Let-else not checking for curly brace of inline const before `else`

[dtolnay]

#![feature(inline_const)]

fn main() {
    let false = const {
        true
    } else {
        return;
    };
}

I believe the intention in let-else is that this code should be considered syntactically invalid, but it compiles successfully today.

If you replace const with unsafe, it correctly fails to parse.

error: right curly brace `}` before `else` in a `let...else` statement not allowed
 --> src/main.rs:6:5
  |
6 |     } else {
  |     ^
  |
help: wrap the expression in parentheses
  |
4 ~     let false = (unsafe {
5 |         true
6 ~     }) else {
  |

[dtolnay]

I believe here is the missing case for ExprKind::ConstBlock:

rust/compiler/rustc_ast/src/util/classify.rs

Lines 49 to 50 in 5f5d248

	Gen(..) | Block(..) | ForLoop(..) | If(..) | Loop(..) | Match(..) | Struct(..)
	| TryBlock(..) | While(..) => break Some(expr),

This should be changed to an exhaustive match so that future new ExprKind variants are forced to touch this code.

Some other unhandled cases that can end in }:

• ExprKind::Become
• ExprKind::FormatArgs
• ExprKind::IncludedBytes
• ExprKind::InlineAsm
• ExprKind::MacCall
• ExprKind::OffsetOf
• ExprKind::Yeet