feat: add #[stageleft::export(Foo, Bar)] to pub-export macro-generated items

[MingweiSamuel]

No description provided.

[MingweiSamuel]

I guess this macro can also apply to other items (not just macros) e.g. a struct macro that has some attributes which generate a newtype (like buildstructor perhaps), ill add those code paths too maybe

Edit: actually I will punt this to later. see branch #50 https://github.com/hydro-project/stageleft/tree/stageleft-export-macro-2

[Copilot]

Pull request overview

This PR adds a new #[stageleft::export(...)] attribute macro that allows marking macro invocations that generate one or more pub items. This prevents "splitbrain" issues where macro-generated types are re-invoked in staged code, causing type mismatches between the original and staged versions.

Changes:

• Adds #[stageleft::export(...)] attribute macro to mark macro invocations that generate public items
• Implements build-time code generation to replace marked macros with re-exports in staged code
• Adds a new test package stageleft_test_no_entry to verify the functionality
• Includes compile-fail test to ensure exported items must be public

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
stageleft_macro/src/lib.rs	Implements the new export attribute macro that validates and marks macro-generated items
stageleft_tool/src/lib.rs	Adds logic to detect stageleft::export attributes and replace marked macros with re-exports in staged code
stageleft/src/lib.rs	Exports the new export macro to make it publicly available
stageleft_test_no_entry/*	New test package demonstrating the export feature with slotmap's new_key_type! macro
stageleft/tests/compile-fail/*	Compile-fail test ensuring exported items must be pub
Cargo.toml, Cargo.lock, stageleft/Cargo.toml	Adds new test package and slotmap dependency

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