Migrate parser to the new span combining scheme

[petrochenkov]

Summary:

I a, b, ..., z are multiple consecutive span nodes that need to be combined into a single new span node, then its combined span should be

span(a).to(span(b)).to(span(c))....to(span(z))`

"Span node" is either an individual token (every token has a span), or some larger AST node having a Span field.

Examples:

-10 // an expression
a + 11 // one more expression
a::<u8>::b // a path

The combined span for these pieces of code will be

// expressions are span nodes because they have their own spans in AST, `-` doesn't have a larger AST node so it's treated as a primitive token
span(expr(-10)) = span(token(-)).to(span(expr(10)));
span(expr(a + 11)) = span(expr(a)).to(span(token(+))).to(span(expr(11)));
// the whole generic arg is a span node because it has its own span in AST
span(path(a::<u8>::b)) = span(ident(a)).to(span(token(::))).to(span(generic_arg(<u8>))).to(span(token(::))).to(span(ident(b)));

Status quo:

Currently the resulting span is typically built like span_first_token.to(span_last_token).
So anything in the middle and the internal structure (i.e. nodes, as opposed to tokens) are ignored.

Why we need to change it:

The to operation will automatically take macro variables into account, and will try to put the resulting span into the best suitable macro context (this was implemented in #119673).
E.g. in $tt + 5 the combined expression span will be put into the context of the macro using $tt as a macro parameter.
Note, that the same thing often happens in the current parser as well, but not consistently, e.g. $a::$b will produce an incorrect resulting path span because the :: in the middle is not considered.
This will also give us some single relatively well predictable rule for combining AST spans.

Implementation:

This work should be parallelizable relatively well (but may require a one time initial setup).
I'll review PRs doing this, they can be assigned to me.
It may be convenient to have a rolling value in the Parser structure for span of the current (or previous?) span node.
The parser has a lot of bespoke diagnostic logic (including snapshotting) that stands in the way of any systematic improvements like this.

How this can be tested:

Make a macro that emits complex nodes using tokens from different contexts, e.g.

macro m($l:tt $op:tt $r:tt) {
    2 + 3;
    $l + 3;
    2 $op 3;
    2 + $r;
    $l $op 3;
    $l + $r;
    2 $op $r;
    $l $op $r;
}

m!(2 + 3);

and emit some diagnostic using those nodes' spans (maybe can add a special internal diagnostic for this testing).

[petrochenkov]

It may also be possible to introduce a smarter n-ary span combining operation to(a1, ..., aN) instead of the current binary to, but it may be more expensive and will make spans better only in very rare cases.
So combining using a chain of binary to operations should be fine for now.

[petrochenkov]

either an individual token

The "token" here is a "token tree" really, despite Rust parser working mostly with flattened token sequences at the moment.
So span(#[word]) should ideally be span(#) to span([word]) and not span(#) to span([) to span(word) to(span(]).

Also span combining for delimited groups can ignore all their internal tokens and only consider their delimiters.
Opening and closing delimiters always have the same context, and cannot be passed to macros separately in any way.
So span([word]) should be span([) to span(]) and not span([) to span(word) to span(]).

[petrochenkov]

@danik292 Yes

[petrochenkov]

I suggest implementing this for some specific AST nodes that are not too complex, e.g. for paths maybe (fn parse_path in rustc_parse), or for something in types (compiler\rustc_parse\src\parser\ty.rs).
The current (and/or previous) node span may need to be kept in struct Parser.

I'd also suggest starting with some testing infra, like a new attribute for showing spans in an AST node.

#[rustc_show_spans(expr)]
fn foo() {
  let x = a + b;
}

should show a diagnostic (probably a note) similar to this

#[rustc_show_spans(expr)]
fn foo() {
  let x = a + b;
          ^
              ^
          ^^^^^
}

See e.g. rustc_effective_visibility (in ./compiler) for an example of adding a new internal attribute.
The logic for showing spans should likely live in the rustc_ast_passes crate.

[noahmbright]

Is the main issue that something like $a::$b would have an incorrect span? I haven't fully understood the problem yet, but if the span is like a start, a length, and a context, how does the problem come from ignoring the middle tokens?