cmd/cue: file embedding not working inputs are CUE files

[jpluscplusm]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.12.0

go version go1.23.5
      -buildmode exe
       -compiler gc
  DefaultGODEBUG asynctimerchan=1,gotypesalias=0,httpservecontentkeepheaders=1,tls3des=1,tlskyber=0,x509keypairleaf=0,x509negativeserial=1
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
cue.lang.version v0.12.0

Does this issue reproduce with the latest stable release?

v0.12.0 is latest.

What did you do?

# Evaluate a CUE package that uses file embedding
exec cue export .:p
cmp stdout stdout.golden

# Evaluate a CUE file that uses file embedding.
exec cue export file.cue
cmp stdout stdout.golden
-- cue.mod/module.cue --
module: "mod.example"
language: version: "v0.10.0"
-- file.cue --
@extern(embed)

package p

files: _ @embed(file=some.txt)
-- some.txt --
hello
-- stdout.golden --
{
    "files": "hello\n"
}

What did you expect to see?

A passing test.

What did you see instead?

# Evaluate a CUE package that uses file embedding (0.012s)
# Evaluate a CUE file that uses file embedding. (0.011s)
> exec cue export file.cue
[stderr]
cannot embed files when not in a module
[exit status 1]
FAIL: /tmp/testscript2371849007/repro.txtar/script.txtar:6: unexpected command failure
error running repro.txtar in /tmp/testscript2371849007/repro.txtar

Assuming that embedding files is permitted when the containing CUE file is referred to directly, instead of via an import path. I scanned through the file embedding proposal and didn't notice anything obvious that would disqualify the failing cue invocation.

Alternatively, if the direct file invocation method is forbidden, I would expect a better error message that didn't refer to being "not in a module", despite definitely having invoked cue whilst "inside a module".

[myitcv]

The expectation that this should work with regular CUE files is correct to my mind. I've updated the repro and description slightly to reflect that.

[mvdan]

agreed to the point I raised a dupe! https://github.com/cue-lang/cue/issues/4141

[mvdan]

I'm going to slightly prioritize this, because embedding is a rather useful feature, so users keep running into this - including myself.