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Represent type-level consts with new-and-improved `hir::ConstArg` ### Summary This is a step toward `min_generic_const_exprs`. We now represent all const generic arguments using an enum that differentiates between const *paths* (temporarily just bare const params) and arbitrary anon consts that may perform computations. This will enable us to cleanly implement the `min_generic_const_args` plan of allowing the use of generics in paths used as const args, while disallowing their use in arbitrary anon consts. Here is a summary of the salient aspects of this change: - Add `current_def_id_parent` to `LoweringContext` This is needed to track anon const parents properly once we implement `ConstArgKind::Path` (which requires moving anon const def-creation outside of `DefCollector`). - Create `hir::ConstArgKind` enum with `Path` and `Anon` variants. Use it in the existing `hir::ConstArg` struct, replacing the previous `hir::AnonConst` field. - Use `ConstArg` for all instances of const args. Specifically, use it instead of `AnonConst` for assoc item constraints, array lengths, and const param defaults. - Some `ast::AnonConst`s now have their `DefId`s created in rustc_ast_lowering rather than `DefCollector`. This is because in some cases they will end up becoming a `ConstArgKind::Path` instead, which has no `DefId`. We have to solve this in a hacky way where we guess whether the `AnonConst` could end up as a path const since we can't know for sure until after name resolution (`N` could refer to a free const or a nullary struct). If it has no chance as being a const param, then we create a `DefId` in `DefCollector` -- otherwise we decide during ast_lowering. This will have to be updated once all path consts use `ConstArgKind::Path`. - We explicitly use `ConstArgHasType` for array lengths, rather than implicitly relying on anon const type feeding -- this is due to the addition of `ConstArgKind::Path`. - Some tests have their outputs changed, but the changes are for the most part minor (including removing duplicate or almost-duplicate errors). One test now ICEs, but it is for an incomplete, unstable feature and is now tracked at https://github.com/rust-lang/rust/issues/127009. ### Followup items post-merge - Use `ConstArgKind::Path` for all const paths, not just const params. - Fix (no github dont close this issue) #127009 - If a path in generic args doesn't resolve as a type, try to resolve as a const instead (do this in rustc_resolve). Then remove the special-casing from `rustc_ast_lowering`, so that all params will automatically be lowered as `ConstArgKind::Path`. - (?) Consider making `const_evaluatable_unchecked` a hard error, or at least trying it in crater r? `@BoxyUwU` |
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