Switch non exhaustive syntax tree enums to use #[non_exhaustive]

[dtolnay]

This follows through on the comments on the __TestExhaustive variants promising that their purpose will be substituted with a deny(reachable) rustc lint once one is available.

syn/src/expr.rs

Lines 247 to 251 in ac68efa

	// Once `deny(reachable)` is available in rustc, Expr will be
	// reimplemented as a non_exhaustive enum.
	// https://github.com/rust-lang/rust/issues/44109#issuecomment-521781237
	#[doc(hidden)]
	__TestExhaustive(crate::private),

That lint is now landing as non_exhaustive_omitted_patterns in Rust 1.57. The correct way to use it for checking exhaustivity of a match is:

match expr {
    Expr::Array(e) => {...}
    Expr::Assign(e) => {...}
    ...
    Expr::Yield(e) => {...}

    #[cfg_attr(test, deny(non_exhaustive_omitted_patterns))]
    _ => { /* some sane fallback */ }
}

[dtolnay]

Closing, to be reopened when 1.57.0 stable is out.