Create highlight group for string delimiters
Create highlight group for string delimiters

Some users wish to highlight string delimiters (e.g., quotation marks)
in a different color from the body of the string itself. This feature is
provided by several other languages' syntax highlighting rules, and is
easy to achieve for Rust as well.

Add a match group for the string delimiters and by default, assign it to
the same highlight as the rest of the string, to preserve existing
behavior. Users who wish to make use of this feature need only adjust
their color scheme files to link the new rustStringDelimiter group to their
preferred syntax highlighting group.

rust-lang/rust.vim