Auto merge of #115416 - c410-f3r:match_cfg, r=Amanieu

Add the `cfg_match!` macro

# Movitation

Adds a match-like version of the `cfg_if` crate without a RFC [for the same reasons that caused `matches!` to be included in the standard library](https://github.com/rust-lang/rust/pull/65479).

* General-purpose (not domain-specific)
* Simple (the implementation is short) and useful (things can become difficult with several `cfg`s)
* Very popular [on crates.io ](https://crates.io/crates/cfg-if) (currently 3th in all-time downloads)
* The two previous points combined make it number three in [left-pad index](https://twitter.com/bascule/status/1184523027888988160) score

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

# Considerations

A match-like syntax feels more natural in the sense that each macro fragment resembles an arm but I personally don't mind switching to any other desired syntax.

The lack of `#[ ... ]` is intended to reduce typing, nevertheless, the same reasoning described above can also be applied to this aspect.

Since blocks are intended to only contain items, anything but `cfg` is not expected to be supported at the current or future time.

~~Credits goes to `@gnzlbg` because most of the code was shamelessly copied from https://github.com/gnzlbg/match_cfg.~~
Credits goes to `@alexcrichton` because most of the code was shamelessly copied from https://github.com/rust-lang/cfg-if.

library/core/src/macros/mod.rs

@@ -321,6 +321,95 @@ pub macro debug_assert_matches($($arg:tt)*) {
     }
 }
 
+/// 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! {
+///     cfg(unix) => {
+///         fn foo() { /* unix specific functionality */ }
+///     }
+///     cfg(target_pointer_width = "32") => {
+///         fn foo() { /* non-unix, 32-bit functionality */ }
+///     }
+///     _ => {
+///         fn foo() { /* fallback implementation */ }
+///     }
+/// }
+/// ```
+#[macro_export]
+#[unstable(feature = "cfg_match", issue = "115585")]
+#[rustc_diagnostic_item = "cfg_match"]
+macro_rules! cfg_match {
+    // with a final wildcard
+    (
+        $(cfg($initial_meta:meta) => { $($initial_tokens:item)* })+
+        _ => { $($extra_tokens:item)* }
+    ) => {
+        cfg_match! {
+            @__items ();
+            $((($initial_meta) ($($initial_tokens)*)),)+
+            (() ($($extra_tokens)*)),
+        }
+    };
+
+    // without a final wildcard
+    (
+        $(cfg($extra_meta:meta) => { $($extra_tokens:item)* })*
+    ) => {
+        cfg_match! {
+            @__items ();
+            $((($extra_meta) ($($extra_tokens)*)),)*
+        }
+    };
+
+    // Internal and recursive macro to emit all the items
+    //
+    // Collects all the previous cfgs in a list at the beginning, so they can be
+    // negated. After the semicolon is all the remaining items.
+    (@__items ($($_:meta,)*);) => {};
+    (
+        @__items ($($no:meta,)*);
+        (($($yes:meta)?) ($($tokens:item)*)),
+        $($rest:tt,)*
+    ) => {
+        // Emit all items within one block, applying an appropriate #[cfg]. The
+        // #[cfg] will require all `$yes` matchers specified and must also negate
+        // all previous matchers.
+        #[cfg(all(
+            $($yes,)?
+            not(any($($no),*))
+        ))]
+        cfg_match! { @__identity $($tokens)* }
+
+        // Recurse to emit all other items in `$rest`, and when we do so add all
+        // our `$yes` matchers to the list of `$no` matchers as future emissions
+        // will have to negate everything we just matched as well.
+        cfg_match! {
+            @__items ($($no,)* $($yes,)?);
+            $($rest,)*
+        }
+    };
+
+    // Internal macro to make __apply work out right for different match types,
+    // because of how macros match/expand stuff.
+    (@__identity $($tokens:item)*) => {
+        $($tokens)*
+    };
+}
+
 /// Returns whether the given expression matches any of the given patterns.
 ///
 /// Like in a `match` expression, the pattern can be optionally followed by `if`

library/core/tests/lib.rs

@@ -96,6 +96,7 @@
 #![feature(const_option_ext)]
 #![feature(const_result)]
 #![cfg_attr(target_has_atomic = "128", feature(integer_atomics))]
+#![cfg_attr(test, feature(cfg_match))]
 #![feature(int_roundings)]
 #![feature(slice_group_by)]
 #![feature(split_array)]
@@ -139,6 +140,7 @@ mod hash;
 mod intrinsics;
 mod iter;
 mod lazy;
+#[cfg(test)]
 mod macros;
 mod manually_drop;
 mod mem;

