internal/cmd/cue-ast-tool: add "join" command

When reducing CUE bug reproducers, I often begin from the evaluation
of a CUE package instance. It is easier to reduce a single CUE file
rather than multiple files, as one can load it into an editor
and quickly delete chunks of it or search through it.

While this is not hard to do, it's tedious for a human;
the package declarations need to be removed, and the imports joined
at the top of the new file. This is, however, easy to do in Go.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ibf89de45ae6ec5d88037f7417d00aa2129cd73e0
Dispatch-Trailer: {"type":"trybot","CL":1200356,"patchset":2,"ref":"refs/changes/56/1200356/2","targetBranch":"master"}

internal/cmd/cue-ast-tool/main.go

@@ -22,8 +22,11 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"slices"
 
+	"cuelang.org/go/cue/ast"
 	"cuelang.org/go/cue/errors"
+	"cuelang.org/go/cue/format"
 	"cuelang.org/go/cue/load"
 	"cuelang.org/go/internal/astinternal"
 )
@@ -39,6 +42,10 @@ usage of cue-ast-tool:
 
       -omitempty    omit empty and invalid values
 
+  cue-ast-tool join [flags] [inputs]
+
+    Join the input as a single file.
+
 See 'cue help inputs' as well.
 `[1:])
 	}
@@ -71,6 +78,36 @@ See 'cue help inputs' as well.
 				os.Stdout.Write(out)
 			}
 		}
+	case "join":
+		// TODO: add a flag drop comments, which is useful when reducing bug reproducers.
+		flag.CommandLine.Parse(args)
+
+		var jointImports []*ast.ImportSpec
+		var jointFields []ast.Decl
+		insts := load.Instances(flag.Args(), &load.Config{})
+		if len(insts) != 1 {
+			log.Fatal("joining multiple instances is not possible yet")
+		}
+		inst := insts[0]
+		if err := inst.Err; err != nil {
+			log.Fatal(errors.Details(err, nil))
+		}
+		for _, file := range inst.Files {
+			jointImports = slices.Concat(jointImports, file.Imports)
+
+			fields := file.Decls[len(file.Preamble()):]
+			jointFields = slices.Concat(jointFields, fields)
+		}
+		// TODO: we should sort and deduplicate imports.
+		joint := &ast.File{Decls: slices.Concat([]ast.Decl{
+			&ast.ImportDecl{Specs: jointImports},
+		}, jointFields)}
+
+		out, err := format.Node(joint)
+		if err != nil {
+			log.Fatal(err)
+		}
+		os.Stdout.Write(out)
 	default:
 		flag.Usage()
 	}