Ralf Jung
7ee19eb038
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.
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