It only has a single use, within code handling indented block comments.
We can replace that with the new `FileMap::col_pos()`, which computes
the col position (BytePos instead of CharPos) based on the record of the
last newline char (which we already record).
This is actually an improvement, because
`trim_whitespace_prefix_and_push_line()` was using `col`, which is a
`CharPos`, as a slice index, which is a byte/char confusion.
The syntax crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), parser, pretty-printer,
lexer, macro expander, and utilities for traversing ASTs.
For more information about how these things work in rustc, see the
rustc guide:
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