gopls/internal/cache: id command-line-arguments packages using GoFiles

Previously, we were using the first CompiledGoFiles to disambiguate the
ID of command-line-arguments packages, but in the presence of cgo
preprocessing there can actually be multiple CompiledGoFiles, leading
to the bug report of golang/go#64557. Fix this by using GoFiles instead.

Fixes golang/go#64557

Change-Id: I3eff976d07da32db1f26ced69228af41a388d9a1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/627776
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Findley <rfindley@google.com>

gopls/internal/cache/load.go

@@ -376,21 +376,23 @@ func buildMetadata(updates map[PackageID]*metadata.Package, pkg *packages.Packag
 
 	if metadata.IsCommandLineArguments(id) {
 		var f string // file to use as disambiguating suffix
-		if len(pkg.CompiledGoFiles) > 0 {
-			f = pkg.CompiledGoFiles[0]
-
-			// If there are multiple files,
-			// we can't use only the first.
-			// (Can this happen? #64557)
-			if len(pkg.CompiledGoFiles) > 1 {
-				bug.Reportf("unexpected files in command-line-arguments package: %v", pkg.CompiledGoFiles)
+		if len(pkg.GoFiles) > 0 {
+			f = pkg.GoFiles[0]
+
+			// If there are multiple files, we can't use only the first. Note that we
+			// consider GoFiles, rather than CompiledGoFiles, as there can be
+			// multiple CompiledGoFiles in the presence of cgo processing, whereas a
+			// command-line-arguments package should always have exactly one nominal
+			// Go source file. (See golang/go#64557.)
+			if len(pkg.GoFiles) > 1 {
+				bug.Reportf("unexpected files in command-line-arguments package: %v", pkg.GoFiles)
 				return nil
 			}
 		} else if len(pkg.IgnoredFiles) > 0 {
 			// A file=empty.go query results in IgnoredFiles=[empty.go].
 			f = pkg.IgnoredFiles[0]
 		} else {
-			bug.Reportf("command-line-arguments package has neither CompiledGoFiles nor IgnoredFiles")
+			bug.Reportf("command-line-arguments package has neither GoFiles nor IgnoredFiles")
 			return nil
 		}
 		id = PackageID(pkg.ID + f)

gopls/internal/test/marker/testdata/definition/standalone_issue64557.txt

@@ -0,0 +1,27 @@
+This test checks that we can load standalone files that use cgo.
+
+-- go.mod --
+module example.com
+
+-- main.go --
+//go:build ignore
+
+package main
+
+import (
+	"C"
+
+	"example.com/a"
+)
+
+func F() {} //@loc(F, "F")
+
+func main() {
+	F() //@def("F", F)
+	println(a.A) //@def("A", A)
+}
+
+-- a/a.go --
+package a
+
+const A = 0 //@loc(A, "A")