Avoid unnecessary heap allocation in `Display` impl for `DataTypeNode`

[nohupped]

Can I remove unnecessary heap allocation in the Display trait for DataTypeNode?

Currently, the Display implementation for DataTypeNode calls .to_string() on a known static string ("UInt8", "Int32", etc), which leads to unnecessary heap allocation even with -C opt-level=3. It performs a heap allocation and deallocation every time fmt() is called. A minimal example in Godbolt.

mov     edi, 5                    ; size = 5
call    __rust_alloc              ; heap allocation
mov     byte ptr [rax + 4], 56    ; manual copying
mov     dword ptr [rax], 1953384789
call    __rust_dealloc            ; cleanup

Using f.write_str("UInt8") or write!(f, "UInt8") avoids this heap allocation, and is also idiomatic.

lea     rsi, [rip + .Lanon.d10afb4a363194822fac1f5885ac38a9.0]
.Lanon.d10afb4a363194822fac1f5885ac38a9.0:
        .ascii  "UInt8"

Perhaps this isn't a hot path, but it's a zero risk improvement that makes the code more idiomatic. Happy to implement this if you're interested.

[serprex]

Yes, good catch