From 02877ac09b4b0188b9212860533cf79f8fdf01f6 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 22 Jun 2019 12:11:01 +0200
Subject: [PATCH 1/2] Lint on 'cfg_attr(,).'

---
 src/libsyntax/config.rs | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/src/libsyntax/config.rs b/src/libsyntax/config.rs
index 3b42e1de61497..1ab367f73c1b3 100644
--- a/src/libsyntax/config.rs
+++ b/src/libsyntax/config.rs
@@ -91,10 +91,10 @@ impl<'a> StripUnconfigured<'a> {
     /// is in the original source file. Gives a compiler error if the syntax of
     /// the attribute is incorrect.
     fn process_cfg_attr(&mut self, attr: ast::Attribute) -> Vec<ast::Attribute> {
-        if !attr.check_name(sym::cfg_attr) {
+        if attr.path != sym::cfg_attr {
             return vec![attr];
         }
-        if attr.tokens.len() == 0 {
+        if attr.tokens.is_empty() {
             self.sess.span_diagnostic
                 .struct_span_err(
                     attr.span,
@@ -108,7 +108,7 @@ impl<'a> StripUnconfigured<'a> {
                        <https://doc.rust-lang.org/reference/conditional-compilation.html\
                        #the-cfg_attr-attribute>")
                 .emit();
-            return Vec::new();
+            return vec![];
         }
 
         let (cfg_predicate, expanded_attrs) = match attr.parse(self.sess, |parser| {
@@ -133,17 +133,18 @@ impl<'a> StripUnconfigured<'a> {
             Ok(result) => result,
             Err(mut e) => {
                 e.emit();
-                return Vec::new();
+                return vec![];
             }
         };
 
-        // Check feature gate and lint on zero attributes in source. Even if the feature is gated,
-        // we still compute as if it wasn't, since the emitted error will stop compilation further
-        // along the compilation.
-        if expanded_attrs.len() == 0 {
-            // FIXME: Emit unused attribute lint here.
+        // Lint on zero attributes in source.
+        if expanded_attrs.is_empty() {
+            return vec![attr];
         }
 
+        // At this point we know the attribute is considered used.
+        attr::mark_used(&attr);
+
         if attr::cfg_matches(&cfg_predicate, self.sess, self.features) {
             // We call `process_cfg_attr` recursively in case there's a
             // `cfg_attr` inside of another `cfg_attr`. E.g.
@@ -159,7 +160,7 @@ impl<'a> StripUnconfigured<'a> {
             }))
             .collect()
         } else {
-            Vec::new()
+            vec![]
         }
     }
 

From af710c9e79345497fdad53873f0acefa0e26fddf Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 22 Jun 2019 12:12:26 +0200
Subject: [PATCH 2/2] Add test for linting on 'cfg_attr(,)'.

---
 .../cfg-attr-empty-is-unused.rs               | 13 ++++++++++++
 .../cfg-attr-empty-is-unused.stderr           | 21 +++++++++++++++++++
 2 files changed, 34 insertions(+)
 create mode 100644 src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.rs
 create mode 100644 src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.stderr

diff --git a/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.rs b/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.rs
new file mode 100644
index 0000000000000..4c96d6e7ca17d
--- /dev/null
+++ b/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.rs
@@ -0,0 +1,13 @@
+// Check that `#[cfg_attr($PREDICATE,)]` triggers the `unused_attribute` lint.
+
+// compile-flags: --cfg TRUE
+
+#![deny(unused)]
+
+#[cfg_attr(FALSE,)] //~ ERROR unused attribute
+fn _f() {}
+
+#[cfg_attr(TRUE,)] //~ ERROR unused attribute
+fn _g() {}
+
+fn main() {}
diff --git a/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.stderr b/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.stderr
new file mode 100644
index 0000000000000..cd3563e66c720
--- /dev/null
+++ b/src/test/ui/conditional-compilation/cfg-attr-empty-is-unused.stderr
@@ -0,0 +1,21 @@
+error: unused attribute
+  --> $DIR/cfg-attr-empty-is-unused.rs:7:1
+   |
+LL | #[cfg_attr(FALSE,)]
+   | ^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/cfg-attr-empty-is-unused.rs:5:9
+   |
+LL | #![deny(unused)]
+   |         ^^^^^^
+   = note: #[deny(unused_attributes)] implied by #[deny(unused)]
+
+error: unused attribute
+  --> $DIR/cfg-attr-empty-is-unused.rs:10:1
+   |
+LL | #[cfg_attr(TRUE,)]
+   | ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+