Conflicting implementations of trait in Rust

Question

I want to implement a custom trait for &'a str and for integer numbers up to i32, but Rust does not allow me to:

use std::convert::Into;

pub trait UiId {
    fn push(&self);
}

impl<'a> UiId for &'a str {
    fn push(&self) {}
}

impl<T: Into<i32>> UiId for T {
    fn push(&self) {}
}

fn main() {}

This fails to compile with the following error:

error[E0119]: conflicting implementations of trait `UiId` for type `&str`:
  --> src/main.rs:11:1
   |
7  | impl<'a> UiId for &'a str {
   | ------------------------- first implementation here
...
11 | impl<T: Into<i32>> UiId for T {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&str`
   |
   = note: upstream crates may add new impl of trait `std::convert::From<&str>` for type `i32` in future versions

&'a str does not implement Into<i32>. Is it possible to implement UiId for &'a str and everything that can be converted into i32 without specifying concrete types? How can I do that?

I think it must be a limitation in how Rust determines whether implementations overlay (or might overlap), but I haven't found anywhere where the rules are spelled out. :-( Oh, for a proper language spec! — Chris Emerson

starblue starblue · Accepted Answer · 2016-08-26T07:52:54

The fact that &'a str does not implement Into<i32> is not taken into account, because there is no guarantee that it couldn't be added later. This would then break your code.

So if this were allowed the possible breakage would make it harder to add implementations to library traits.

Unfortunately I couldn't find documentation for that, neither in The Rust Programming Language Book nor in the Reference Manual.

The best I could find is RFC 1023, which says that a crate [...] cannot rely that Type: !Trait holds unless Type or Trait is local.

Conflicting implementations of trait in Rust

2 Answers