Parse string literals to find their length.

This makes the double brackets somewhat unnecessary. Now they are just
used to find the macro invocation, though we should be able to get that
with the data in `Position`.

src/lib.rs

@@ -274,13 +274,80 @@ impl Expect {
             line_start += line.len();
         }
         let (literal_start, line_indent) = target_line.unwrap();
-        let literal_length =
-            file[literal_start..].find("]]").expect("Couldn't find matching `]]` for `expect![[`.");
-        let literal_range = literal_start..literal_start + literal_length;
+
+        let literal_len = locate_end(&file[literal_start..])
+            .expect("Couldn't find matching `]]` for `expect![[`.");
+        let literal_range = literal_start..literal_start + literal_len;
         Location { line_indent, literal_range }
     }
 }
 
+fn locate_end(lit_to_eof: &str) -> Option<usize> {
+    if lit_to_eof.chars().skip_while(|c| c.is_whitespace()).take(2).eq([']', ']'].iter().cloned()) {
+        // expect![[ ]]
+        Some(0)
+    } else {
+        // expect![["foo"]]
+        find_str_lit_len(lit_to_eof)
+    }
+}
+
+/// Parses a string literal, returning the byte index of its last character
+/// (either a quote or a hash).
+fn find_str_lit_len(str_lit_to_eof: &str) -> Option<usize> {
+    use StrLitKind::*;
+    #[derive(Clone, Copy)]
+    enum StrLitKind {
+        Normal,
+        Raw(usize),
+    }
+
+    fn try_find_n_hashes(
+        s: &mut impl Iterator<Item = char>,
+        desired_hashes: usize,
+    ) -> Option<(usize, Option<char>)> {
+        let mut n = 0;
+        loop {
+            match s.next()? {
+                '#' => n += 1,
+                c => return Some((n, Some(c))),
+            }
+
+            if n == desired_hashes {
+                return Some((n, None));
+            }
+        }
+    }
+
+    let mut s = str_lit_to_eof.chars();
+    let kind = match s.next()? {
+        '"' => Normal,
+        'r' => {
+            let (n, c) = try_find_n_hashes(&mut s, usize::MAX)?;
+            if c != Some('"') { return None; }
+            Raw(n)
+        }
+        _ => return None,
+    };
+
+    let mut oldc = None;
+    loop {
+        let c = oldc.take().or_else(|| s.next())?;
+        match (c, kind) {
+            ('\\', Normal) => { let _escaped = s.next()?; }
+            ('"', Normal | Raw(0)) => break,
+            ('"', Raw(n)) => {
+                let (seen, c) = try_find_n_hashes(&mut s, n)?;
+                if seen == n { break; }
+                oldc = c;
+            }
+            _ => {}
+        }
+    }
+
+    Some(str_lit_to_eof.len() - s.as_str().len())
+}
+
 impl ExpectFile {
     /// Checks if file contents is equal to `actual`.
     pub fn assert_eq(&self, actual: &str) {
@@ -661,4 +728,48 @@ line1
 "#]],
         );
     }
+
+    #[test]
+    fn test_locate() {
+        macro_rules! check_locate {
+            ($( [[$s:literal]] ),* $(,)?) => {$({
+                let lit = stringify!($s);
+                let with_trailer = format!("{} \t]]\n", lit);
+                assert_eq!(locate_end(&with_trailer), Some(lit.len()));
+            })*};
+        }
+
+        // Check that we handle string literals containing "]]" correctly.
+        check_locate!(
+            [[r#"{ arr: [[1, 2], [3, 4]], other: "foo" } "#]],
+            [["]]"]],
+            [["\"]]"]],
+            [[r#""]]"#]],
+        );
+
+        // Check `expect![[  ]]` as well.
+        assert_eq!(locate_end(" ]]"), Some(0));
+    }
+
+    #[test]
+    fn test_find_str_lit_len() {
+        macro_rules! check_str_lit_len {
+            ($( $s:literal ),* $(,)?) => {$({
+                let lit = stringify!($s);
+                assert_eq!(find_str_lit_len(lit), Some(lit.len()));
+            })*}
+        }
+
+        check_str_lit_len![
+            r##"foa\""#"##,
+            r##"
+
+                asdf][]]""""#
+            "##,
+            "",
+            "\"",
+            "\"\"",
+            "#\"#\"#",
+        ];
+    }
 }