Canonicalize types based on hashing than HirIds and TyTy equality #1265
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When we compile types for GCC we need to make sure we canonicalize them,
this means that for any user defined type such as Structs or Tuples we want
all expressions or declarations to use the same tree. Even if we have
duplicate tree's we can end up confusing the middle-end and lose out on
optimizations as we will be using view_convert_exprs to maintain the
consistancy which will marshall between the objects unnessecarily.
The old method for doing this kept mappings of HirIds which was fine for
simple cases but when generics are involved new Id's are generated so this
meant we ended up having a vector of pairs {TyTy::BaseType, tree} and we
did a liner search to find the relevant TyTy::BaseType that we equaled to.
This was not only slow but did not handle the cases for generic associated
types or nested generics. So we needed a more general faster implementation
therefore hashing. This patch takes the gcc/tree.h type_hash_canon code
and adds in hashing for RECORD and UNION types. This means we will generate
a duplicate type then hash it and look for an existing type.
This patch will also allow us to fix how we implement monomorphization of
functions by nesting the hashes behind DefId's.
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This patch updates how we canonicalize the types. The old method was not
able to handle more complex cases involving generic associated types. So
this now uses a moe generic hashing system which now speeds up the lookups.
The commit message gives much more detail here.