internal/filetypes: avoid struct-valued defaults

Various places in `internal/filetypes/types.cue` use a pattern like
this:

	encodings: cue: {
		*forms.schema | _
	}

where the `*forms.schema` value is a struct. This seems unlikely to be
what's intended: it means that if a single value inside `encoding.cue`
is inconsistent with `forms.schema`, all the rest of the fields in
`forms.schema` will be discarded, leaving no defaults at all.

There are four places in the code that this pattern is used. As it happens,
none of the attributes which are set by the struct-valued
default can possibly conflict with anything (all values are set to false
and there is nowhere that sets a non-default true value),
so this change should not change observable behaviour in any way.

Also remove a redundant field definition inside `forms.schema`.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I47e1a78ae4ac868966099bf85c0e5731aa7b78b6
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1200763
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

internal/filetypes/types.cue

@@ -104,9 +104,7 @@ modes: export: {
 		docs:       true | *false
 		attributes: true | *false
 	}
-	encodings: cue: {
-		*forms.data | _
-	}
+	encodings: cue: forms.data
 }
 
 // TODO(mvdan): this "output" mode appears to be unused at the moment.
@@ -118,9 +116,7 @@ modes: output: {
 		docs:       true | *false
 		attributes: true | *false
 	}
-	encodings: cue: {
-		*forms.data | _
-	}
+	encodings: cue: forms.data
 }
 
 // eval is a legacy mode
@@ -132,9 +128,7 @@ modes: eval: {
 		docs:       true | *false
 		attributes: true | *false
 	}
-	encodings: cue: {
-		*forms.final | _
-	}
+	encodings: cue: forms.final
 }
 
 modes: def: {
@@ -145,9 +139,7 @@ modes: def: {
 		docs:       *true | false
 		attributes: *true | false
 	}
-	encodings: cue: {
-		*forms.schema | _
-	}
+	encodings: cue: forms.schema
 }
 
 // A Encoding indicates a file format for representing a program.
@@ -266,7 +258,6 @@ forms: schema: {
 	constraints:  *true | false
 	keepDefaults: *true | false
 	imports:      *true | false
-	optional:     *true | false
 }
 
 forms: final: {