rust/src/grammar/raw-string-literal-ambiguity.md
2014-07-21 10:59:57 -07:00

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Rust's lexical grammar is not context-free. Raw string literals are the source
of the problem. Informally, a raw string literal is an `r`, followed by `N`
hashes (where N can be zero), a quote, any characters, then a quote followed
by `N` hashes. This grammar describes this as best possible:
R -> 'r' S
S -> '"' B '"'
S -> '#' S '#'
B -> . B
B -> ε
Where `.` represents any character, and `ε` the empty string. Consider the
string `r#""#"#`. This string is not a valid raw string literal, but can be
accepted as one by the above grammar, using the derivation:
R : #""#"#
S : ""#"
S : "#
B : #
B : ε
(Where `T : U` means the rule `T` is applied, and `U` is the remainder of the
string.) The difficulty arises from the fact that it is fundamentally
context-sensitive. In particular, the context needed is the number of hashes.
I know of no way to resolve this, but also have not come up with a proof that
it is not context sensitive. Such a proof would probably use the pumping lemma
for context-free languages, but I (cmr) could not come up with a proof after
spending a few hours on it, and decided my time best spent elsewhere. Pull
request welcome!