cmd/go: report non-Go files as package error

This change modifies cmd/go/list to format the error correctly in case
-e flag is set. It also fixes a bug where the package loader was only
ever checking the first pattern if it had the go extension. This caused
and error when a file without .go extension was not the first argument.

Fixes #29899

Change-Id: I029bf4465ad4ad054434b8337c1d2a59369783da
Reviewed-on: https://go-review.googlesource.com/c/go/+/166398
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/load/pkg.go

@@ -1935,8 +1935,12 @@ func Packages(args []string) []*Package {
 // cannot be loaded at all.
 // The packages that fail to load will have p.Error != nil.
 func PackagesAndErrors(patterns []string) []*Package {
-	if len(patterns) > 0 && strings.HasSuffix(patterns[0], ".go") {
-		return []*Package{GoFilesPackage(patterns)}
+	if len(patterns) > 0 {
+		for _, p := range patterns {
+			if strings.HasSuffix(p, ".go") {
+				return []*Package{GoFilesPackage(patterns)}
+			}
+		}
 	}
 
 	matches := ImportPaths(patterns)
@@ -2048,7 +2052,14 @@ func GoFilesPackage(gofiles []string) *Package {
 
 	for _, f := range gofiles {
 		if !strings.HasSuffix(f, ".go") {
-			base.Fatalf("named files must be .go files")
+			pkg := new(Package)
+			pkg.Internal.Local = true
+			pkg.Internal.CmdlineFiles = true
+			pkg.Name = f
+			pkg.Error = &PackageError{
+				Err: fmt.Sprintf("named files must be .go files: %s", pkg.Name),
+			}
+			return pkg
 		}
 	}
 

src/cmd/go/testdata/script/list_test_non_go_files.txt

+env GO111MODULE=off
+
+# issue 29899: handling files with non-Go extension
+go list -e -test -json -- c.c x.go
+stdout '"Err": "named files must be .go files: c.c"'
+
+! go list -test -json -- c.c x.go
+stderr 'can''t load package: named files must be .go files: c.c'
+
+-- x.go --
+package main
+-- c.c --
+package c