Make type hashing :total (#52427)

Since types can't really be deleted and the hash of a type is already
being cached inside of the type object itself, it should be legal to
mark the `ccall` retrieving that hash as `:total`, permitting hashing of
types to be concretely evaluated.

I'm unsure if the test I've added is good as written, or whether this
should be done differently.

---------

Co-authored-by: Sukera <Seelengrab@users.noreply.github.com>
Co-authored-by: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>

base/hashing.jl

@@ -29,7 +29,9 @@ See also: [`objectid`](@ref), [`Dict`](@ref), [`Set`](@ref).
 """
 hash(x::Any) = hash(x, zero(UInt))
 hash(w::WeakRef, h::UInt) = hash(w.value, h)
-hash(T::Type, h::UInt) = hash_uint(3h - ccall(:jl_type_hash, UInt, (Any,), T))
+
+# Types can't be deleted, so marking as total allows the compiler to look up the hash
+hash(T::Type, h::UInt) = hash_uint(3h - @assume_effects :total ccall(:jl_type_hash, UInt, (Any,), T))
 
 ## hashing general objects ##
 

test/hashing.jl

@@ -302,3 +302,5 @@ struct AUnionParam{T<:Union{Nothing,Float32,Float64}} end
     # test hashing of rational with odd denominator
     @test hash(5//3) == hash(big(5)//3)
 end
+
+@test Core.Compiler.is_foldable_nothrow(Base.infer_effects(hash, Tuple{Type{Int}, UInt}))