Recover shorthand struct literals in if conditions

[chenyukang]

Fixes #147877

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

r? @khyperia

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[fmease]

This modifies the stable grammar of Rust and is thus not acceptable.

It regresses stable code like the following:

fn scope(cond: bool) {
    let u = ();

    if cond {
        u
    }

    {
        println!();
    }
}

or smaller:

#[cfg(false)] fn f() { if x { x } {} }

View changes since the review

[chenyukang]

aha, yes! I didn't notice is_likely_struct_lit is on happy path, let me think about it.

[chenyukang]

I updated the code.

This fix adds a lightweight extra { check after parsing each if or while body in error_on_ambiguous_struct_literal. If another block follows, it inspects the parsed AST to detect whether a shorthand struct literal was misinterpreted as the body.

I am not entirely satisfied with adding work to the normal parsing path, but I have not found a cleaner recovery point.