Improve error recovery for some built-in macros

src/libsyntax_ext/concat.rs

@@ -18,6 +18,7 @@ pub fn expand_syntax_ext(
     };
     let mut accumulator = String::new();
     let mut missing_literal = vec![];
+    let mut has_errors = false;
     for e in es {
         match e.node {
             ast::ExprKind::Lit(ref lit) => match lit.node {
@@ -41,6 +42,9 @@ pub fn expand_syntax_ext(
                     cx.span_err(e.span, "cannot concatenate a byte string literal");
                 }
             },
+            ast::ExprKind::Err => {
+                has_errors = true;
+            }
             _ => {
                 missing_literal.push(e.span);
             }
@@ -50,6 +54,9 @@ pub fn expand_syntax_ext(
         let mut err = cx.struct_span_err(missing_literal, "expected a literal");
         err.note("only literals (like `\"foo\"`, `42` and `3.14`) can be passed to `concat!()`");
         err.emit();
+        return base::DummyResult::expr(sp);
+    } else if has_errors {
+        return base::DummyResult::expr(sp);
     }
     let sp = sp.apply_mark(cx.current_expansion.mark);
     base::MacEager::expr(cx.expr_str(sp, Symbol::intern(&accumulator)))