Rollup merge of #133720 - c410-f3r:cfg-match-foo-bar-baz, r=joshtriplett

[cfg_match] Adjust syntax

A year has passed since the creation of #115585 and the feature, as expected, is not moving forward. Let's change that.

This PR proposes changing the arm's syntax from  `cfg(SOME_CONDITION) => { ... }` to `SOME_CODITION => {}`.

```rust
match_cfg! {
   unix => {
        fn foo() { /* unix specific functionality */ }
    }
    target_pointer_width = "32" => {
        fn foo() { /* non-unix, 32-bit functionality */ }
    }
    _ => {
        fn foo() { /* fallback implementation */ }
    }
}
```

Why? Because after several manual migrations in https://github.com/rust-lang/rust/pull/116342 it became clear,  at least for me, that `cfg` prefixes are unnecessary, verbose and redundant.

Again, everything is just a proposal to move things forward. If the shown syntax isn't ideal, feel free to close this PR or suggest other alternatives.

1 files changed, 52 insertions, 0 deletions

diff --git a/library/core/src/macros/mod.rs b/library/core/src/macros/mod.rs
index 402b436d28e..5c04e5a40df 100644
--- a/library/core/src/macros/mod.rs
+++ b/library/core/src/macros/mod.rs
@@ -224,6 +224,7 @@ pub macro assert_matches {
 ///     }
 /// }
 /// ```
+#[cfg(bootstrap)]
 #[unstable(feature = "cfg_match", issue = "115585")]
 #[rustc_diagnostic_item = "cfg_match"]
 pub macro cfg_match {
@@ -284,6 +285,57 @@ pub macro cfg_match {
     }
 }
 
+/// A macro for defining `#[cfg]` match-like statements.
+///
+/// It is similar to the `if/elif` C preprocessor macro by allowing definition of a cascade of
+/// `#[cfg]` cases, emitting the implementation which matches first.
+///
+/// This allows you to conveniently provide a long list `#[cfg]`'d blocks of code
+/// without having to rewrite each clause multiple times.
+///
+/// Trailing `_` wildcard match arms are **optional** and they indicate a fallback branch when
+/// all previous declarations do not evaluate to true.
+///
+/// # Example
+///
+/// ```
+/// #![feature(cfg_match)]
+///
+/// cfg_match! {
+///     unix => {
+///         fn foo() { /* unix specific functionality */ }
+///     }
+///     target_pointer_width = "32" => {
+///         fn foo() { /* non-unix, 32-bit functionality */ }
+///     }
+///     _ => {
+///         fn foo() { /* fallback implementation */ }
+///     }
+/// }
+/// ```
+#[cfg(not(bootstrap))]
+#[unstable(feature = "cfg_match", issue = "115585")]
+#[rustc_diagnostic_item = "cfg_match"]
+pub macro cfg_match {
+    ({ $($tt:tt)* }) => {{
+        cfg_match! { $($tt)* }
+    }},
+    (_ => { $($output:tt)* }) => {
+        $($output)*
+    },
+    (
+        $cfg:meta => $output:tt
+        $($( $rest:tt )+)?
+    ) => {
+        #[cfg($cfg)]
+        cfg_match! { _ => $output }
+        $(
+            #[cfg(not($cfg))]
+            cfg_match! { $($rest)+ }
+        )?
+    },
+}
+
 /// Asserts that a boolean expression is `true` at runtime.
 ///
 /// This will invoke the [`panic!`] macro if the provided expression cannot be