librustc: Forbid pattern bindings after `@`s, for memory safety.
This is an alternative to upgrading the way rvalues are handled in the borrow check. Making rvalues handled more like lvalues in the borrow check caused numerous problems related to double mutable borrows and rvalue scopes. Rather than come up with more borrow check rules to try to solve these problems, I decided to just forbid pattern bindings after `@`. This affected fewer than 10 lines of code in the compiler and libraries. This breaks code like: match x { y @ z => { ... } } match a { b @ Some(c) => { ... } } Change this code to use nested `match` or `let` expressions. For example: match x { y => { let z = y; ... } } match a { Some(c) => { let b = Some(c); ... } } Closes #14587. [breaking-change]
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