given Vec<foo:Bar> (with single ':') in a struct field, rustc provides an unintuitive affiliated types error

[inanna-malick]

Given the following code: (https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c485d59d73a93f4845ea71f4dd4a08b9

mod my_module {
    struct A;
}

// FAILS WITH UNEXPECTED ASSOCIATED TYPES ERRORS
struct MyStruct {
    // NOTE: Vec<> wrapper required to cause associated types error
    my_field: Vec<my_module:A>,
}

The current output is:

error: associated type bounds are not allowed within structs, enums, or unions
 --> src/lib.rs:8:19
  |
8 |     my_field: Vec<my_module: A>,
  |                   ^^^^^^^^^^^^

error[E0405]: cannot find trait `A` in this scope
 --> src/lib.rs:8:30
  |
6 | struct MyStruct {
  |                - help: you might be missing a type parameter: `<A>`
7 |     // NOTE: Vec<> wrapper required to cause associated types error
8 |     my_field: Vec<my_module: A>,
  |                              ^ not found in this scope

error[E0658]: associated type bounds are unstable
 --> src/lib.rs:8:19
  |
8 |     my_field: Vec<my_module: A>,
  |                   ^^^^^^^^^^^^
  |
  = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information

error[E0107]: this struct takes at least 1 generic argument but 0 generic arguments were supplied
 --> src/lib.rs:8:15
  |
8 |     my_field: Vec<my_module: A>,
  |               ^^^ expected at least 1 generic argument
  |
help: add missing generic argument
  |
8 |     my_field: Vec<T, my_module: A>,
  |                   ++

error[E0229]: associated type bindings are not allowed here
 --> src/lib.rs:8:19
  |
8 |     my_field: Vec<my_module: A>,
  |                   ^^^^^^^^^^^^ associated type not allowed here

Some errors have detailed explanations: E0107, E0229, E0405, E0658.

Ideally the output would point out the potential that the user had meant to type '::' instead of ':'.

Rationale: I hit this and I spent a few minutes trying to figure out what was going on before I realized that I had made a type and written ':' instead of '::' - it's the kind of small typo the eye easily glazes over

cc @estebank per https://twitter.com/ekuber/status/1479928446218293249, thanks so much for putting so much time into ergonomics <3

[TaKO8Ki]

It seems that this problem has already been solved in nightly.

ref: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=c816dd29ce8ce457fbdaa7f0eb66d2d3

[estebank]

Looking at the output of that link, I think it'd be nice to also handle this (in the parser):

error: expected `,`, or `}`, found `:`
  --> src/lib.rs:18:24
   |
18 |     my_field: my_module:A,
   |                        ^

It would also be nice to reduce the number of errors we see for the original case to maybe one or two at most:

error[E0405]: cannot find trait `A` in this scope
  --> src/lib.rs:64:27
   |
64 |   my_field: Vec<my_module:A>,
   |                           ^ not found in this scope
   |
help: you might have meant to write a path instead of an associated type bound
   |
64 |   my_field: Vec<my_module::A>,
   |                          ~~
help: you might be missing a type parameter
   |
63 | struct MyStruct2<A> {
   |                 +++

error[E0658]: associated type bounds are unstable
  --> src/lib.rs:64:17
   |
64 |   my_field: Vec<my_module:A>,
   |                 ^^^^^^^^^^^
   |
   = note: see issue #52662 <https://github.com/rust-lang/rust/issues/52662> for more information
   = help: add `#![feature(associated_type_bounds)]` to the crate attributes to enable

error[E0107]: this struct takes at least 1 generic argument but 0 generic arguments were supplied
  --> src/lib.rs:64:13
   |
64 |   my_field: Vec<my_module:A>,
   |             ^^^ expected at least 1 generic argument
   |
help: add missing generic argument
   |
64 |   my_field: Vec<T, my_module:A>,
   |                 ++

error[E0229]: associated type bindings are not allowed here
  --> src/lib.rs:64:17
   |
64 |   my_field: Vec<my_module:A>,
   |                 ^^^^^^^^^^^ associated type not allowed here