The linker is told once, not twice

novox/hq 04-ISSUES/161. A repeated flag is not a merged one. The Go
command takes the last -ldflags and drops the first, so passing the
toolchain's flags and then the system stamp as a second one produced a
binary that knew its system and had lost -s -w: 12.2MB against 8.5MB,
with its debug info intact.

My own comment said the linker "accepts and merges" them. It does not,
and I found out by reading the file the build produced rather than by
reading the comment again.

Linker flags are now the toolchain's own list, composed into one flag with
the stamp. A test refuses a compile line that carries -ldflags itself,
because that is what makes two.

internal/builder/system_stamp_test.go

@@ -47,3 +47,30 @@ func TestTheStampIsTheOneThingTakenFromTheArtifact(t *testing.T) {
 		t.Fatalf("the compile line takes something from the module: %q", joined)
 	}
 }
+
+func TestTheLinkerIsToldOnceNotTwice(t *testing.T) {
+	// A repeated flag is not a merged one: the Go command takes the last -ldflags and drops the
+	// first. Passing the toolchain's flags and then the stamp separately produced a binary that knew
+	// its system and had lost -s -w — 12.2MB against 8.5MB, with its debug info (04-ISSUES/161).
+	chain, err := ToolchainFor("go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, arg := range chain.Compile {
+		if arg == "-ldflags" {
+			t.Fatal("the compile line carries -ldflags, so composing one here makes two")
+		}
+	}
+	if len(chain.LinkerFlags) == 0 {
+		t.Fatal("the go toolchain passes no linker flags, so the binary keeps its debug info")
+	}
+	var stripped bool
+	for _, f := range chain.LinkerFlags {
+		if f == "-s" {
+			stripped = true
+		}
+	}
+	if !stripped {
+		t.Fatalf("the go toolchain does not strip: %v", chain.LinkerFlags)
+	}
+}