Allow `self` as an identifier in patterns

Proposal

Allow the use of self as an identifier in identifier patterns (foo @ Foo { bar, baz }).

Motivation

As seen in the following search results within the rust-lang/rust repository, there are numerous instances where developers want to destructure a struct in a let statement with self on the right-hand side.

Search results in the rust-lang/rust repository

For example, consider the following code in library/std/src/sys/pal/sgx/net.rs:

fn try_into_inner(self) -> Result<FileDesc, Socket> {
    let Socket { inner, local_addr } = self;
    Arc::try_unwrap(inner).map_err(|inner| Socket { inner, local_addr })
}

If self were allowed as an identifier in identifier patterns, this could be written more concisely as:

fn try_into_inner(self @ Socket { inner, local_addr }) -> Result<FileDesc, Socket> {
    Arc::try_unwrap(inner).map_err(|inner| Socket { inner, local_addr })
}

This is not only more concise but also more natural from a language specification standpoint.

The proposal illustrates the use case in library/std/src/sys/pal/sgx/net.rs; start by reviewing the Rust rules for identifier and @ patterns alongside the RFC process. Compare the existing let Socket { inner, local_addr } = self form with the proposed function-parameter form, and consider the language specification and compiler tests needed to establish the accepted syntax.