1 files changed, 93 insertions, 0 deletions
diff --git a/compiler/rustc_lexer/src/cursor.rs b/compiler/rustc_lexer/src/cursor.rs
new file mode 100644
index 000000000..21557a9c8
--- /dev/null
+++ b/compiler/rustc_lexer/src/cursor.rs
@@ -0,0 +1,93 @@
+use std::str::Chars;
+
+/// Peekable iterator over a char sequence.
+///
+/// Next characters can be peeked via `first` method,
+/// and position can be shifted forward via `bump` method.
+pub(crate) struct Cursor<'a> {
+    initial_len: usize,
+    /// Iterator over chars. Slightly faster than a &str.
+    chars: Chars<'a>,
+    #[cfg(debug_assertions)]
+    prev: char,
+}
+
+pub(crate) const EOF_CHAR: char = '\0';
+
+impl<'a> Cursor<'a> {
+    pub(crate) fn new(input: &'a str) -> Cursor<'a> {
+        Cursor {
+            initial_len: input.len(),
+            chars: input.chars(),
+            #[cfg(debug_assertions)]
+            prev: EOF_CHAR,
+        }
+    }
+
+    /// Returns the last eaten symbol (or `'\0'` in release builds).
+    /// (For debug assertions only.)
+    pub(crate) fn prev(&self) -> char {
+        #[cfg(debug_assertions)]
+        {
+            self.prev
+        }
+
+        #[cfg(not(debug_assertions))]
+        {
+            EOF_CHAR
+        }
+    }
+
+    /// Peeks the next symbol from the input stream without consuming it.
+    /// If requested position doesn't exist, `EOF_CHAR` is returned.
+    /// However, getting `EOF_CHAR` doesn't always mean actual end of file,
+    /// it should be checked with `is_eof` method.
+    pub(crate) fn first(&self) -> char {
+        // `.next()` optimizes better than `.nth(0)`
+        self.chars.clone().next().unwrap_or(EOF_CHAR)
+    }
+
+    /// Peeks the second symbol from the input stream without consuming it.
+    pub(crate) fn second(&self) -> char {
+        // `.next()` optimizes better than `.nth(1)`
+        let mut iter = self.chars.clone();
+        iter.next();
+        iter.next().unwrap_or(EOF_CHAR)
+    }
+
+    /// Checks if there is nothing more to consume.
+    pub(crate) fn is_eof(&self) -> bool {
+        self.chars.as_str().is_empty()
+    }
+
+    /// Returns amount of already consumed symbols.
+    pub(crate) fn len_consumed(&self) -> u32 {
+        (self.initial_len - self.chars.as_str().len()) as u32
+    }
+
+    /// Resets the number of bytes consumed to 0.
+    pub(crate) fn reset_len_consumed(&mut self) {
+        self.initial_len = self.chars.as_str().len();
+    }
+
+    /// Moves to the next character.
+    pub(crate) fn bump(&mut self) -> Option<char> {
+        let c = self.chars.next()?;
+
+        #[cfg(debug_assertions)]
+        {
+            self.prev = c;
+        }
+
+        Some(c)
+    }
+
+    /// Eats symbols while predicate returns true or until the end of file is reached.
+    pub(crate) fn eat_while(&mut self, mut predicate: impl FnMut(char) -> bool) {
+        // It was tried making optimized version of this for eg. line comments, but
+        // LLVM can inline all of this and compile it down to fast iteration over bytes.
+        while predicate(self.first()) && !self.is_eof() {
+            self.bump();
+        }
+    }
+}