internal/cuetxtar: do not load the root cuelang.org/go CUE module

When loading cuetxtar files, such as cue/testdata/eval/let.txtar,
we want to use cue/load to load the list of CUE files in the txtar.
cue/load will attempt to walk the chain of parent directories to find
a valid CUE module, and since the vast majority of these txtar files
contain only CUE files, we'd load cuelang.org/go repeatedly.

This didn't cause any failures or issues per se, but reading parent
directories works against Go's per-package test caching,
and loading the root cue.mod/module.cue over and over again is wasteful.

As measured by benchcmd and benchstat, adt.TestEvalV3 sees a moderate
speed-up, as every subtest begins with a cue/load call via cuetxtar.

               │     old      │                 new                 │
               │    sec/op    │   sec/op     vs base                │
    TestEvalV3   544.4m ± 10%   452.1m ± 6%  -16.94% (p=0.000 n=12)

               │      old      │                 new                 │
               │  user-sec/op  │ user-sec/op  vs base                │
    TestEvalV3   1007.7m ± 12%   753.5m ± 5%  -25.23% (p=0.000 n=12)

               │     old     │              new               │
               │ sys-sec/op  │  sys-sec/op   vs base          │
    TestEvalV3   45.15m ± 9%   52.16m ± 19%  ~ (p=0.266 n=12)

               │      old       │                  new                   │
               │ peak-RSS-bytes │ peak-RSS-bytes  vs base                │
    TestEvalV3     80.03Mi ± 2%     36.43Mi ± 2%  -54.47% (p=0.000 n=12)

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Ife342fd0b4dd09f350aace8b668c2def7725c453
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1218102
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mvdan

internal/cuetxtar/txtar.go

@@ -329,6 +329,9 @@ func loadWithConfig(a *txtar.Archive, dir string, cfg load.Config, args ...strin
 		overlay[filepath.Join(dir, f.Name)] = load.FromBytes(f.Data)
 	}
 
+	// Don't walk the chain of parent directories; this busts Go's test cache,
+	// and also leads to loading the root module "cuelang.org/go" repeatedly for no reason.
+	cfg.ModuleRoot = "."
 	cfg.Dir = dir
 	cfg.Overlay = overlay