fix: Add recursion limit when hovering on const values

[Wilfred]

This fixes #21503

AI disclosure: I minimised the crash manually and wrote the unit test, then asked Claude to write a fix. I've reviewed all the changes and they seem reasonable.

I'm in two minds about this solution: the sample code shouldn't produce an infinite recursion as far as I can see, so maybe there's a better fix. However, I also suspect it'd be possible to write a genuine infinite recursion const function, in which case you wouldn't want r-a to stack overflow even though the code wouldn't compile.

Let me know what you think.

[ChayimFriedman2]

I don't think it can be possible to create an infinite recursion. When evaluating yes, but not when rendering.

[Wilfred]

@ChayimFriedman2 It shouldn't be possible, but it definitely is today unfortunately -- see the test added.

Perhaps the type parameters aren't being substituted correctly? It looks like rendering is infinitely recursing on Option<()>::SIGNATURE instead of recursing into ()::SIGNATURE as it should.

[Veykril]

Didn't we have a similar issue with impl Trait where trait Trait<T = Self> overflowing. How did we solve that? Feels like we could take a similar approach here as well.

[ShoyuVanilla]

#16877
I'm not sure is this what you are mentioning but you've introduced generic_args_sans_defaults 😄

[Veykril]

Ah, right, we didn't solve it I worked around it 😆

[Wilfred]

I see there isn't any substitution in the environment here, that feels like it might be the problem?

rust-analyzer/crates/hir-ty/src/display.rs

Lines 940 to 944 in 2e35358

	render_variant_after_name(
	var_id.fields(f.db),
	f,
	field_types,
	f.db.trait_environment(def.into()),

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