"expected pattern" when parsing slice patterns

[estebank]

The parser cannot cope with the following code using slice patterns:

if let &[ref start.., ref end] = &variant_spans[..] {

[matklad]

The relevant code is here:

https://github.com/rust-analyzer/rust-analyzer/blob/4e8664d9d37021e85d72b6228fa45e7edf4a1a74/crates/ra_parser/src/grammar/patterns.rs#L225-L254

Note that comments are executable tests

[tomjakubowski]

That particular case parses without errors with cargo run --bin ra_cli parse.

fn main() {
    if let &[ref start.., ref end] = &variant_spans[..] {}
}

          LET_KW@[19; 22) "let"
          WHITESPACE@[22; 23) " "
          REF_PAT@[23; 46)
            AMP@[23; 24) "&"
            SLICE_PAT@[24; 46)
              L_BRACK@[24; 25) "["
              RANGE_PAT@[25; 36)
                BIND_PAT@[25; 34)
                  REF_KW@[25; 28) "ref"
                  WHITESPACE@[28; 29) " "
                  NAME@[29; 34)
                    IDENT@[29; 34) "start"
                DOTDOT@[34; 36) ".."
              COMMA@[36; 37) ","
              WHITESPACE@[37; 38) " "
              BIND_PAT@[38; 45)
                REF_KW@[38; 41) "ref"
                WHITESPACE@[41; 42) " "
                NAME@[42; 45)
                  IDENT@[42; 45) "end"
              R_BRACK@[45; 46) "]"
          WHITESPACE@[46; 47) " "

But it's a misparse: the RANGE_PAT is wrong, and should probably match a new SyntaxKind for "subslice patterns".

Another test case produces a RANGE_PAT but this one does error the parser:

fn main() {
    let [a, b..] = [];
}

      LET_STMT@[16; 34)
        LET_KW@[16; 19) "let"
        WHITESPACE@[19; 20) " "
        SLICE_PAT@[20; 28)
          L_BRACK@[20; 21) "["
          BIND_PAT@[21; 22)
            NAME@[21; 22)
              IDENT@[21; 22) "a"
          COMMA@[22; 23) ","
          WHITESPACE@[23; 24) " "
          RANGE_PAT@[24; 28)
            BIND_PAT@[24; 25)
              NAME@[24; 25)
                IDENT@[24; 25) "b"
            DOTDOT@[25; 27) ".."
            ERROR@[27; 28)
              R_BRACK@[27; 28) "]"
        WHITESPACE@[28; 29) " "
        EQ@[29; 30) "="

[matklad]

@estebank hm, I am now confused, what ref start.. means exactly? Is it a "subslice pattern" from rust-lang/rfcs#2359? Shouldn't it be spelled s ref start @ .. really soon?

[estebank]

I believe you're correct in all your understanding. I don't think the syntax accepted in the RFC has been implemented, so rustc is using the previous syntax I'm showing here. It might be acceptable for rust-analyzer not to support it and skip straight to the finalized syntax, but I wanted to flag it.

[Centril]

The syntax in the RFC was implemented in rust-lang/rust#62550. cc rust-lang/rust#62254