1.2 KiB
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!