encoding/yaml: Panic when unmarshalling YAML content that is only comments

[robharrop]

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

$ cue version
cue version v0.4.3 darwin/arm64

Also tested on cue version v0.4.4-0.20220706163258-2795dcc4db87

Does this issue reproduce with the latest release?

Yes.

What did you do?

Attempt to unmarshal YAML content that is just a comment using encoding/yaml.

Given test.cue with:

import "encoding/yaml"

out: yaml.Unmarshal("#test")

Running cue eval test.cue

What did you expect to see?

Certainly not a panic. Likely an empty output or bottom?

What did you see instead?

A panic:

panic: assertion failed: nil return value: unspecified error [recovered]
	panic: assertion failed: nil return value: unspecified error

goroutine 1 [running]:
cuelang.org/go/cmd/cue/cmd.recoverError(0x140005bfe70)
	cuelang.org/go/cmd/cue/cmd/root.go:229 +0x98
panic({0x1035261c0, 0x1400071c860})
	runtime/panic.go:838 +0x204
cuelang.org/go/internal/core/adt.Assertf(...)
	cuelang.org/go/internal/core/adt/context.go:93
cuelang.org/go/internal/core/adt.(*OpContext).evaluateRec(0x14000719ad0, 0x1400011f7c0, {0x103610a68, 0x14000113140}, 0x78?)
	cuelang.org/go/internal/core/adt/context.go:582 +0x280
...

[critterjohnson]

This actually applies to not just comments, but any time yaml.Unmarshal() is called on anything that returns no actual value (like an empty string).