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Note that raw string literals are context-sensitive #1185

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9f40cee
Note that raw string literals are context-sensitive
Gankra Mar 25, 2022
53e0a8b
Link the old context-sensitive proof.
Gankra Mar 25, 2022
05fd5e7
fixup newline
Gankra Mar 25, 2022
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10 changes: 10 additions & 0 deletions src/tokens.md
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Expand Up @@ -208,6 +208,16 @@ the characters `U+0022` (double-quote) (except when followed by at least as
many `U+0023` (`#`) characters as were used to start the raw string literal) or
`U+005C` (`\`) do not have any special meaning.

**Note that this is a _context-sensitive_ grammar, as opposed to _context-free_.**
This is because strings like `r###"I contain only 2 "##s so I'm ok"###` require
a parser to properly count the number of opening #'s and compare that count to two
different values. In practical terms this is very easy for a parser to do, but
a context-free language can't because the only way to express "counting" is
as a destructive operation which forces you to forget the count. This allows
for the "comparison" of two counts (such (as (balanced) (parens))) but not 3+.

See [the proof for more formal details](https://github.com/rust-lang/rust/blob/5187be620c76a313a19b9b596e1bce3a80a345dd/src/grammar/raw-string-literal-ambiguity.md).

Examples for string literals:

```rust
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