fix a bug in compiletest JSON parsing for duplicate errors

In some cases, we give multiple primary spans, in which case we would
report one `//~` annotation per primary span. That was very confusing
because these things are reported to the user as a single error.

UI tests would be better here.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/tools/compiletest/src/json.rs b/src/tools/compiletest/src/json.rs
index d9da1bdc348..06cbd9a3df4 100644
--- a/src/tools/compiletest/src/json.rs
+++ b/src/tools/compiletest/src/json.rs
@@ -97,6 +97,7 @@ fn push_expected_errors(expected_errors: &mut Vec<Error>,
     let primary_spans: Vec<_> = spans_in_this_file.iter()
         .cloned()
         .filter(|span| span.is_primary)
+        .take(1) // sometimes we have more than one showing up in the json; pick first
         .collect();
     let primary_spans = if primary_spans.is_empty() {
         // subdiagnostics often don't have a span of their own;