Add a new attr indicates pub struct with private fields could be contructed externally

[mu001999]

Fixes #126169

[rustbot]

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[compiler-errors]

#126169 (comment) notes #126289. Please add a test and/or support for that.

[mu001999]

@compiler-errors updated the test

[compiler-errors]

Also, this doesn't follow the usual process for adding new attributes to the compiler.

This either needs to be FCP'd by T-lang as an insta-stable attribute, or it needs to be feature gated appropriately.

[compiler-errors]

@rustbot author

Please add feature gating. I also wonder if we should bikeshed this a bit first before adding it, like on i-rl-o.

[mu001999]

@rustbot ready

[mu001999]

I also wonder if we should bikeshed this a bit first before adding it, like on i-rl-o.

@compiler-errors Sorry for the delay, I opened one on i-rl-o ;).

link: https://internals.rust-lang.org/t/externally-constructed-or-anything-else/21049

[Soveu]

Is this a good place for this check? In function check_item, around line 838 you have a variable may_construct_self and in visit_node there is unconditionally_treats_fields_as_live

[mu001999]

Oh, seems checking it in create_and_seed_worklist is good enough. And I opened another PR which just checks the repr(c) or repr(transparent), see #127104.

[compiler-errors]

I believe the other approach to this is #127104? If so, I'm gonna close this.