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We have four macros for generating trivial traversal (fold/visit) and lift impls. - `rustc_ir::TrivialTypeTraversalImpls` - `rustc_middle::TrivialTypeTraversalImpls` - `rustc_middle::TrivialLiftImpls` - `rustc_middle::TrivialTypeTraversalAndLiftImpls` The first two are very similar. The last one just combines the second and third one. The macros themselves are ok, but their use is a mess. This commit does the following. - Removes types that no longer need a lift and/or traversal impl from the macro calls. - Consolidates the macro calls into the smallest number of calls possible, with each one mentioning as many types as possible. - Orders the types within those macro calls alphabetically, and makes the module qualification more consistent. - Eliminates `rustc_middle::mir::type_foldable`, because the macro calls were merged and the manual `TypeFoldable` impls are better placed in `structural_impls.rs`, alongside all the other ones. This makes the code more concise. Moving forward, it also makes it more obvious where new types should be added.
For more information about how rustc works, see the rustc dev guide.