cmd/internal/testdir: filter out errors outside input file set

When an errorcheck test uses -m and instantiates an imported generic
function, the errors will include -m messages from the imported package
(since the new function has not previously been walked). These errors
cannot be matched since we can't write errors in files outside the test
input.

To fix this (and enable the other CLs in this stack), drop any unmatched
errors that occur in files outside those in the input set.

Change-Id: I2fcf0dd4693125d2e5823ea4437011730d8b1b1f
Reviewed-on: https://go-review.googlesource.com/c/go/+/672515
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>

Author: jakebailey

src/cmd/internal/testdir/testdir_test.go

@@ -1242,6 +1242,24 @@ func (t test) errorCheck(outStr string, wantAuto bool, fullshort ...string) (err
 		}
 	}
 
+	if len(out) > 0 {
+		// If a test uses -m and instantiates an imported generic function,
+		// the errors will include messages for the instantiated function
+		// with locations in the other package. Filter those out.
+		localOut := make([]string, 0, len(out))
+	outLoop:
+		for _, errLine := range out {
+			for j := 0; j < len(fullshort); j += 2 {
+				full, short := fullshort[j], fullshort[j+1]
+				if strings.HasPrefix(errLine, full+":") || strings.HasPrefix(errLine, short+":") {
+					localOut = append(localOut, errLine)
+					continue outLoop
+				}
+			}
+		}
+		out = localOut
+	}
+
 	if len(out) > 0 {
 		errs = append(errs, fmt.Errorf("Unmatched Errors:"))
 		for _, errLine := range out {