pre-RFC `const` function arguments

language design
22

This RFC does not proposing adding const as part of patterns. AFAIK const as part of patterns doesn't work yet (what would this even mean?), e.g.

struct A { a: i32, b: i32 }
let A{ a, b }: A = A{ a: 2, b: 3 }; // OK
const A{ a, b }: A = A { a: 2, b: 3 }; // ~ERROR

I can imagine that we might want to make that work someday though, so this RFC should not prevent that from happening.

From a syntactic point-of-view, if we ever get const in patterns, we might be able to allow this in const-function arguments without breaking backwards compatibility like this:

fn foo(const A{ a, b }: A) {}

The question is whether const generics would play along. I have no idea.