Introduce NonterminalKind for more type-safe mbe parsing

[matklad]

It encapsulate the (part of) the interface between the parser and
macro by example (macro_rules) parser.

The second bit is somewhat more general parse_ast_fragment, which is
the reason why we keep some parse_xxx functions as public.

[rust-highfive]

r? @ecstatic-morse

(rust_highfive has picked a reviewer for you, use r? to override)

[matklad]

r? @petrochenkov

I also want to change MetaVarDecl(Span, Ident /* name to bind */, Ident /* kind of nonterminal */), to MetaVarDecl(Span, Ident /* name to bind */, Option<MbeFragment>),

[matklad]

I also want to change MetaVarDecl(Span, Ident /* name to bind /, Ident / kind of nonterminal /), to MetaVarDecl(Span, Ident / name to bind */, Option),

Ah, MetaVarDecl is also serializable. I think it makes sense to keep it as is then and not to mix serialization into parser's interface.

[mark-i-m]

This code is quite out of cache for me, but any improvements are very well! 🎉

[matklad]

Switched to NonterminalKind! This actually I think made it possible to use it in the MetaVarDecl, but that would be a somewhat bigger refactoring with higher risk of breaking something, so it might make sense to review the two separately.

[matklad]

Yup, so the second commit uses NonterminalKind for MetaVarDecl as well, ready for review now!

[matklad]

The remaining weirdness with Option is due to #40107 I belive. I'll tackle that next, seems fine to try to hard error now.

[petrochenkov]

@bors r+

[bors]

📌 Commit 1a2d07e has been approved by petrochenkov

[matklad]

Are there any word of mouth and/or written guidelines about the ordering of mods and uses? I've fixed this negatively, by not mixing uses and modules in random order, but i am wondering what the positive fix would look like. In rust-analyer, we are somewhat picky about this: https://github.com/rust-analyzer/rust-analyzer/blob/master/docs/dev/style.md#order-of-imports

[petrochenkov]

I don't think there's an official guideline, but the most common order is

use crate::things;

use other_crate::things;

use std::things;

mod foo;

/* Other items */

[bors]

⌛ Testing commit 1a2d07e with merge 8ae5d23b3623b4622364808137967732e0783d70...

[rust-log-analyzer]

The job i686-msvc-1 of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

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[bors]

💔 Test failed - checks-actions

[matklad]

Seems to be a spurious network error:

    Updating crates.io index
error: failed to get `cc` as a dependency of package `bootstrap v0.0.0 (D:\a\rust\rust\src\bootstrap)`

Caused by:
  failed to fetch `https://github.com/rust-lang/crates.io-index`

Caused by:
  error inflating zlib stream; class=Zlib (5)
failed to run: D:\a\rust\rust\build\i686-pc-windows-msvc\stage0\bin\cargo.exe build --manifest-path D:\a\rust\rust\src/bootstrap/Cargo.toml --locked
Build completed unsuccessfully in 0:00:16
make: *** [Makefile:60: prepare] Error 1
Command failed. Attempt 5/5:
    Updating crates.io index
error: failed to get `cc` as a dependency of package `bootstrap v0.0.0 (D:\a\rust\rust\src\bootstrap)`

Caused by:
  failed to fetch `https://github.com/rust-lang/crates.io-index`

Caused by:
  error inflating zlib stream; class=Zlib (5)

@bors retry

[bors]

⌛ Testing commit 1a2d07e with merge f042d74...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: petrochenkov
Pushing f042d74 to master...