disallow | from the follow-set of pat2021

[nikomatsakis]

In Edition 2021, the following should error:

macro_rules! foo {
    ($x:pat | $y:pat) => { }
}

fn main() { }

for the same reason that the following already errors:

macro_rules! foo {
    ($x:expr | $y:expr) => { }
}

fn main() { }

cc @rust-lang/project-edition-2021

[nikomatsakis]

@hi-rustin I was wondering if you were interested in fixing this as a follow-up to #83468 -- I don't think you included it in that PR, right?

[Rustin170506]

@rustbot claim

[Rustin170506]

I think the problem is here:

rust/compiler/rustc_expand/src/mbe/macro_rules.rs

Line 1098 in 36bcf40

NonterminalKind::Stmt | NonterminalKind::Expr => {

, but if we allow |, then

macro_rules! foo {
    ($x:expr |) => { }
} 

will also be allowed. This does not seem right. But in the current function we can't yet tell what token comes next in |. Do you have any suggestions?

[nikomatsakis]

Actually, the code may already be correct? It appears that | is not allowed for pat2021, which is the goal:

rust/compiler/rustc_expand/src/mbe/macro_rules.rs

Lines 1119 to 1129 in 36bcf40

	NonterminalKind::Pat2021 { .. } => {
	const TOKENS: &[&str] = &["`=>`", "`,`", "`=`", "`if`", "`in`"];
	match tok {
	TokenTree::Token(token) => match token.kind {
	FatArrow | Comma | Eq => IsInFollow::Yes,
	Ident(name, false) if name == kw::If || name == kw::In => IsInFollow::Yes,
	_ => IsInFollow::No(TOKENS),
	},
	_ => IsInFollow::No(TOKENS),
	}
	}

Note that | is allowed for pat2015. Maybe we just need a test -- or maybe there already is one!

[Rustin170506]

Actually, the code may already be correct? It appears that | is not allowed for pat2021, which is the goal:

rust/compiler/rustc_expand/src/mbe/macro_rules.rs

Lines 1119 to 1129 in 36bcf40

	NonterminalKind::Pat2021 { .. } => {
	const TOKENS: &[&str] = &["`=>`", "`,`", "`=`", "`if`", "`in`"];
	match tok {
	TokenTree::Token(token) => match token.kind {
	FatArrow | Comma | Eq => IsInFollow::Yes,
	Ident(name, false) if name == kw::If || name == kw::In => IsInFollow::Yes,
	_ => IsInFollow::No(TOKENS),
	},
	_ => IsInFollow::No(TOKENS),
	}
	}

Note that | is allowed for pat2015. Maybe we just need a test -- or maybe there already is one!

Yes this part of the logic I have added, but my question is about ($x:expr | $y:expr) => { }. I will add an additional test for pat2021.

[nikomatsakis]

@hi-rustin I expect $x:expr | $y:expr to be an error -- it is, right?

[Rustin170506]

@hi-rustin I expect $x:expr | $y:expr to be an error -- it is, right?

Yes. I see what you mean. I misunderstood your description in the issue.