Normalize identifiers in librustc_parse.

Cargo.lock

@@ -3753,6 +3753,7 @@ dependencies = [
  "smallvec 1.0.0",
  "syntax",
  "syntax_pos",
+ "unicode-normalization",
 ]
 
 [[package]]
@@ -4935,9 +4936,12 @@ dependencies = [
 
 [[package]]
 name = "unicode-normalization"
-version = "0.1.7"
+version = "0.1.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6a0180bc61fc5a987082bfa111f4cc95c4caff7f9799f3e46df09163a937aa25"
+checksum = "b561e267b2326bb4cebfc0ef9e68355c7abe6c6f522aeac2f5bf95d56c59bdcf"
+dependencies = [
+ "smallvec 1.0.0",
+]
 
 [[package]]
 name = "unicode-segmentation"

src/librustc_parse/Cargo.toml

@@ -20,3 +20,4 @@ rustc_lexer = { path = "../librustc_lexer" }
 rustc_target = { path = "../librustc_target" }
 smallvec = { version = "1.0", features = ["union", "may_dangle"] }
 rustc_error_codes = { path = "../librustc_error_codes" }
+unicode-normalization = "0.1.11"

src/librustc_parse/lexer/mod.rs

@@ -220,8 +220,7 @@ impl<'a> StringReader<'a> {
                 if is_raw_ident {
                     ident_start = ident_start + BytePos(2);
                 }
-                // FIXME: perform NFKC normalization here. (Issue #2253)
-                let sym = self.symbol_from(ident_start);
+                let sym = self.nfc_symbol_from(ident_start);
                 if is_raw_ident {
                     let span = self.mk_sp(start, self.pos);
                     if !sym.can_be_raw() {
@@ -466,6 +465,20 @@ impl<'a> StringReader<'a> {
         Symbol::intern(self.str_from_to(start, end))
     }
 
+    /// As symbol_from, with the text normalized into Unicode NFC form.
+    fn nfc_symbol_from(&self, start: BytePos) -> Symbol {
+        use unicode_normalization::{is_nfc_quick, IsNormalized, UnicodeNormalization};
+        debug!("taking an normalized ident from {:?} to {:?}", start, self.pos);
+        let sym = self.str_from(start);
+        match is_nfc_quick(sym.chars()) {
+            IsNormalized::Yes => Symbol::intern(sym),
+            _ => {
+                let sym_str: String = sym.chars().nfc().collect();
+                Symbol::intern(&sym_str)
+            }
+        }
+    }
+
     /// Slice of the source text spanning from `start` up to but excluding `end`.
     fn str_from_to(&self, start: BytePos, end: BytePos) -> &str
     {