scriggo: add UnexpandedTransformer and rename TreeTransformer

[gazerro]

scriggo: add `UnexpandedTransformer` and rename `TreeTransformer`

Introduce a new build option for templates that allows applying a
transformation to the AST of each parsed file before expansion.

This is particularly useful when changes need to be applied prior to
expansion, such as rewriting paths.

As part of this change, 'BuildOptions.TreeTransformer' has been renamed
to ExpandedTransformer to better reflect when it is executed.

[zapateo]

I don't quite get the verb tense in this documentation. Shouldn't it be something like "it is invoked..."?

[gazerro]

Changed in commit 931249c.

[zapateo]

I don't quite understand this code. This particular line performs a type assertion but silently fails if the assertion fails. Is this intentional? Wouldn't it be better to fail?

[gazerro]

Yes, this is intentional and the code is correct. It relies on the value assigned to render, which is nil when the node is not a Render. In other words, the logic is effectively saying: "as long as render is nil, keep traversing".

Alternatively, it could be written more explicitly like this:

astutil.Inspect(tree, func(node ast.Node) bool {
    if render != nil {
        return false
    }
    var ok bool
    render, ok = node.(*ast.Render)
    return !ok
}

However, I'm not convinced this version is actually clearer than the current implementation.