Leading pipes in patterns need to be preserved in the AST

[Aaron1011]

The following code:

fn main() {
     match () { | () => () }
}

is currently pretty-printed as:

fn main() { match () { () => (), } }

If this occurs in a macro invocation, it leads to an instance of #43081

[danielhenrymantilla]

Extra info (but it boils down to what @Aaron1011 said):

Using

fn main ()
{
    match () { | () => () }
    let () = 42; // I was interested in seeing the error message being correctly spanned
}

I get the following log:

[INFO  rustc_parse] cached tokens found, but they're not "probably equal", going with stringified version
[INFO  rustc_parse] cached tokens: TokenStream([(Token(Token { kind: Ident("fn", false), span: examples/main.rs:4:1: 4:3 (#0) }), NonJoint), (Token(Token { kind: Ident("main", false), span: examples/main.rs:4:4: 4:8 (#0) }), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:4:9: 4:10 (#0), close: examples/main.rs:4:10: 4:11 (#0) }, Paren, TokenStream([])), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:5:1: 5:2 (#0), close: examples/main.rs:8:1: 8:2 (#0) }, Brace, TokenStream([(Token(Token { kind: Ident("match", false), span: examples/main.rs:6:5: 6:10 (#0) }), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:6:11: 6:12 (#0), close: examples/main.rs:6:12: 6:13 (#0) }, Paren, TokenStream([])), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:6:14: 6:15 (#0), close: examples/main.rs:6:27: 6:28 (#0) }, Brace, TokenStream([(Token(Token { kind: BinOp(Or), span: examples/main.rs:6:16: 6:17 (#0) }), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:6:18: 6:19 (#0), close: examples/main.rs:6:19: 6:20 (#0) }, Paren, TokenStream([])), NonJoint), (Token(Token { kind: FatArrow, span: examples/main.rs:6:21: 6:23 (#0) }), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:6:24: 6:25 (#0), close: examples/main.rs:6:25: 6:26 (#0) }, Paren, TokenStream([])), NonJoint)])), NonJoint), (Token(Token { kind: Ident("let", false), span: examples/main.rs:7:5: 7:8 (#0) }), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:7:9: 7:10 (#0), close: examples/main.rs:7:10: 7:11 (#0) }, Paren, TokenStream([])), NonJoint), (Token(Token { kind: Eq, span: examples/main.rs:7:12: 7:13 (#0) }), NonJoint), (Token(Token { kind: Literal(Lit { kind: Integer, symbol: "42", suffix: None }), span: examples/main.rs:7:14: 7:16 (#0) }), Joint), (Token(Token { kind: Semi, span: examples/main.rs:7:16: 7:17 (#0) }), NonJoint)])), NonJoint)])
[INFO  rustc_parse] reparsed tokens: TokenStream([(Token(Token { kind: Ident("fn", false), span: examples/main.rs:1:1: 1:1 (#0) }), NonJoint), (Token(Token { kind: Ident("main", false), span: examples/main.rs:1:1: 1:1 (#0) }), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:1:1: 1:1 (#0), close: examples/main.rs:1:1: 1:1 (#0) }, Paren, TokenStream([])), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:1:1: 1:1 (#0), close: examples/main.rs:1:1: 1:1 (#0) }, Brace, TokenStream([(Token(Token { kind: Ident("match", false), span: examples/main.rs:1:1: 1:1 (#0) }), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:1:1: 1:1 (#0), close: examples/main.rs:1:1: 1:1 (#0) }, Paren, TokenStream([])), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:1:1: 1:1 (#0), close: examples/main.rs:1:1: 1:1 (#0) }, Brace, TokenStream([(Delimited(DelimSpan { open: examples/main.rs:1:1: 1:1 (#0), close: examples/main.rs:1:1: 1:1 (#0) }, Paren, TokenStream([])), NonJoint), (Token(Token { kind: FatArrow, span: examples/main.rs:1:1: 1:1 (#0) }), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:1:1: 1:1 (#0), close: examples/main.rs:1:1: 1:1 (#0) }, Paren, TokenStream([])), NonJoint), (Token(Token { kind: Comma, span: examples/main.rs:1:1: 1:1 (#0) }), NonJoint)])), NonJoint), (Token(Token { kind: Ident("let", false), span: examples/main.rs:1:1: 1:1 (#0) }), NonJoint), (Delimited(DelimSpan { open: examples/main.rs:1:1: 1:1 (#0), close: examples/main.rs:1:1: 1:1 (#0) }, Paren, TokenStream([])), NonJoint), (Token(Token { kind: Eq, span: examples/main.rs:1:1: 1:1 (#0) }), NonJoint), (Token(Token { kind: Literal(Lit { kind: Integer, symbol: "42", suffix: None }), span: examples/main.rs:1:1: 1:1 (#0) }), Joint), (Token(Token { kind: Semi, span: examples/main.rs:1:1: 1:1 (#0) }), NonJoint)])), NonJoint)])

Rendered diff: https://www.textcompare.org/index.html?id=5f4d8b98086a55001745f791

[petrochenkov]

This applies to all trailing separators in theory (except that the separator is leading rather than trailing in this case), but in other cases the difference is fixed up by fuzzy comparison in tokenstream_probably_equal_for_proc_macro.