cue: allow omitting leading "#" for LookupDef

This is mostly to smooth the transition to #-style definitions.

Case in point: the internal filetypes package was upgraded and
to new-style definitions (included in this CL). The conversion was
automatic, but the code now requires a leading `#` prefix for
definitions. This change allows using `cue fix` without breaking
code.

Note that now a field name can never be a valid definition name
if it does not start with `#`.

Change-Id: Id69a9601e8d244c27a0855f454b730ce9baf6b9f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6240
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/types.go

@@ -1524,6 +1524,13 @@ func (v Value) LookupDef(name string) Value {
 			return newChildValue(&o, i)
 		}
 	}
+	if !strings.HasPrefix(name, "#") {
+		alt := v.LookupDef("#" + name)
+		// Use the original error message if this resulted in an error as well.
+		if alt.Err() == nil {
+			return alt
+		}
+	}
 	return newErrValue(v, ctx.mkErr(v.path.v,
 		"defintion %q not found", name))
 }

internal/filetypes/filetypes_test.go

@@ -42,7 +42,7 @@ func TestFromFile(t *testing.T) {
 	}{{
 		name: "must specify encoding",
 		in:   build.File{},
-		out:  `FileInfo.encoding: non-concrete value string`,
+		out:  `#FileInfo.encoding: non-concrete value string`,
 	}, {
 		// Default without any
 		name: "cue",

internal/filetypes/types.cue

@@ -23,23 +23,23 @@ package build
 // There
 
 // A File corresponds to a Go build.File.
-File :: {
+#File: {
 	filename:        string
-	encoding:        Encoding
-	interpretation?: Interpretation
-	form?:           Form
+	encoding:        #Encoding
+	interpretation?: #Interpretation
+	form?:           #Form
 	tags?: {[string]: string}
 }
 
 // Default is the file used for stdin and stdout. The settings depend
 // on the file mode.
-Default :: File & {
+#Default: #File & {
 	filename: *"-" | string
 }
 
 // A FileInfo defines how a file is encoded and interpreted.
-FileInfo :: {
-	File
+#FileInfo: {
+	#File
 
 	// For each of these fields it is explained what a true value means
 	// for encoding/decoding.
@@ -68,12 +68,11 @@ modes: _
 // In input mode, settings flags are interpreted as what is allowed to occur
 // in the input. The default settings, therefore, tend to be permissive.
 modes: input: {
-	Default :: {
+	#Default: {
 		encoding: *"cue" | _
 		...
 	}
-
-	FileInfo :: x, x = {
+	#FileInfo: x, let x = {
 		docs:       *true | false
 		attributes: *true | false
 	}
@@ -86,12 +85,11 @@ modes: input: {
 }
 
 modes: export: {
-	Default :: {
+	#Default: {
 		encoding: *"json" | _
 		...
 	}
-
-	FileInfo :: x, x = {
+	#FileInfo: x, let x = {
 		docs:       true | *false
 		attributes: true | *false
 	}
@@ -101,7 +99,7 @@ modes: export: {
 }
 
 modes: ouptut: {
-	FileInfo :: x, x = {
+	#FileInfo: x, let x = {
 		docs:       true | *false
 		attributes: true | *false
 	}
@@ -112,11 +110,11 @@ modes: ouptut: {
 
 // eval is a legacy mode
 modes: eval: {
-	Default :: {
+	#Default: {
 		encoding: *"cue" | _
 		...
 	}
-	FileInfo :: x, x = {
+	#FileInfo: x, let x = {
 		docs:       true | *false
 		attributes: true | *false
 	}
@@ -126,12 +124,11 @@ modes: eval: {
 }
 
 modes: def: {
-	Default :: {
+	#Default: {
 		encoding: *"cue" | _
 		...
 	}
-
-	FileInfo :: x, x = {
+	#FileInfo: x, let x = {
 		docs:       *true | false
 		attributes: *true | false
 	}
@@ -159,16 +156,15 @@ extensions: {
 }
 
 // A Encoding indicates a file format for representing a program.
-Encoding :: !="" // | error("no encoding specified")
+#Encoding: !="" // | error("no encoding specified")
 
 // An Interpretation determines how a certain program should be interpreted.
 // For instance, data may be interpreted as describing a schema, which itself
 // can be converted to a CUE schema.
-Interpretation :: string
-
-Form :: string
+#Interpretation: string
+#Form:           string
 
-file: FileInfo & {
+file: #FileInfo & {
 
 	filename: "foo.json"
 	form:     "schema"
@@ -219,7 +215,7 @@ tags: {
 }
 
 // forms defines schema for all forms. It does not include the form ID.
-forms: [Name=string]: FileInfo
+forms: [Name=string]: #FileInfo
 
 forms: "": _
 
@@ -327,7 +323,7 @@ encodings: code: {
 	stream: false
 }
 
-interpretations: [Name=string]: FileInfo
+interpretations: [Name=string]: #FileInfo
 
 interpretations: "": _