Auto merge of rust-lang#143461 - folkertdev:cfg-select-builtin-macro, r=petrochenkov

make `cfg_select` a builtin macro

tracking issue: rust-lang#115585

This parses mostly the same as the `macro cfg_select` version, except:

1. wrapping in double brackets is no longer supported (or needed): `cfg_select {{ /* ... */ }}` is now rejected.
2. in an expression context, the rhs is no longer wrapped in a block, so that this now works:
  ```rust
  fn main() {
      println!(cfg_select! {
          unix => { "foo" }
          _ => { "bar" }
      });
  }
  ```
3. a single wildcard rule is now supported: `cfg_select { _ => 1 }` now works

I've also added an error if none of the rules evaluate to true, and warnings for any arms that follow the `_` wildcard rule.

cc `@traviscross` if I'm missing any feature that should/should not be included
r? `@petrochenkov` for the macro logic details

Author: bors

core/src/macros/mod.rs

@@ -230,32 +230,16 @@ pub macro assert_matches {
 /// ```
 /// #![feature(cfg_select)]
 ///
-/// let _some_string = cfg_select! {{
+/// let _some_string = cfg_select! {
 ///     unix => { "With great power comes great electricity bills" }
 ///     _ => { "Behind every successful diet is an unwatched pizza" }
-/// }};
+/// };
 /// ```
 #[unstable(feature = "cfg_select", issue = "115585")]
 #[rustc_diagnostic_item = "cfg_select"]
-#[rustc_macro_transparency = "semitransparent"]
-pub macro cfg_select {
-    ({ $($tt:tt)* }) => {{
-        $crate::cfg_select! { $($tt)* }
-    }},
-    (_ => { $($output:tt)* }) => {
-        $($output)*
-    },
-    (
-        $cfg:meta => $output:tt
-        $($( $rest:tt )+)?
-    ) => {
-        #[cfg($cfg)]
-        $crate::cfg_select! { _ => $output }
-        $(
-            #[cfg(not($cfg))]
-            $crate::cfg_select! { $($rest)+ }
-        )?
-    },
+#[rustc_builtin_macro]
+pub macro cfg_select($($tt:tt)*) {
+    /* compiler built-in */
 }
 
 /// Asserts that a boolean expression is `true` at runtime.

coretests/tests/macros.rs

@@ -48,11 +48,12 @@ fn matches_leading_pipe() {
 fn cfg_select_basic() {
     cfg_select! {
         target_pointer_width = "64" => { fn f0_() -> bool { true }}
+        _ => {}
     }
 
     cfg_select! {
         unix => { fn f1_() -> bool { true } }
-        any(target_os = "macos", target_os = "linux") => { fn f1_() -> bool { false }}
+        _ => { fn f1_() -> bool { false }}
     }
 
     cfg_select! {
@@ -70,6 +71,8 @@ fn cfg_select_basic() {
 
     #[cfg(unix)]
     assert!(f1_());
+    #[cfg(not(unix))]
+    assert!(!f1_());
 
     #[cfg(target_pointer_width = "32")]
     assert!(!f2_());
@@ -183,6 +186,12 @@ fn _accepts_expressions() -> i32 {
     }
 }
 
+fn _accepts_only_wildcard() -> i32 {
+    cfg_select! {
+        _ => { 1 }
+    }
+}
+
 // The current implementation expands to a macro call, which allows the use of expression
 // statements.
 fn _allows_stmt_expr_attributes() {
@@ -195,12 +204,12 @@ fn _allows_stmt_expr_attributes() {
 }
 
 fn _expression() {
-    let _ = cfg_select!({
+    let _ = cfg_select!(
         windows => {
             " XP"
         }
         _ => {
             ""
         }
-    });
+    );
 }