internal/cmd/cue-ast: 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 <[email protected]>
Change-Id: Ibf89de45ae6ec5d88037f7417d00aa2129cd73e0
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1200356
Reviewed-by: Roger Peppe <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

internal/cmd/cue-ast/main.go

@@ -22,8 +22,12 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"slices"
 
+	"cuelang.org/go/cue/ast"
+	"cuelang.org/go/cue/ast/astutil"
 	"cuelang.org/go/cue/errors"
+	"cuelang.org/go/cue/format"
 	"cuelang.org/go/cue/load"
 	"cuelang.org/go/internal/astinternal"
 )
@@ -39,6 +43,11 @@ usage of cue-ast:
 
       -omitempty    omit empty and invalid values
 
+  cue-ast join [flags] [inputs]
+
+    Join the input package instance as a single file.
+    Joining multiple package instances is not supported yet.
+
 See 'cue help inputs' as well.
 `[1:])
 	}
@@ -71,6 +80,42 @@ 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)}
+
+		// Sanitize the resulting file so that, for example,
+		// multiple packages imported as the same name avoid collisions.
+		if err := astutil.Sanitize(joint); err != nil {
+			log.Fatal(err)
+		}
+
+		out, err := format.Node(joint)
+		if err != nil {
+			log.Fatal(err)
+		}
+		os.Stdout.Write(out)
 	default:
 		flag.Usage()
 	}