cmd/go: write changes to go.mod and go.sum after loading the command-…

…line-arguments package

This entrypoint was missed in CL 349600, and the behavior happened not
to be covered by existing tests.

Fixes golang#52331.

Change-Id: Iccf12e8e633215abe4bfa1c3ca2fe3a8391b5ba5
Reviewed-on: https://go-review.googlesource.com/c/go/+/401536
Run-TryBot: Bryan Mills <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

src/cmd/go/internal/modload/load.go

@@ -717,6 +717,12 @@ func ImportFromFiles(ctx context.Context, gofiles []string) {
 		},
 	})
 	requirements = loaded.requirements
+
+	if !ExplicitWriteGoMod {
+		if err := commitRequirements(ctx); err != nil {
+			base.Fatalf("go: %v", err)
+		}
+	}
 }
 
 // DirImportPath returns the effective import path for dir,

src/cmd/go/testdata/script/mod_run_issue52331.txt

@@ -0,0 +1,35 @@
+# Regression test for https://go.dev/issue/52331: 'go run -mod=mod'
+# failed to write go.mod and go.sum with the resolved dependencies.
+
+[short] skip
+
+! go run main.go
+# stderr '^main\.go:6:2: no required module provides package example\.com/version; to add it:\n\tgo get example\.com/version\n\z'
+
+go run -mod=mod main.go
+cmp go.mod go.mod.want
+grep -count=1 '^example\.com/version v1.1.0 h1:' go.sum
+grep -count=1 '^example\.com/version v1.1.0/go.mod h1:' go.sum
+
+-- go.mod --
+module example
+
+go 1.17
+-- go.mod.want --
+module example
+
+go 1.17
+
+require example.com/version v1.1.0 // indirect
+-- main.go --
+package main
+
+import (
+	"fmt"
+
+	"example.com/version"
+)
+
+func main() {
+	fmt.Println(version.V)
+}