Emit pre-expansion feature gate warning for box'ed struct field pats

compiler/rustc_parse/src/parser/pat.rs

@@ -1749,6 +1749,9 @@ fn parse_pat_field(&mut self, lo: Span, attrs: AttrVec) -> PResult<'a, PatField>
         } else {
             // Parsing a pattern of the form `(box) (ref) (mut) fieldname`.
             let is_box = self.eat_keyword(exp!(Box));
+            if is_box {
+                self.psess.gated_spans.gate(sym::box_patterns, self.prev_token.span);
+            }
             let boxed_span = self.token.span;
             let mutability = self.parse_mutability();
             let by_ref = self.parse_byref();

tests/ui/feature-gates/feature-gate-box_patterns.rs

@@ -1,4 +1,9 @@
 fn main() {
     let box x = Box::new('c'); //~ ERROR box pattern syntax is experimental
-    println!("x: {}", x);
+    let _: char = x;
+
+    struct Packet { x: Box<i32> }
+
+    let Packet { box x } = Packet { x: Box::new(0) }; //~ ERROR box pattern syntax is experimental
+    let _: i32 = x;
 }

tests/ui/feature-gates/feature-gate-box_patterns.stderr

@@ -8,6 +8,16 @@ LL |     let box x = Box::new('c');
    = help: add `#![feature(box_patterns)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 1 previous error
+error[E0658]: box pattern syntax is experimental
+  --> $DIR/feature-gate-box_patterns.rs:7:18
+   |
+LL |     let Packet { box x } = Packet { x: Box::new(0) };
+   |                  ^^^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

tests/ui/feature-gates/soft-feature-gate-box_patterns.rs

@@ -0,0 +1,17 @@
+// For historical reasons, box patterns don't have a proper pre-expansion feature gate.
+// We're now at least issuing a *warning* for those that only exist before macro expansion.
+// FIXME(#154045): Turn their post-expansion feature gate into a proper pre-expansion one.
+//                 As part of this, move these test cases into `feature-gate-box_patterns.rs`.
+//@ check-pass
+
+fn main() {
+    #[cfg(false)]
+    let box x;
+    //~^ WARN box pattern syntax is experimental
+    //~| WARN unstable syntax can change at any point in the future
+
+    #[cfg(false)]
+    let Packet { box x };
+    //~^ WARN box pattern syntax is experimental
+    //~| WARN unstable syntax can change at any point in the future
+}

tests/ui/feature-gates/soft-feature-gate-box_patterns.stderr

@@ -0,0 +1,26 @@
+warning: box pattern syntax is experimental
+  --> $DIR/soft-feature-gate-box_patterns.rs:9:9
+   |
+LL |     let box x;
+   |         ^^^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: box pattern syntax is experimental
+  --> $DIR/soft-feature-gate-box_patterns.rs:14:18
+   |
+LL |     let Packet { box x };
+   |                  ^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: 2 warnings emitted
+