Document when associated types may be omitted from dyn Trait type

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
68/100
Issue type
Documentation
Clarity
Clearly specified
Activity status
Stale
Tech stack
rust
Domain
documentation

Research direction

Start with the Reference's trait object types chapter linked in the issue and compare its current wording with the three Rust examples. Document when associated types may be omitted, including the Self: Sized case, and explain the rejected form. Done means the chapter clearly covers all shown cases.

Written by the indexing model from the issue text.

Description

We accept this,

pub trait Tr {
    type Ty;
}
fn f(_: &dyn Tr<Ty = ()>) {}

and we accept this (as of https://github.com/rust-lang/rust/pull/112319),

pub trait Tr {
    type Ty where Self: Sized;
}
fn f(_: &dyn Tr) {}

but we of course don't accept this:

pub trait Tr {
    type Ty;
}
fn f(_: &dyn Tr) {} //~ ERROR

The Reference should speak to this in our chapter on trait object types, but is silent on this matter.

cc @ehuss https://github.com/rust-lang/reference/issues/1932 https://github.com/rust-lang/rust/issues/127251

Dominant language
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