Add support for diff3 format

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -2579,12 +2579,16 @@ impl<'a> Parser<'a> {
         };
         let mut spans = Vec::with_capacity(3);
         spans.push(start);
+        let mut middlediff3 = None;
         let mut middle = None;
         let mut end = None;
         loop {
             if self.token.kind == TokenKind::Eof {
                 break;
             }
+            if let Some(span) = self.diff_marker(&TokenKind::OrOr, &TokenKind::BinOp(token::Or)) {
+                middlediff3 = Some(span);
+            }
             if let Some(span) = self.diff_marker(&TokenKind::EqEq, &TokenKind::Eq) {
                 middle = Some(span);
             }
@@ -2597,6 +2601,9 @@ impl<'a> Parser<'a> {
         }
         let mut err = self.struct_span_err(spans, "encountered diff marker");
         err.span_label(start, "after this is the code before the merge");
+        if let Some(middle) = middlediff3 {
+            err.span_label(middle, "");
+        }
         if let Some(middle) = middle {
             err.span_label(middle, "");
         }

src/test/ui/parser/diff-markers/enum-2.rs

@@ -2,8 +2,10 @@ enum E {
     Foo {
 <<<<<<< HEAD //~ ERROR encountered diff marker
         x: u8,
+|||||||
+        z: (),
 =======
-        x: i8,
+        y: i8,
 >>>>>>> branch
     }
 }

src/test/ui/parser/diff-markers/enum-2.stderr

@@ -4,9 +4,12 @@ error: encountered diff marker
 LL | <<<<<<< HEAD
    | ^^^^^^^ after this is the code before the merge
 LL |         x: u8,
+LL | |||||||
+   | -------
+LL |         z: (),
 LL | =======
    | -------
-LL |         x: i8,
+LL |         y: i8,
 LL | >>>>>>> branch
    | ^^^^^^^ above this are the incoming code changes
    |