tools/fix: correct the ast walker so sub-expressions get fixed

Originally, the ast walker in fix was using the "before" function. But
this would mean recursion would stop as soon as it does any replacement.
This means it wouldn't work for nested expressions.

By simply switching to the "after" function, we ensure we don't do any
replacement until we've bottomed out in the AST.

Signed-off-by: Matthew Sackman <[email protected]>
Change-Id: I5372b319429f48c222b2a8b4a6507873f19250c1
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1200495
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: cuematthew

cmd/cue/cmd/testdata/script/fix.txtar

@@ -2,8 +2,7 @@ exec cue fix ./...
 # Make sure we fix all files in a directory, even if they're a mix of packages (or no packages).
 cmp p/one.cue p/one.cue.fixed
 cmp p/two.cue p/two.cue.fixed
-# TODO: the following cmp fails because of a bug in fix/fix.go
-! cmp p/three.cue p/three.cue.fixed
+cmp p/three.cue p/three.cue.fixed
 # TODO: the added imports have unnecessary name-mangling. https://cuelang.org/issue/3408
 -- p/one.cue --
 package one

tools/fix/fix.go

@@ -44,7 +44,10 @@ func File(f *ast.File, o ...Option) *ast.File {
 		f(&options)
 	}
 
-	f = astutil.Apply(f, func(c astutil.Cursor) bool {
+	// Make sure we use the "after" function, and not the "before",
+	// because "before" will stop recursion early which creates
+	// problems with nested expressions.
+	f = astutil.Apply(f, nil, func(c astutil.Cursor) bool {
 		n := c.Node()
 		switch n := n.(type) {
 		case *ast.BinaryExpr:
@@ -94,7 +97,7 @@ func File(f *ast.File, o ...Option) *ast.File {
 			}
 		}
 		return true
-	}, nil).(*ast.File)
+	}).(*ast.File)
 
 	if options.simplify {
 		f = simplify(f)