Avoid aborting after expansion from BuildReducedGraphVisitor errors.

src/libsyntax/ext/base.rs

@@ -708,6 +708,7 @@ pub struct ExtCtxt<'a> {
     pub ecfg: expand::ExpansionConfig<'a>,
     pub crate_root: Option<&'static str>,
     pub resolver: &'a mut Resolver,
+    pub resolve_err_count: usize,
     pub current_expansion: ExpansionData,
 }
 
@@ -722,6 +723,7 @@ impl<'a> ExtCtxt<'a> {
             ecfg: ecfg,
             crate_root: None,
             resolver: resolver,
+            resolve_err_count: 0,
             current_expansion: ExpansionData {
                 mark: Mark::root(),
                 depth: 0,

src/libsyntax/ext/expand.rs

@@ -200,7 +200,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
             _ => unreachable!(),
         };
 
-        if self.cx.parse_sess.span_diagnostic.err_count() > err_count {
+        if self.cx.parse_sess.span_diagnostic.err_count() - self.cx.resolve_err_count > err_count {
             self.cx.parse_sess.span_diagnostic.abort_if_errors();
         }
 
@@ -271,8 +271,10 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         self.cx.cfg = crate_config;
 
         if self.monotonic {
+            let err_count = self.cx.parse_sess.span_diagnostic.err_count();
             let mark = self.cx.current_expansion.mark;
             self.cx.resolver.visit_expansion(mark, &result.0);
+            self.cx.resolve_err_count += self.cx.parse_sess.span_diagnostic.err_count() - err_count;
         }
 
         result