Improve completion of macro_rules partial inputs

[A4-Tacks]

rust-analyzer version: rust-analyzer 1.91.0-nightly (040a98a 2025-08-20)

rustc version: rustc 1.91.0-nightly (040a98af7 2025-08-20)

editor or extension: coc-rust-analyzer

code snippet to reproduce:

macro_rules! vec_like {
    ($($e:expr),+ $(,)?) => {
        [$($e),+]
    };
}

fn main() {
    vec_like!("".|);
}

| is cursor

Unable to complete when entering the dot, as it cannot be fully parsed as expr

But there are no other matching branches at this point,
should we try inputting the incomplete expr into the macro_rules?

[ChayimFriedman2]

That's because of speculative expansion, see #t-compiler/rust-analyzer > What does speculative expansion buy us? @ 💬. So either get rid of speculative expansion or wait for Salsa to gain speculative execution abilities then switch to that.