cmd/go: report scan error position in 'go list -e'tls

This CL extracts some error handling code into a common method for
presenting errors encountered when loading package data.

Fixes #36087
Fixes #36762

Change-Id: I87c8d41e3cc6e6afa152d9c067bc60923bf19fbe
Reviewed-on: https://go-review.googlesource.com/c/go/+/210938
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

1 files changed, 32 insertions, 4 deletions

diff --git a/src/cmd/go/testdata/script/list_parse_err.txt b/src/cmd/go/testdata/script/list_parse_err.txt
index 5aacaa88fa..3c5345801a 100644
--- a/src/cmd/go/testdata/script/list_parse_err.txt
+++ b/src/cmd/go/testdata/script/list_parse_err.txt
@@ -1,17 +1,45 @@
-# 'go list' should report imports, even if some files have parse errors
+# 'go list' without -e should fail and print errors on stderr.
+! go list ./p
+stderr '^p[/\\]b.go:2:2: expected ''package'', found ''EOF''$'
+! go list -f '{{range .Imports}}{{.}} {{end}}' ./p
+stderr '^p[/\\]b.go:2:2: expected ''package'', found ''EOF''$'
+! go list -test ./t
+stderr '^can''t load test package: t[/\\]t_test.go:8:1: expected declaration, found ʕ'
+! go list -test -f '{{range .Imports}}{{.}} {{end}}' ./t
+stderr '^can''t load test package: t[/\\]t_test.go:8:1: expected declaration, found ʕ'
+
+# 'go list -e' should report imports, even if some files have parse errors
 # before the import block.
-go list -e -f '{{range .Imports}}{{.}} {{end}}'
+go list -e -f '{{range .Imports}}{{.}} {{end}}' ./p
 stdout '^fmt '
 
+# 'go list' should report the position of the error if there's only one.
+go list -e -f '{{.Error.Pos}} => {{.Error.Err}}' ./p
+stdout 'b.go:[0-9:]+ => expected ''package'', found ''EOF'''
+
+# 'go test' should report the position of the error if there's only one.
+go list -e -test -f '{{if .Error}}{{.Error.Pos}} => {{.Error.Err}}{{end}}' ./t
+stdout 't_test.go:[0-9:]+ => expected declaration, found ʕ'
+
 -- go.mod --
 module m
 
 go 1.13
 
--- a.go --
+-- p/a.go --
 package a
 
 import "fmt"
 
--- b.go --
+-- p/b.go --
 // no package statement
+
+-- t/t_test.go --
+package t
+
+import "testing"
+
+func Test(t *testing.T) {}
+
+// scan error
+ʕ◔ϖ◔ʔ