add guard_patterns unstable feature, without unstable book chapter or tracking issue for now

Author: max-niederman

compiler/rustc_feature/src/unstable.rs

@@ -501,6 +501,8 @@ declare_features! (
     (incomplete, generic_const_items, "1.73.0", Some(113521)),
     /// Allows registering static items globally, possibly across crates, to iterate over at runtime.
     (unstable, global_registration, "1.80.0", Some(125119)),
+    /// Allows using guards in patterns.
+    (unstable, guard_patterns, "CURRENT_RUSTC_VERSION", None),
     /// Allows using `..=X` as a patterns in slices.
     (unstable, half_open_range_patterns_in_slices, "1.66.0", Some(67264)),
     /// Allows `if let` guard in match arms.

compiler/rustc_span/src/symbol.rs

@@ -984,6 +984,7 @@ symbols! {
         global_registration,
         globs,
         gt,
+        guard_patterns,
         half_open_range_patterns,
         half_open_range_patterns_in_slices,
         hash,

tests/ui/feature-gates/feature-gate-guard-patterns.rs

@@ -0,0 +1,29 @@
+fn match_guards_still_work() {
+    match 0 {
+        0 if guard(0) => {},
+        _ => {},
+    }
+}
+
+fn other_guards_dont() {
+    match 0 {
+        (0 if guard(0)) | 1 => {},
+        //~^ ERROR: guard patterns are unstable
+        _ => {},
+    }
+
+    let ((x if guard(x)) | x) = 0;
+    //~^ ERROR: guard patterns are unstable
+
+    if let (x if guard(x)) = 0 {}
+    //~^ ERROR: guard patterns are unstable
+    while let (x if guard(x)) = 0 {}
+    //~^ ERROR: guard patterns are unstable
+}
+
+fn even_as_function_parameters(((x if guard(x), _) | (_, x)): (i32, i32)) {}
+//~^ ERROR: guard patterns are unstable
+
+fn guard<T>(x: T) -> bool {
+    unimplemented!()
+}