Rollup merge of #154475 - fmease:soft-gate-box-struct-field-pat, r=Kivooeo

Emit a pre-expansion feature gate warning for `box`'ed struct field patterns

While the following code triggers a feature gate *warning*:

```rs
fn f() {
    #[cfg(false)]
    let box x; //~ WARN box pattern syntax is experimental
}
```

the code below does not (on stable & main):

```rs
fn f() {
    #[cfg(false)]
    let Struct { box x };
}
```

This is an oversight as both are part of the unstable feature `box_patterns` (that isn't properly gated pre expansion for historical reasons). Of course, both forms lead to a feature gate error *post expansion*.

This is a bug fix and doesn't need any input from T-compiler or T-lang. For context, emitting warnings in these cases is legitimized by [MCP 535](https://github.com/rust-lang/compiler-team/issues/535)[^1].

Part of rust-lang/rust#154045.

[^1]: In case you're wondering why the MCP talks about a *lint* even though the feature gate warnings as seen today don't reference any lint by name, read https://github.com/rust-lang/rust/issues/154045#issuecomment-4144034419.

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();