Add support for repetition to proc_macro::quote

[tgross35]

I'm opening a standalone issue to have something with "help wanted" labels in case anyone is interested in picking this up.

Our proc_macro::quote does not support repetition, unlike quote from the quote crate. As mentioned many times on the tracking issue, this is something we should support or at least account for before proc_macro::quote can be stabilized.

This should use the syntax:

proc_macro::quote! {
    $( CONTENTS )SEP*
}

Where CONTENTS is the thing to be repeated and SEP is an optional single-character separator. Expansion should work for anything that implements IntoIterator. This matches the quote crate's logic (except quote::quote uses # rather than $).

It's probably easiest to just copy quote's logic here, which uses an extension trait to facilitate this.

• quote crate source repo: https://github.com/dtolnay/quote.
• Link to source for our quote that needs to be updated: https://github.com/rust-lang/rust/blob/fa58ce343ad498196d799a7381869e79938e952a/library/proc_macro/src/quote.rs
• Link to tests that should be updated: https://github.com/rust-lang/rust/blob/c02a4f0852e6665cf3df3867982021383f5615df/tests/ui/proc-macro/quote/auxiliary/basic.rs (plus any new tests in tests/ui/proc-macro/quote for things we should reject)
• Tracking issue: Tracking issue for the quote! macro in proc_macro #54722

[tgross35]

Cc @dtolnay in case you have any notes here

[tgross35]

Also fyi @SpriteOvO if you might be interested in this, since you have done a lot of other work with quote!

[dtolnay]

I recommend copying from the quote crate. It is tricky to get something that has all the behaviors that people expect are "obvious". If you see $($thing)*,—

• You cannot turn it into for x in $thing {...} because this is incompatible with $($thing)* $($thing)* where thing is something non-copyable like a Vec, a ubiquitous use case.

• You cannot turn it into for x in &$thing {...} because this is incompatible with thing being an Iterator, a ubiquitous use case.

• You cannot turn it into for x in &mut $thing {...} because this requires let mut thing in the caller's code which is highly unexpected.

• On top of all the above, in $($thing1 $thing2)* the correct behavior depends on which one of these is an iterator/collection and which one is a single token (Ident) or TokenStream. For a TokenStream, even though TokenStream has an IntoIterator impl, using that here produces an extremely unexpected result.