Add a lint for duplicated attributes.

Cargo.lock

@@ -3678,6 +3678,7 @@ dependencies = [
  "rustc_expand",
  "rustc_feature",
  "rustc_lexer",
+ "rustc_lint_defs",
  "rustc_parse",
  "rustc_parse_format",
  "rustc_session",

compiler/rustc_builtin_macros/Cargo.toml

@@ -15,6 +15,7 @@ rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_feature = { path = "../rustc_feature" }
 rustc_lexer = { path = "../rustc_lexer" }
+rustc_lint_defs = { path = "../rustc_lint_defs" }
 rustc_parse = { path = "../rustc_parse" }
 rustc_target = { path = "../rustc_target" }
 rustc_session = { path = "../rustc_session" }

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",
+            );
+        }
+    }
+}

compiler/rustc_lint_defs/src/builtin.rs

@@ -3092,6 +3092,7 @@ declare_lint_pass! {
         TEXT_DIRECTION_CODEPOINT_IN_COMMENT,
         DEREF_INTO_DYN_SUPERTRAIT,
         DEPRECATED_CFG_ATTR_CRATE_TYPE_NAME,
+        DUPLICATE_MACRO_ATTRIBUTES,
     ]
 }
 
@@ -3629,3 +3630,32 @@ declare_lint! {
         reference: "issue #89460 <https://github.com/rust-lang/rust/issues/89460>",
     };
 }
+
+declare_lint! {
+    /// The `duplicate_macro_attributes` lint detects when a `#[test]`-like built-in macro
+    /// attribute is duplicated on an item. This lint may trigger on `bench`, `cfg_eval`, `test`
+    /// and `test_case`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,ignore (needs --test)
+    /// #[test]
+    /// #[test]
+    /// fn foo() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// A duplicated attribute may erroneously originate from a copy-paste and the effect of it
+    /// being duplicated may not be obvious or desireable.
+    ///
+    /// For instance, doubling the `#[test]` attributes registers the test to be run twice with no
+    /// change to its environment.
+    ///
+    /// [issue #90979]: https://github.com/rust-lang/rust/issues/90979
+    pub DUPLICATE_MACRO_ATTRIBUTES,
+    Warn,
+    "duplicated attribute"
+}

src/test/ui/attributes/duplicated-attributes.rs

@@ -0,0 +1,41 @@
+// Test that, if an item is annotated with a builtin attribute more than once, a warning is
+// emitted.
+// Tests https://github.com/rust-lang/rust/issues/90979
+
+// check-pass
+// compile-flags: --test
+
+#![feature(test)]
+#![feature(cfg_eval)]
+
+#[test]
+#[test]
+//~^ WARNING duplicated attribute
+fn f() {}
+
+// The following shouldn't trigger an error. The attribute is not duplicated.
+#[test]
+fn f2() {}
+
+// The following shouldn't trigger an error either. The second attribute is not #[test].
+#[test]
+#[inline]
+fn f3() {}
+
+extern crate test;
+use test::Bencher;
+
+#[bench]
+#[bench]
+//~^ WARNING duplicated attribute
+fn f4(_: &mut Bencher) {}
+
+#[cfg_eval]
+#[cfg_eval]
+//~^ WARNING duplicated attribute
+struct S;
+
+#[cfg_eval]
+struct S2;
+
+fn main() {}

src/test/ui/attributes/duplicated-attributes.stderr

@@ -0,0 +1,22 @@
+warning: duplicated attribute
+  --> $DIR/duplicated-attributes.rs:12:1
+   |
+LL | #[test]
+   | ^^^^^^^
+   |
+   = note: `#[warn(duplicate_macro_attributes)]` on by default
+
+warning: duplicated attribute
+  --> $DIR/duplicated-attributes.rs:29:1
+   |
+LL | #[bench]
+   | ^^^^^^^^
+
+warning: duplicated attribute
+  --> $DIR/duplicated-attributes.rs:34:1
+   |
+LL | #[cfg_eval]
+   | ^^^^^^^^^^^
+
+warning: 3 warnings emitted
+