cfg_attr_multi: Feature gate

Author: Havvy

src/doc/unstable-book/src/language-features/cfg-attr-multi.md

@@ -0,0 +1,20 @@
+# `cfg_attr_multi`
+
+The tracking issue for this feature is: [#555666]
+The RFC for this feature is: [#2539]
+
+[#555666]: https://github.com/rust-lang/rust/issues/555666
+[#2539]: https://github.com/rust-lang/rfcs/pull/2539
+
+------------------------
+
+This feature flag lets you put multiple attributes into a `cfg_attr` attribute.
+
+Example:
+
+```rust,ignore
+#[cfg_attr(all(), must_use, optimize)]
+```
+
+Because `cfg_attr` resolves before procedural macros, this does not affect
+macro resolution at all.

src/libsyntax/config.rs

@@ -9,7 +9,14 @@
 // except according to those terms.
 
 use attr::HasAttrs;
-use feature_gate::{feature_err, EXPLAIN_STMT_ATTR_SYNTAX, Features, get_features, GateIssue};
+use feature_gate::{
+    feature_err,
+    EXPLAIN_STMT_ATTR_SYNTAX,
+    Features,
+    get_features,
+    GateIssue,
+    emit_feature_err,
+};
 use {fold, attr};
 use ast;
 use source_map::Spanned;
@@ -97,6 +104,13 @@ impl<'a> StripUnconfigured<'a> {
             return vec![attr];
         }
 
+        let gate_cfg_attr_multi = if let Some(ref features) = self.features {
+            !features.cfg_attr_multi
+        } else {
+            false
+        };
+        let cfg_attr_span = attr.span;
+
         let (cfg_predicate, expanded_attrs) = match attr.parse(self.sess, |parser| {
             parser.expect(&token::OpenDelim(token::Paren))?;
 
@@ -123,6 +137,26 @@ impl<'a> StripUnconfigured<'a> {
             }
         };
 
+        // 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 futher
+        // along the compilation.
+        match (expanded_attrs.len(), gate_cfg_attr_multi) {
+            (0, false) => {
+                // FIXME: Emit unused attribute lint here.
+            },
+            (1, _) => {},
+            (_, true) => {
+                emit_feature_err(
+                    self.sess,
+                    "cfg_attr_multi",
+                    cfg_attr_span,
+                    GateIssue::Language,
+                    "cfg_attr with zero or more than one attributes is experimental",
+                );
+            },
+            (_, false) => {}
+        }
+
         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.

src/libsyntax/feature_gate.rs

@@ -515,6 +515,9 @@ declare_features! (
 
     // Allows `impl Trait` in bindings (`let`, `const`, `static`)
     (active, impl_trait_in_bindings, "1.30.0", Some(34511), None),
+
+    // #[cfg_attr(predicate, multiple, attributes, here)]
+    (active, cfg_attr_multi, "1.31.0", Some(555666), None),
 );
 
 declare_features! (

src/test/ui/conditional-compilation/cfg-attr-multi-false.rs

@@ -4,6 +4,7 @@
 // compile-pass
 
 #![warn(unused_must_use)]
+#![feature(cfg_attr_multi)]
 
 #[cfg_attr(any(), deprecated, must_use)]
 struct Struct {}

src/test/ui/conditional-compilation/cfg-attr-multi-invalid-1.rs

@@ -10,6 +10,7 @@
 //
 // compile-flags: --cfg broken
 
+#![feature(cfg_attr_multi)]
 #![cfg_attr(broken, no_core, no_std)] //~ ERROR no_core is experimental
 
 fn main() { }

src/test/ui/conditional-compilation/cfg-attr-multi-invalid-1.stderr

@@ -1,5 +1,5 @@
 error[E0658]: no_core is experimental (see issue #29639)
-  --> $DIR/cfg-attr-multi-invalid-1.rs:13:21
+  --> $DIR/cfg-attr-multi-invalid-1.rs:14:21
    |
 LL | #![cfg_attr(broken, no_core, no_std)] //~ ERROR no_core is experimental
    |                     ^^^^^^^

src/test/ui/conditional-compilation/cfg-attr-multi-invalid-2.rs

@@ -10,6 +10,7 @@
 //
 // compile-flags: --cfg broken
 
+#![feature(cfg_attr_multi)]
 #![cfg_attr(broken, no_std, no_core)] //~ ERROR no_core is experimental
 
 fn main() { }

src/test/ui/conditional-compilation/cfg-attr-multi-invalid-2.stderr

@@ -1,5 +1,5 @@
 error[E0658]: no_core is experimental (see issue #29639)
-  --> $DIR/cfg-attr-multi-invalid-2.rs:13:29
+  --> $DIR/cfg-attr-multi-invalid-2.rs:14:29
    |
 LL | #![cfg_attr(broken, no_std, no_core)] //~ ERROR no_core is experimental
    |                             ^^^^^^^

src/test/ui/conditional-compilation/cfg-attr-multi-true.rs

@@ -5,6 +5,7 @@
 // compile-pass
 
 #![warn(unused_must_use)]
+#![feature(cfg_attr_multi)]
 
 #[cfg_attr(all(), deprecated, must_use)]
 struct MustUseDeprecated {}

src/test/ui/conditional-compilation/cfg-attr-multi-true.stderr

@@ -1,31 +1,31 @@
 warning: use of deprecated item 'MustUseDeprecated'
-  --> $DIR/cfg-attr-multi-true.rs:12:6
+  --> $DIR/cfg-attr-multi-true.rs:13:6
    |
 LL | impl MustUseDeprecated { //~ warning: use of deprecated item
    |      ^^^^^^^^^^^^^^^^^
    |
    = note: #[warn(deprecated)] on by default
 
 warning: use of deprecated item 'MustUseDeprecated'
-  --> $DIR/cfg-attr-multi-true.rs:19:5
+  --> $DIR/cfg-attr-multi-true.rs:20:5
    |
 LL |     MustUseDeprecated::new(); //~ warning: use of deprecated item
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
 warning: use of deprecated item 'MustUseDeprecated'
-  --> $DIR/cfg-attr-multi-true.rs:13:17
+  --> $DIR/cfg-attr-multi-true.rs:14:17
    |
 LL |     fn new() -> MustUseDeprecated { //~ warning: use of deprecated item
    |                 ^^^^^^^^^^^^^^^^^
 
 warning: use of deprecated item 'MustUseDeprecated'
-  --> $DIR/cfg-attr-multi-true.rs:14:9
+  --> $DIR/cfg-attr-multi-true.rs:15:9
    |
 LL |         MustUseDeprecated {} //~ warning: use of deprecated item
    |         ^^^^^^^^^^^^^^^^^
 
 warning: unused `MustUseDeprecated` which must be used
-  --> $DIR/cfg-attr-multi-true.rs:19:5
+  --> $DIR/cfg-attr-multi-true.rs:20:5
    |
 LL |     MustUseDeprecated::new(); //~ warning: use of deprecated item
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^