seatgen: a string's enum is one Go constant per value

A caller names a value the seat definition gives, such as the removal
reason retention, and never writes it by hand.

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

modules/mesh-build-go/buildstep/seatgen/gogen.go

@@ -496,6 +496,7 @@ func goTypes(b *bytes.Buffer, tp *typer, also map[string]*typ) error {
 		fmt.Fprintf(b, "// %s %s\n", t.name, oneLine(desc))
 		if t.t.kind != "struct" {
 			fmt.Fprintf(b, "type %s = %s\n\n", t.name, goType(t.t, kinds, true))
+			goEnum(b, t.name, t.name, t.t)
 			continue
 		}
 		fmt.Fprintf(b, "type %s struct {\n", t.name)
@@ -510,10 +511,26 @@ func goTypes(b *bytes.Buffer, tp *typer, also map[string]*typ) error {
 			fmt.Fprintf(b, "\t%s %s `json:%q`\n", ident(f.json), goType(f.t, kinds, f.required), tag)
 		}
 		b.WriteString("}\n\n")
+		for _, f := range t.t.fields {
+			goEnum(b, t.name+"'s "+f.json, t.name+ident(f.json), f.t)
+		}
 	}
 	return nil
 }
 
+// goEnum writes a string's enum as one constant per value, named prefix and the value (RemoveInputReasonRetention), so a
+// caller names a value the definition gives and never writes it by hand.
+func goEnum(b *bytes.Buffer, what, prefix string, t *typ) {
+	if t.kind != "string" || len(t.enum) == 0 {
+		return
+	}
+	fmt.Fprintf(b, "// The values of %s.\nconst (\n", what)
+	for _, v := range t.enum {
+		fmt.Fprintf(b, "\t%s%s = %q\n", prefix, ident(v), v)
+	}
+	b.WriteString(")\n\n")
+}
+
 func goType(t *typ, kinds map[string]*typ, required bool) string {
 	switch t.kind {
 	case "string":

modules/mesh-build-go/buildstep/seatgen/layout_test.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"go/parser"
 	"go/token"
+	re "regexp"
 	"strings"
 	"testing"
 
@@ -71,6 +72,10 @@ func TestEachSeatDefinedWholeIsAGoModuleOfItsOwn(t *testing.T) {
 		if !strings.Contains(src, "type Ref struct") || !strings.Contains(src, "type PublishedEvent") {
 			t.Errorf("%s does not hold the part's types whole", seat)
 		}
+		// A string's enum is a constant per value, so a caller names the definition's value (A3t: the removal reason).
+		if !re.MustCompile(`RefReasonRetention\s+= "retention"`).MatchString(src) || !re.MustCompile(`RefReasonTagGone\s+= "tag-gone"`).MatchString(src) {
+			t.Errorf("%s does not name the part's enum values:\n%s", seat, src)
+		}
 	}
 	if !strings.Contains(string(files["go/seats/seats.go"]), "OneStore") {
 		t.Error("the seat set does not name the seat at the seat list's grain")

modules/mesh-build-go/buildstep/seatgen/testdata/contracts/parts/registry.json

@@ -1,3 +1,3 @@
 {"format":1,"part":"registry","verbs":[{"name":"publish","input":{"$ref":"#/defs/Ref"},"answer":{"type":"object"}}],
  "events":[{"name":"published","body":{"$ref":"#/defs/Ref"}}],
- "defs":{"Ref":{"type":"object","properties":{"digest":{"type":"string"}},"required":["digest"]}}}
+ "defs":{"Ref":{"type":"object","properties":{"digest":{"type":"string"},"reason":{"type":"string","enum":["tag-gone","retention"]}},"required":["digest"]}}}