Walk support deleting Node

[sluongng]

I have a use case where in Golang, we often use Gazelle to generate a bunch of go_repository target in a separate starlark file under a macro. Example:

# deps.bzl

def repo_deps():
    go_repository(
        name = "cc_mvdan_interfacer",
        build_external = "external",
        importpath = "mvdan.cc/interfacer",
        sum = "h1:WX1yoOaKQfddO/mLzdV4wptyWgoH/6hwLs7QHTixo0I=",
        version = "v0.0.0-20180901003855-c20040233aed",
    )
    go_repository(
        name = "cc_mvdan_lint",
        build_external = "external",
        importpath = "mvdan.cc/lint",
        sum = "h1:DxJ5nJdkhDlLok9K6qO+5290kphDJbHOQO1DFFFTeBo=",
        version = "v0.0.0-20170908181259-adc824a0674b",
    )

or a more advance use case:

# deps.bzl

def repo_deps(enable_feature = True):
    if enable_feature:
        go_repository(
            name = "cc_mvdan_interfacer",
            build_external = "external",
            importpath = "mvdan.cc/interfacer",
            sum = "h1:WX1yoOaKQfddO/mLzdV4wptyWgoH/6hwLs7QHTixo0I=",
            version = "v0.0.0-20180901003855-c20040233aed",
        )
    go_repository(
        name = "cc_mvdan_lint",
        build_external = "external",
        importpath = "mvdan.cc/lint",
        sum = "h1:DxJ5nJdkhDlLok9K6qO+5290kphDJbHOQO1DFFFTeBo=",
        version = "v0.0.0-20170908181259-adc824a0674b",
    )

I want to write a program where I would parse up these deps.bzl file and programatically delete rules under a certain condition: i.e. comparing known name or comparing known version etc...

The problem is that the current API https://pkg.go.dev/github.com/bazelbuild/buildtools@v0.0.0-20220531122519-a43aed7014c8/build#File.DelRules does not support deleting rules inside macro, but assumed that all rules are loaded on the top level of the AST.

I tried tinkering with the Walk function families but none of which seems to allow me to delete a node by setting the Expr pointer to nil?

It seems like the only way to delete a node is to update it's parent node to exclude it from the []Expr body/stmt slice. But as shown in the advance case above, the parent node could be almost anything: a DefStmt, an IfStmt, ForStmt etc... So ideally it would be awesome if Walk api would allow you to set pointer of the node to nil to delete it.

---

Overall, it would be nice if we can have a way to query and edit Starlark AST similar to how tree-sitter does it today. 🙏

[sluongng]

https://github.com/sluongng/nogo-analyzer/blob/f2aa5a39d502b23cc573e98f8ebeba1489c9e029/private/cmd/deps-prune/main.go#L57-L84

Here is my current implementation, which only support the case where go_repository target was defined within defStmt and does not handle any other cases.