Add AST -> HIR translation for lookarounds

Author: shilangyu

regex-syntax/src/hir/literal.rs

@@ -2456,7 +2456,6 @@ mod tests {
     }
 
     #[test]
-    #[ignore = "Missing parser support for lookaround"]
     fn lookaround() {
         assert_eq!(exact(["ab"]), e(r"a(?<=qwa)b"));
         assert_eq!(exact(["ab"]), e(r"a(?<!qwe)b"));

regex-syntax/src/hir/print.rs

@@ -485,15 +485,29 @@ mod tests {
     }
 
     #[test]
-    #[ignore = "Missing parser support for lookaround"]
     fn print_look_around() {
-        roundtrip("(?<=)", "(?<=(?:))");
-        roundtrip("(?<!)", "(?<!(?:))");
+        // we do not want to do a roundtrip: printed lookarounds are not
+        // can contain capture groups which are unsupported by the parser.
+        // TODO(shilangyu): is this a problem that some regexes are not
+        //                  roundtrippable?
+        fn test(given: &str, expected: &str) {
+            let builder = ParserBuilder::new();
+            let hir = builder.build().parse(given).unwrap();
+
+            let mut printer = Printer::new();
+            let mut dst = String::new();
+            printer.print(&hir, &mut dst).unwrap();
+
+            assert_eq!(expected, dst);
+        }
+
+        test("(?<=)", "(?<=(?:))");
+        test("(?<!)", "(?<!(?:))");
 
-        roundtrip("(?<=a)", "(?<=a)");
-        roundtrip("(?<!a)", "(?<!a)");
+        test("(?<=a)", "(?<=a)");
+        test("(?<!a)", "(?<!a)");
 
-        roundtrip("(?<=(?<!(?<!(?<=a))))", "(?<=(?<!(?<!(?<=a))))");
+        test("(?<=(?<!(?<!(?<=a))))", "(?<=(?<!(?<!(?<=a))))");
     }
 
     #[test]

regex-syntax/src/hir/translate.rs

@@ -354,9 +354,6 @@ impl<'t, 'p> Visitor for TranslatorI<'t, 'p> {
                     .unwrap_or_else(|| self.flags());
                 self.push(HirFrame::Group { old_flags });
             }
-            Ast::LookAround(ref x) => {
-                todo!("translation from AST to HIR");
-            }
             Ast::Concat(_) => {
                 self.push(HirFrame::Concat);
             }
@@ -449,8 +446,16 @@ impl<'t, 'p> Visitor for TranslatorI<'t, 'p> {
                 self.trans().flags.set(old_flags);
                 self.push(HirFrame::Expr(self.hir_capture(x, expr)));
             }
-            Ast::LookAround(_) => {
-                todo!("translation from AST to HIR");
+            Ast::LookAround(ref x) => {
+                let expr = Box::new(self.pop().unwrap().unwrap_expr());
+                self.push(HirFrame::Expr(Hir::lookaround(match x.kind {
+                    ast::LookAroundKind::PositiveLookBehind => {
+                        hir::LookAround::PositiveLookBehind(expr)
+                    }
+                    ast::LookAroundKind::NegativeLookBehind => {
+                        hir::LookAround::NegativeLookBehind(expr)
+                    }
+                })));
             }
             Ast::Concat(_) => {
                 let mut exprs = vec![];