library/core/tests/macros.rs

@@ -1,3 +1,25 @@
+trait Trait {
+    fn blah(&self);
+}
+
+#[allow(dead_code)]
+struct Struct;
+
+impl Trait for Struct {
+    cfg_match! {
+        cfg(feature = "blah") => {
+            fn blah(&self) {
+                unimplemented!();
+            }
+        }
+        _ => {
+            fn blah(&self) {
+                unimplemented!();
+            }
+        }
+    }
+}
+
 #[test]
 fn assert_eq_trailing_comma() {
     assert_eq!(1, 1,);
@@ -18,3 +40,135 @@ fn assert_ne_trailing_comma() {
 fn matches_leading_pipe() {
     matches!(1, | 1 | 2 | 3);
 }
+
+#[test]
+fn cfg_match_basic() {
+    cfg_match! {
+        cfg(target_pointer_width = "64") => { fn f0_() -> bool { true }}
+    }
+
+    cfg_match! {
+        cfg(unix) => { fn f1_() -> bool { true }}
+        cfg(any(target_os = "macos", target_os = "linux")) => { fn f1_() -> bool { false }}
+    }
+
+    cfg_match! {
+        cfg(target_pointer_width = "32") => { fn f2_() -> bool { false }}
+        cfg(target_pointer_width = "64") => { fn f2_() -> bool { true }}
+    }
+
+    cfg_match! {
+        cfg(target_pointer_width = "16") => { fn f3_() -> i32 { 1 }}
+        _ => { fn f3_() -> i32 { 2 }}
+    }
+
+    #[cfg(target_pointer_width = "64")]
+    assert!(f0_());
+
+    #[cfg(unix)]
+    assert!(f1_());
+
+    #[cfg(target_pointer_width = "32")]
+    assert!(!f2_());
+    #[cfg(target_pointer_width = "64")]
+    assert!(f2_());
+
+    #[cfg(not(target_pointer_width = "16"))]
+    assert_eq!(f3_(), 2);
+}
+
+#[test]
+fn cfg_match_debug_assertions() {
+    cfg_match! {
+        cfg(debug_assertions) => {
+            assert!(cfg!(debug_assertions));
+            assert_eq!(4, 2+2);
+        }
+        _ => {
+            assert!(cfg!(not(debug_assertions)));
+            assert_eq!(10, 5+5);
+        }
+    }
+}
+
+#[cfg(target_pointer_width = "64")]
+#[test]
+fn cfg_match_no_duplication_on_64() {
+    cfg_match! {
+        cfg(windows) => {
+            fn foo() {}
+        }
+        cfg(unix) => {
+            fn foo() {}
+        }
+        cfg(target_pointer_width = "64") => {
+            fn foo() {}
+        }
+    }
+    foo();
+}
+
+#[test]
+fn cfg_match_options() {
+    cfg_match! {
+        cfg(test) => {
+            use core::option::Option as Option2;
+            fn works1() -> Option2<u32> { Some(1) }
+        }
+        _ => { fn works1() -> Option<u32> { None } }
+    }
+
+    cfg_match! {
+        cfg(feature = "foo") => { fn works2() -> bool { false } }
+        cfg(test) => { fn works2() -> bool { true } }
+        _ => { fn works2() -> bool { false } }
+    }
+
+    cfg_match! {
+        cfg(feature = "foo") => { fn works3() -> bool { false } }
+        _ => { fn works3() -> bool { true } }
+    }
+
+    cfg_match! {
+        cfg(test) => {
+            use core::option::Option as Option3;
+            fn works4() -> Option3<u32> { Some(1) }
+        }
+    }
+
+    cfg_match! {
+        cfg(feature = "foo") => { fn works5() -> bool { false } }
+        cfg(test) => { fn works5() -> bool { true } }
+    }
+
+    assert!(works1().is_some());
+    assert!(works2());
+    assert!(works3());
+    assert!(works4().is_some());
+    assert!(works5());
+}
+
+#[test]
+fn cfg_match_two_functions() {
+    cfg_match! {
+        cfg(target_pointer_width = "64") => {
+            fn foo1() {}
+            fn bar1() {}
+        }
+        _ => {
+            fn foo2() {}
+            fn bar2() {}
+        }
+    }
+
+    #[cfg(target_pointer_width = "64")]
+    {
+        foo1();
+        bar1();
+    }
+    #[cfg(not(target_pointer_width = "64"))]
+    {
+        foo2();
+        bar2();
+    }
+}

library/std/src/lib.rs

@@ -665,6 +665,9 @@ pub use core::{
 )]
 pub use core::concat_bytes;
 
+#[unstable(feature = "cfg_match", issue = "115585")]
+pub use core::cfg_match;
+
 #[stable(feature = "core_primitive", since = "1.43.0")]
 pub use core::primitive;