macro_rules follow logic allocates uselessly in the happy path

[oli-obk]

E.g. in

rust/src/libsyntax/ext/tt/macro_rules.rs

Line 1045 in ab7cf71

let tokens = vec!["`=>`", "`,`", "`=`", "`|`", "`if`", "`in`"];

a vec is created, only to be thrown away if the macro actually works out. I think we can just turn most places into an early return in the happy path and then only allocate in the error path.

Do not work on this issue before #61541 is merged.

Also,

rust/src/libsyntax/ext/tt/macro_rules.rs

Line 1015 in ab7cf71

fn is_in_follow(tok: &quoted::TokenTree, frag: &str) -> IsInFollow {

hasn't had its docs updated to reflect the source changes.

@oli-obk Are there any reasons to not use constant/static slices and convert them to vecs on demand?

Or maybe even convert

enum IsInFollow {
    Yes,
    No(Vec<&'static str>),
    Invalid(String, &'static str),
}

to something like

enum IsInFollow {
    Yes,
    No(&'static [&'static str]),
    Invalid(String, &'static str),
}

so there's no need to allocate vecs (In all cases) anymore?

[oli-obk]

That's a great idea! I haven't checked whether it'll work everywhere, but I think it should.