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2024 reserved ## breaks some proc-macros #133887
Description
The prohibition of ## in Rust 2024 breaks some proc-macros. For example, the peg makes use of ## for calling methods:
#![warn(rust_2024_guarded_string_incompatible_syntax)]
use peg::RuleResult;
trait StrExt {
fn ident_start(&self, pos: usize) -> RuleResult<()>;
}
impl StrExt for str {
fn ident_start(&self, pos: usize) -> RuleResult<()> {
let Some(ch) = self[pos..].chars().next() else {
return RuleResult::Failed;
};
RuleResult::Matched(pos + ch.len_utf8(), ())
}
}
peg::parser!(grammar parser() for str {
pub(crate) rule simple_literal_start()
= [ 'A'..='Z' ] / ##ident_start()
});Here this will suggest to add a space to supposedly maintain compatibility with Rust 2024:
@@ -15,5 +15,5 @@
peg::parser!(grammar parser() for str {
pub(crate) rule simple_literal_start()
- = [ 'A'..='Z' ] / ##ident_start()
+ = [ 'A'..='Z' ] / # #ident_start()
});However, this breaks peg because the peg macro uses Jointness to detect the ##IDENT() syntax. When the ## tokens are separated, they change to have Alone spacing.
I don't know what we can do here, other than remove the ## reservation in 2024, but that has other implications.
Meta
rustc 1.85.0-nightly (c44b3d50f 2024-12-03)
binary: rustc
commit-hash: c44b3d50fea96a3e0417e8264c16ea21a0a3fca2
commit-date: 2024-12-03
host: aarch64-unknown-linux-gnu
release: 1.85.0-nightly
LLVM version: 19.1.4
Tracking:
Reacted by León Orell Valerian Liehr
Activity
traviscross commentedon Dec 5, 2024
Looks like they added that in kevinmehall/rust-peg@1f3418c, though interestingly, I'm struggling to find that feature anywhere in the documentation.
Any idea how many crates are affected?
Worst case, we could just reserve
###+instead.cc @pitaj
pitaj commentedon Dec 5, 2024
I guess we should have run crater before merging the
##version of that feature.Anyways, it will be pretty easy to change it to reserve
#",##",###instead.pitaj commentedon Dec 5, 2024
#133924 removes the reservation of
##, and instead reserves#",##", and###.traviscross commentedon Dec 11, 2024
See the notes on this from today's lang call here:
#",##",####133924 (comment)cc @kevinmehall
traviscross commentedon Dec 18, 2024
We discussed this in the lang call today and confirmed, in light of the very much appreciated feedback from @kevinmehall, that we're comfortable proceeding as planned with the
##reservation.