Currently, we deal with escape sequences twice: once when we lex a
string, and a second time when we unescape literals. This PR aims to
remove this duplication, by introducing a new `unescape` mode as a
single source of truth for character escaping rules
When a format string has escaped whitespace characters format
arguments were shifted by one per each escaped character. Account
for these escaped characters when synthesizing the spans.
Fix#55155.
- Point at opening mismatched formatting brace
- Account for differences between raw and regular strings
- Account for differences between the code snippet and `InternedString`
- Add more tests
When encountering format string errors in a raw string, or regular
string literal with embedded newlines, account for the positional
change to use correct spans.
:drive by fix: 🚗
Prevent fmt::Arguments from being shared across threads
Fixes#45197
This is a **breaking change**! Without doing this it's very easy to create race conditions.
There's probably a way to do this without breaking valid use cases, but it would require quite an overhaul of the formatting machinery.
On cases of malformed format strings where a `{` hasn't been properly
escaped, like `println!("{");`, present a note explaining how to escape
the `{` char.
