Overly specific PartialEq impl for Option and Result

[bugaevc]

I tried this code:

// This works:
let s1: String = "foo".to_owned();
let s2: &str = "foo";
s1 == s2;

// But these don't:
let o1: Option<String> = Some("foo".to_owned());
let o2: Option<&str> = Some("foo");
o1 == o2;

let r1: Result<String, ()> = Ok("foo".to_owned());
let r2: Result<&str, ()> = Ok("foo");
r1 == r2;

(playground)

This is because PartialEq impls are #[derive]d for Option and Result, which gives them conservative bounds.

Instead of current

impl<T> PartialEq<Option<T>> for Option<T>
where
    T: PartialEq<T>

I'd expect

impl<A, B> PartialEq<Option<B>> for Option<A>
where
    A: PartialEq<B>

and likewise, for Result

impl<T1, T2, E1, E2> PartialEq<Result<T2, E2>> for Result<T1, E1>
where
    T1: PartialEq<T2>,
    E1: PartialEq<E2>,

Note that this is similar to what Vec<T> already implements:

impl<A, B> PartialEq<Vec<B>> for Vec<A>
where
    A: PartialEq<B>

Meta

rustc --version --verbose:

rustc 1.45.2 (d3fb005a3 2020-07-31)
binary: rustc
commit-hash: d3fb005a39e62501b8b0b356166e515ae24e2e54
commit-date: 2020-07-31
host: x86_64-unknown-linux-gnu
release: 1.45.2
LLVM version: 10.0

[cuviper]

I just tried it for Option, and it immediately showed a type-inference issue:

error[E0282]: type annotations needed
   --> library/test/src/cli.rs:293:66
    |
293 |     let mut report_time_colored = report_time && colored_opt_str == Some("colored".into());
    |                                                                  ^^      ---------------- this method call resolves to `T`
    |                                                                  |
    |                                                                  cannot infer type for type parameter `B`

The left is Option<String>, so this wouldn't even need to convert .into() anymore, but it's a bad sign that we didn't get far. I changed that, and got an even more troubling one:

error[E0282]: type annotations needed
  --> library/term/src/terminfo/searcher/tests.rs:15:37
   |
15 |     assert!(get_dbpath_for_term("") == None);
   |                                     ^^ cannot infer type for type parameter `B`

It doesn't know what kind of None this is! I changed that to is_none(), but then I hit a whole bunch of assert_eq!(something, None) in the core tests. These may not want just is_none() because assert_eq! will debug-print the different values.

So, I think this is an infeasible change.

[tamird]

Reached this as well and found the same problem with Result; not even the standard library compiles because of the same inference failure that @cuviper reports above.

error[E0282]: type annotations needed
   --> /home/tamird/.cargo/registry/src/github.com-1ecc6299db9ec823/tracing-subscriber-0.2.16/src/filter/env/field.rs:285:60
    |
285 |             Some((ValueMatch::U64(ref e), ref matched)) if Ok(value) == (*e).try_into() => {
    |                                                            ^^ cannot infer type for type parameter `E` declared on the enum `Result`

[chris-morgan]

Duplicate of #20063 (and several more exact duplicates and similar cases which have been closed as duplicate of that isue).