Comment by WhyNotHugo

If you look at how macros are mostly used, though, a lot of that stuff could be replaced directly with reflection. Most derive macros, for example, aren't really interested in the syntax of the type they're deriving for, they're interested in its shape, and the syntax is being used as a proxy for that. Similarly, a lot of macros get used to express relationships between types that cannot be expressed at the type system level, and are therefore expressed at a syntactic level - stuff like "this trait is derived for all tuples based on a simple pattern".

There are also proc macros just for creating DSLs, but Rust is already mostly expressive enough that you don't really need this. There are some exceptions, like sqlx, that really do embed a full, existing DSL, but these are much rarer and - I suspect - more of a novelty than a deeply foundational feature of Rust.

They are disjoint, but the things you can use them for overlap a lot. In particular, I'd dare say a majority of existing `#[derive]`-style macros might be easier to implement in a hypothetical reflection layer instead.

Instead of taking a raw token stream of a struct, parsing it with `syn` (duplicating the work the compiler does later), generating the proper methods and carefully generating trait checks for the compiler to check in a later phase (for example, `#[derive(Eq)] struct S(u16)` creates an invisible never-called method just to do `let _: ::core::cmp::AssertParamIsEq<u16>;` so the compiler can show an error 20s after an incorrectly used macro finished), just directly iterate fields and check `field.type.implements_trait(Eq)` inside the derive macro itself.

That said, that's just wishful thinking - with how complex trait solving is, supporting injecting custom code in the middle of it (checking existing traits and adding new trait impls) might make compile time even worse, assuming it's even possible at all. It’s also not a clear perf win if a reflection function were to run on each instantiation of a generic type.