !ecx.ecfg.should_test {
         return vec![];
@@ -55,6 +56,7 @@ pub fn expand_test(
     item: Annotatable,
 ) -> Vec<Annotatable> {
     check_builtin_macro_attribute(cx, meta_item, sym::test);
+    warn_on_duplicate_attribute(&cx, &item, sym::test);
     expand_test_or_bench(cx, attr_sp, item, false)
 }
 
@@ -65,6 +67,7 @@ pub fn expand_bench(
     item: Annotatable,
 ) -> Vec<Annotatable> {
     check_builtin_macro_attribute(cx, meta_item, sym::bench);
+    warn_on_duplicate_attribute(&cx, &item, sym::bench);
     expand_test_or_bench(cx, attr_sp, item, true)
 }
 

compiler/rustc_builtin_macros/src/util.rs

@@ -1,6 +1,7 @@
-use rustc_ast::MetaItem;
-use rustc_expand::base::ExtCtxt;
+use rustc_ast::{Attribute, MetaItem};
+use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_feature::AttributeTemplate;
+use rustc_lint_defs::builtin::DUPLICATE_MACRO_ATTRIBUTES;
 use rustc_parse::validate_attr;
 use rustc_span::Symbol;
 
@@ -10,3 +11,33 @@ pub fn check_builtin_macro_attribute(ecx: &ExtCtxt<'_>, meta_item: &MetaItem, na
     let attr = ecx.attribute(meta_item.clone());
     validate_attr::check_builtin_attribute(&ecx.sess.parse_sess, &attr, name, template);
 }
+
+/// Emit a warning if the item is annotated with the given attribute. This is used to diagnose when
+/// an attribute may have been mistakenly duplicated.
+pub fn warn_on_duplicate_attribute(ecx: &ExtCtxt<'_>, item: &Annotatable, name: Symbol) {
+    let attrs: Option<&[Attribute]> = match item {
+        Annotatable::Item(item) => Some(&item.attrs),
+        Annotatable::TraitItem(item) => Some(&item.attrs),
+        Annotatable::ImplItem(item) => Some(&item.attrs),
+        Annotatable::ForeignItem(item) => Some(&item.attrs),
+        Annotatable::Expr(expr) => Some(&expr.attrs),
+        Annotatable::Arm(arm) => Some(&arm.attrs),
+        Annotatable::ExprField(field) => Some(&field.attrs),
+        Annotatable::PatField(field) => Some(&field.attrs),
+        Annotatable::GenericParam(param) => Some(&param.attrs),
+        Annotatable::Param(param) => Some(&param.attrs),
+        Annotatable::FieldDef(def) => Some(&def.attrs),
+        Annotatable::Variant(variant) => Some(&variant.attrs),
+        _ => None,
+    };
+    if let Some(attrs) = attrs {
+        if let Some(attr) = ecx.sess.find_by_name(attrs, name) {
+            ecx.parse_sess().buffer_lint(
+                DUPLICATE_MACRO_ATTRIBUTES,
+                attr.span,
+                ecx.current_expansion.lint_node_id,
+                "duplicated attribute",
+            );
+        }
+    }
+}