cmd/go: teach the build cache about -trimpath

Fixes #31896

Change-Id: I228a809568cd37c599987f9f1e99df5c229e6c9b
Reviewed-on: https://go-review.googlesource.com/c/go/+/176112
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/work/exec.go

@@ -211,6 +211,9 @@ func (b *Builder) buildActionID(a *Action) cache.ActionID {
 	fmt.Fprintf(h, "goos %s goarch %s\n", cfg.Goos, cfg.Goarch)
 	fmt.Fprintf(h, "import %q\n", p.ImportPath)
 	fmt.Fprintf(h, "omitdebug %v standard %v local %v prefix %q\n", p.Internal.OmitDebug, p.Standard, p.Internal.Local, p.Internal.LocalPrefix)
+	if cfg.BuildTrimpath {
+		fmt.Fprintln(h, "trimpath")
+	}
 	if p.Internal.ForceLibrary {
 		fmt.Fprintf(h, "forcelibrary\n")
 	}
@@ -1116,6 +1119,9 @@ func (b *Builder) linkActionID(a *Action) cache.ActionID {
 	fmt.Fprintf(h, "buildmode %s goos %s goarch %s\n", cfg.BuildBuildmode, cfg.Goos, cfg.Goarch)
 	fmt.Fprintf(h, "import %q\n", p.ImportPath)
 	fmt.Fprintf(h, "omitdebug %v standard %v local %v prefix %q\n", p.Internal.OmitDebug, p.Standard, p.Internal.Local, p.Internal.LocalPrefix)
+	if cfg.BuildTrimpath {
+		fmt.Fprintln(h, "trimpath")
+	}
 
 	// Toolchain-dependent configuration, shared with b.linkSharedActionID.
 	b.printLinkerConfig(h, p)

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

+env GO111MODULE=on
+
+# Set up fresh GOCACHE.
+env GOCACHE=$WORK/gocache
+mkdir $GOCACHE
+
+cd $WORK
+go build -o a.out
+
+# Varying -trimpath should cause a rebuild.
+go build -x -o a.out -trimpath
+stderr '(compile|gccgo)( |\.exe)'
+stderr 'link( |\.exe)'
+
+-- $WORK/hello.go --
+package main
+func main() { println("hello") }
+
+-- $WORK/go.mod --
+module m