Go: in a go.work workspace or with a relative replace, workers cannot build, so every mutant counts as KILLED #27

What happens

In a Go module that is part of a go.work workspace, or that points at a sibling module with a relative replace directory, no mutant can build inside its worker. Every mutant fails with [setup failed] and counts as KILLED, so the package scores 100% even though no test ran (the symptom in #18, with a different cause, like #15).

Repro

mutator a5a0cb8, crapper 6fd92c4, Go 1.27.3, Linux.

repo/
  go.work         use ./lib ./svc
  lib/go.mod      module example.com/acme/lib
  lib/lib.go      package lib; func Zero() int { return 0 }
  svc/go.mod      module example.com/acme/svc
                  require example.com/acme/lib v0.0.0
                  replace example.com/acme/lib => ../lib
  svc/calc/calc.go
  svc/calc/calc_test.go

svc/calc/calc.go:

package calc

import "example.com/acme/lib"

func Max(a, b int) int {
	if a > b {
		return a
	}
	return b + lib.Zero()
}

svc/calc/calc_test.go:

package calc

import "testing"

func TestMax(t *testing.T) {
	if Max(2, 1) != 2 || Max(1, 2) != 2 {
		t.Fatal("wrong max")
	}
}

Both mutation sites are equivalent mutants (>= gives the same Max, and b - 0 == b + 0), so both should survive. go test ./calc passes in svc/.

$ cd svc && mutator --no-coverage --mutate-all calc/calc.go
KILLED    calc/calc.go:6 > -> >=
KILLED    calc/calc.go:9 + -> -
Total     2   0   0   2  100.0%

$ GOWORK=off mutator --no-coverage --mutate-all calc/calc.go
Total     2   0   0   2  100.0%

Control: the same function in a single module with no workspace and no replace reports both sites SURVIVED.

Cause, as far as I can tell

The worker is a directory under target/mutation-workers/ with the module's files linked into it. Running go test there, by hand in an equivalent directory:

• With the workspace active, Go resolves the worker to the original module, whose directory does not contain the worker path:

  main module (example.com/acme/svc) does not contain package example.com/acme/svc/target/probe/calc
  FAIL	./calc [setup failed]
  

• With GOWORK=off, the relative replace is resolved from the worker directory, where ../lib does not exist:

  calc/calc.go:3:8: example.com/acme/lib@v0.0.0: replacement directory ../lib does not exist
  FAIL	example.com/acme/svc/calc [setup failed]
  

Why it matters

Monorepos commonly use go.work, and modules often keep relative replace lines so they also build without the workspace. On those layouts, mutator reports 100% for every package and exits 0.

Possible fix

• Give the worker its own workspace: write a go.work into the worker that uses the worker directory plus the other workspace modules by absolute path, and set GOWORK to it.
• For relative replace directories, run with -modfile pointing at a copy of go.mod whose replace paths are made absolute.
• Independently of this, Mutants that do not compile count as KILLED, which inflates the score (Go: + on strings) #18's suggestion (report [setup failed] and [build failed] as their own outcome, left out of the score) would turn this from a silent 100% into a visible error.