Fix `let` keyword removal suggestion in structs
(1.) Fixes a bug where, given this code:
```rust
struct Foo {
let x: i32,
}
```
We were parsing the field name as `let` instead of `x`, which causes issues later on in the type-checking phase.
(2.) Also, suggestions for `let: i32` as a field regressed, displaying this extra `help:` which is removed by this PR
```
help: remove the let, the `let` keyword is not allowed in struct field definitions
|
2 - let: i32,
2 + : i32,
```
(3.) Makes the suggestion text a bit more succinct, since we don't need to re-explain that `let` is not allowed in this position (since it's in a note that follows). This causes the suggestion to render inline as well.
cc `@gimbles,` this addresses a few nits I mentioned in your PR.
It's now only used in one function. Also, the "should we glue the
tokens?" check is only necessary when pushing a `TokenTree::Token`, not
when pushing a `TokenTree::Delimited`.
As part of this, we now do the "should we glue the tokens?" check
immediately, which avoids having look back at the previous token. It
also puts all the logic dealing with token gluing in a single place.
Remove `expr_parentheses_needed` from `ParseSess`
Not sure why this method needed to exist on `ParseSess`, but we can achieve the same behavior by just inlining it everywhere.
Group together more size assertions.
Also add a few more assertions for some relevant token-related types.
And fix an erroneous comment in `rustc_errors`.
r? `@lqd`
Improve errors for incomplete functions in struct definitions
Given the following code:
```rust
fn main() {}
struct Foo {
fn
}
```
[playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=29139f870511f6918324be5ddc26c345)
The current output is:
```
Compiling playground v0.0.1 (/playground)
error: functions are not allowed in struct definitions
--> src/main.rs:4:5
|
4 | fn
| ^^
|
= help: unlike in C++, Java, and C#, functions are declared in `impl` blocks
= help: see https://doc.rust-lang.org/book/ch05-03-method-syntax.html for more information
error: could not compile `playground` due to previous error
```
In this case, rustc should suggest escaping `fn` to use it as an identifier.
Migrate more of rustc_parse to SessionDiagnostic
Still far from complete, but I thought I'd add a checkpoint here because rebasing was starting to get annoying.
