Do not accept the following
```rust
macro_rules! lexes {($($_:tt)*) => {}}
lexes!(🐛"foo");
```
Before, invalid emoji identifiers were gated during parsing instead of lexing in all cases, but this didn't account for macro expansion of literal prefixes.
Fix#123696.
Remove unncessary check for macro call
Since `macro_rules` is a contextual keyword, it is an `IDENT` token and thus `is_path_start` already identifies it correctly. You can tell the previous check is unnecessary because the relevant tests still pass.
internal: Improve readability of the parser code
The code is basically equivalent to the previous version, but it improves the readability by making it much more simpler and concise.
Previously, when parsing an argument list with a missing argument (e.g.,
`(a, , b)` in `foo(a, , b)`), the parser would stop upon an unexpected
token (at the second comma in the example), resulting in an incorrect
parse tree.
This commit improves error handling in such cases, ensuring a more
accurate parse tree is built.
`unescape_literal` becomes `unescape_unicode`, and `unescape_c_string`
becomes `unescape_mixed`. Because rfc3349 will mean that C string
literals will no longer be the only mixed utf8 literals.
Detect `NulInCStr` error earlier.
By making it an `EscapeError` instead of a `LitError`. This makes it like the other errors produced when checking string literals contents, e.g. for invalid escape sequences or bare CR chars.
NOTE: this means these errors are issued earlier, before expansion, which changes behaviour. It will be possible to move the check back to the later point if desired. If that happens, it's likely that all the string literal contents checks will be delayed together.
One nice thing about this: the old approach had some code in `report_lit_error` to calculate the span of the nul char from a range. This code used a hardwired `+2` to account for the `c"` at the start of a C string literal, but this should have changed to a `+3` for raw C string literals to account for the `cr"`, which meant that the caret in `cr"` nul error messages was one short of where it should have been. The new approach doesn't need any of this and avoids the off-by-one error.
r? ```@fee1-dead```
