loop_match

The tracking issue for this feature is: #132306

The #[loop_match] and #[const_continue] attributes can be used to improve the code generation of logic that fits this shape:

loop {
    state = 'blk: {
        match state {
            State::A => {
                break 'blk State::B
            }
            State::B => { /* ... */ }
            /* ... */
        }
    }
}

Here the loop itself can be annotated with #[loop_match], and any break 'blk with #[const_continue] if the value is know at compile time:

#[loop_match]
loop {
    state = 'blk: {
        match state {
            State::A => {
                #[const_continue]
                break 'blk State::B
            }
            State::B => { /* ... */ }
            /* ... */
        }
    }
}

The observable behavior of this loop is exactly the same as without the extra attributes. The difference is in the generated output: normally, when the state is A, control flow moves from the A branch, back to the top of the loop, then to the B branch. With the attributes, The A branch will immediately jump to the B branch.

Removing the indirection can be beneficial for stack usage and branch prediction, and enables other optimizations by clearly splitting out the control flow paths that your program will actually use.