rustc-ap-rustc_error_codes 638.0.0

Automatically published version of the package `rustc_error_codes` in the rust-lang/rust repository from commit 30ca215b4e38b32aa7abdd635c5e2d56f5724494 The publishing script for this crate lives at: https://github.com/alexcrichton/rustc-auto-publish
Documentation 
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#### Note: this error code is no longer emitted by the compiler.

Sub-bindings, e.g. `ref x @ Some(ref y)` are now allowed under
`#![feature(bindings_after_at)]` and checked to make sure that
memory safety is upheld.

--------------

In certain cases it is possible for sub-bindings to violate memory safety.
Updates to the borrow checker in a future version of Rust may remove this
restriction, but for now patterns must be rewritten without sub-bindings.

Before:

```compile_fail
match Some("hi".to_string()) {
    ref op_string_ref @ Some(s) => {},
    None => {},
}
```

After:

```
match Some("hi".to_string()) {
    Some(ref s) => {
        let op_string_ref = &Some(s);
        // ...
    },
    None => {},
}
```

The `op_string_ref` binding has type `&Option<&String>` in both cases.

See also https://github.com/rust-lang/rust/issues/14587