Fix ICE in const param defaults with lifetime-dependent types

[lapla-cogito]

fixes#142913

The ICE happened because const_param_default instantiated the default expression using the const parameter’s own identity arguments, which are empty. When the default’s type referenced earlier generics, the subsequent instantiate call saw parameters but no arguments and panicked. In this PR, supplying the slice of the parent’s generic arguments up to the parameter being defaulted prevents that mismatch.

[rustbot]

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[BoxyUwU]

r? BoxyUwU

[BoxyUwU]

In theory anonymous constants aren't supposed to work with generic_const_parameter_types because they aren't allowed to make use of generic parameters so can't actually construct a value of a type that references generics. Feeding a type of &'a Foo to the default would kinda be wrong 🤔

I think in lower_const_arg in ty lowering we try to detect feeding a type referencing generic parameters for an anon const and if so we error. I'm not sure why that's not happening. I would expect fixing whatever's going wrong there to also fix this ICE. Could you take a look into that for me?

[BoxyUwU]

@rustbot author

[rustbot]

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[rust-bors]

☔ The latest upstream changes (presumably #150798) made this pull request unmergeable. Please resolve the merge conflicts.

[BoxyUwU]

hiya, sorry this fix wound up being merged as part of a different PR. I still think it would be good to look into what I mentioned above though. Going to close this for now but if you're interested in taking a look into that, that'd be great