fix: parse field expr keyword field to field-expr node

[A4-Tacks]

This makes the completion more coherent and helpful for climbing trees

Example

const _=x.ref

Before this PR

CONST
  CONST_KW "const"
  WHITESPACE " "
  UNDERSCORE "_"
  EQ "="
  FIELD_EXPR
    PATH_EXPR
      PATH
        PATH_SEGMENT
          NAME_REF
            IDENT "x"
    DOT "."
ERROR
  REF_KW "ref"
ERROR
  SEMICOLON ";"

After this PR

CONST
  CONST_KW "const"
  WHITESPACE " "
  UNDERSCORE "_"
  EQ "="
  FIELD_EXPR
    PATH_EXPR
      PATH
        PATH_SEGMENT
          NAME_REF
            IDENT "x"
    DOT "."
    ERROR
      REF_KW "ref"
  SEMICOLON ";"

---

struct S { indent: u8 }
fn foo(_: &mut u8) {}
fn main() {
    let mut s = S { indent: 2 };
    foo(s.in$0)
}

Before this PR

ty: (), name: ?

indent

After this PR

ty: &mut u8, name: ?

&mut indent
indent

[ChayimFriedman2]

I feel this is a wrong step to take. The reason completion fails is a bug in completion (specifically in the expansion phase, which speculative execution will fix). We shouldn't touch parsing to fix that.

[A4-Tacks]

I added a bug fix example

[ChayimFriedman2]

I'm aware of this bug and that this fixes it, that's why I said:

The reason completion fails is a bug in completion (specifically in the expansion phase, which speculative execution will fix). We shouldn't touch parsing to fix that.

[A4-Tacks]

But some steps are on the original tree, not speculative tree, like expected type and name

And this does make incomplete parsing more reasonable, it won't get worse

[ChayimFriedman2]

But some steps are on the original tree, not speculative tree, like expected type and name

I don't understand what you mean by that. What I'm saying is that once we have speculative execution, this bug will be fixed without any change to the parser.

And this does make incomplete parsing more reasonable, it won't get worse

Changing parsing to accept more code has many implications, and I'm opposed to that without a very good reason, especially not to aid a specific IDE feature such as completion.

[A4-Tacks]

Okay, I've come up with a worse case:

const _={
    x.
    let y = 2;
}