cmd/cue: go get Go fields with the json omitempty tag as optional

"omitzero" was added in Go 1.24 as an improvement over "omitempty",
which works in a more intuitive way with structs and arrays,
and supports calling an IsZero method for custom implementations
such as time.Time.

Fixes #3869.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I9b4c58544a86cf961434d581012aafd0fd91a2c6
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1213251
Reviewed-by: Roger Peppe <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: mvdan

Diff for: cmd/cue/cmd/get_go.go

@@ -1474,8 +1474,11 @@ func (e *extractor) isOptional(f *types.Var, doc *ast.CommentGroup, tag string)
 		}
 	}
 
+	// Go 1.24 added the "omitzero" option to encoding/json, an improvement over "omitempty".
+	// Note that, as of mid 2025, YAML libraries don't seem to have picked up "omitzero" yet.
 	// TODO: also when the type is a list or other kind of pointer.
 	return hasFlag(tag, "json", "omitempty", 1) ||
+		hasFlag(tag, "json", "omitzero", 1) ||
 		hasFlag(tag, "yaml", "omitempty", 1)
 }
 

Diff for: cmd/cue/cmd/testdata/script/get_go_types.txtar

@@ -331,7 +331,7 @@ import (
 		CustomJSON: #CustomJSON
 	} @go(,struct{CustomJSON})
 	optionalOmitEmptyJSON?: string    @go(OptionalOmitEmptyJSON)
-	optionalOmitZeroJSON:   time.Time @go(OptionalOmitZeroJSON)
+	optionalOmitZeroJSON?:  time.Time @go(OptionalOmitZeroJSON)
 	optionalOmitEmptyYAML?: string    @go(OptionalOmitEmptyYAML)
 
 	// +optional