mGCA: Make trait object types with type-level associated consts dyn compatible if all of the latter are specified via bindings#150843

Draft

fmease wants to merge 9 commits intorust-lang:mainfrom

Commits on Jan 15, 2026

Under mGCA make traits with assoc consts dyn compatible but require all assoc consts to be specified via bindings
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Fix v0 symbol mangling for assoc const bindings
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Under mGCA permit certain const projections in dyn-compatible traits
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Generalize diag for missing assoc types to account for assoc consts
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Generalize diag for conflicting assoc type bindings to account for assoc consts
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Introduce `AssocTag::descr` & refactor in the vicinity
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Fix handling of const params defaults that ref `Self` & generalize diag
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Force users to specify assoc consts from supertraits that would otherwise ref `Self`
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Reject const projections behind trait aliases that mention `Self`
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