Add start/end params to str.startswith/endswith, allow named default in dict.pop

[twitchax]

Summary

This PR makes three Bazel-compatible improvements to Starlark built-in methods:

1. str.startswith(prefix[, start[, end]]) — Add optional start and end parameters, matching the Starlark spec and Python semantics. The substring s[start:end] is tested instead of the full string.

2. str.endswith(suffix[, start[, end]]) — Same as above for endswith().

3. dict.pop(key[, default]) — Allow default to be passed as a named keyword argument (e.g., d.pop("key", default=None)), not just positional. This matches Bazel's Java Starlark implementation and Python's dict.pop().

Motivation

These changes are needed for Bazel compatibility. Real-world Bazel BUILD files and .bzl macros use these features extensively:

# startswith with start/end (common in monorepo BUILD files)
[b for b in GLOBAL_BINARIES if b.startswith("//", 0, 2)]

# dict.pop with named default (common in rule macros)  
workdir = kwargs.pop("workdir", default = None)

Related Issues

• bazelbuild/starlark#56 — The Starlark spec was updated to include start/end params for startswith/endswith, but starlark-rust never implemented them.
• Conformance test suite inconsistency — The bazelbuild/starlark conformance tests document this with: # _inconsistency_: rust fails when default=None

Implementation Details

startswith / endswith

Reuses the existing convert_str_indices infrastructure (already used by find(), count(), index(), rfind(), rindex()) to slice the string before testing. Supports both single string and tuple-of-strings prefix/suffix arguments.

dict.pop

Removes #[starlark(require = pos)] from the default parameter, allowing it to be passed as either positional or named. The key parameter remains positional-only.

Testing

• All 981 existing tests pass (828 lib + 153 doc)
• New tests added for each change:
  • test_startswith_with_start_end — 7 assertions covering basic, negative indices, and tuple prefix
  • test_endswith_with_start_end — 4 assertions covering basic, negative indices, and tuple suffix
  • test_dict_pop_default_named — 3 assertions covering missing key, existing key, and fallback value

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[twitchax]

Full disclosure: identified and implemented mostly with LLM. Hope that's OK. Happy to make artisanal edits.

[Copilot]

Pull request overview

This PR adds Bazel-compatible enhancements to Starlark string and dict methods by implementing optional start/end parameters for str.startswith() and str.endswith(), and allowing dict.pop() to accept default as a named keyword argument. These changes align starlark-rust with the official Starlark specification and Bazel's Java implementation.

Changes:

• Add optional start and end parameters to str.startswith() and str.endswith() methods, enabling substring range checking
• Allow dict.pop() to accept default as a named keyword argument (e.g., default=None) in addition to positional argument
• Add comprehensive test coverage for all three enhancements

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
starlark/src/values/types/string/methods.rs	Adds start and end parameters to startswith() and endswith() methods using convert_str_indices() infrastructure; updates documentation with new examples; adds test cases for new functionality
starlark/src/values/types/dict/methods.rs	Removes require = pos constraint from dict.pop() default parameter to allow named keyword usage; adds test cases verifying named parameter functionality

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[meta-codesync]

@cjhopman has imported this pull request. If you are a Meta employee, you can view this in D102404513. (Because this pull request was imported automatically, there will not be any future comments.)