[compiler-v2] Ability constraints are not shallow #12674
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wrwg
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Mar 26, 2024
This concludes moving all ability checking into the type inference algorithm by representing checks as type constraints. It also introduces some new constraints to deal with some other checks which have done previously ad-hoc (whether types are allowed to be references, tuples, phantom). The PR turned out to be a larger refactoring of the checker as expected. On the good side, a lot of redundant ad-hoc code could be removed and replaced by more generic code. Closes #12437 and closes #12674
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wrwg
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Mar 28, 2024
This concludes moving all ability checking into the type inference algorithm by representing checks as type constraints. It also introduces some new constraints to deal with some other checks which have done previously ad-hoc (whether types are allowed to be references, tuples, phantom). The PR turned out to be a larger refactoring of the checker as expected. On the good side, a lot of redundant ad-hoc code could be removed and replaced by more generic code. Closes #12437 and closes #12674
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The current type constraint mechanism of v2 assumes that a constraint can be resolved if a shallow binding of a type variable is provided. However, that is not true for ability constraints. Consider e.g.
Box<tvar>-- we don't know the abilities of the shallow outer type unless we know the typetvar.Since this seems to be a rare scenario, for now we bail out with an error "type annotation needed" if coming along this case. Instead, we may do full constraint derivation (which we need to do for traits anyway, eventually), by breaking down ability constraints for structured types into constraints of the components.
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