diagnostic about typoed path separator could be better

[comex]

Given this code, where a path separator is typoed as a single instead of double colon:

struct Foo { a: () }

fn main() {
    Foo:new();        
}

the compiler currently outputs:

error[E0423]: expected value, found struct `Foo`
 --> src/main.rs:4:5
  |
1 | struct Foo { a: () }
  | -------------------- `Foo` defined here
...
4 |     Foo:new();        
  |     ^^^ help: use struct literal syntax instead: `Foo { a: val }`

error[E0412]: cannot find type `new` in this scope
 --> src/main.rs:4:9
  |
4 |     Foo:new();        
  |        -^^^ not found in this scope
  |        |
  |        help: maybe you meant to write a path separator here: `::`

This output is quite decent already: the second diagnostic correctly identifies that I meant to write a path separator and suggests a fix. However, before I read the second diagnostic, I naturally read the first diagnostic, which is entirely unhelpful. It would be better if the suggestion could be on the first diagnostic, or if the diagnostics could be combined in some way.

[nsunderland1]

Another example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=cb829e88ed7b85f4d0bc0b6fc2b6f388

#[allow(unused)]
use tokio;
#[allow(unused)]
use std::time::Duration;

fn main() {
    tokio::time::sleep(Duration:from_millis(200))
}

The full output is this:

Compiling playground v0.0.1 (/playground)
error: invalid `struct` delimiters or `fn` call arguments
 [--> src/main.rs:7:5
](https://play.rust-lang.org/#)  |
7 |     tokio::time::sleep(Duration:from_millis(200))
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
help: if `tokio::time::sleep` is a struct, use braces as delimiters
  |
7 |     tokio::time::sleep { Duration:from_millis(200) }
  |                        ~                           ~
help: if `tokio::time::sleep` is a function, use the arguments directly
  |
7 -     tokio::time::sleep(Duration:from_millis(200))
7 +     tokio::time::sleep(from_millis(200))
  | 

error: could not compile `playground` due to previous error

This case doesn't suggest fixing the path separator at all, which is super confusing.

[fmease]

Since the removal of the original type ascription syntax, the compiler output is:

error: path separator must be a double colon
 --> src/main.rs:4:8
  |
4 |     Foo:new();        
  |        ^
  |
help: use a double colon instead
  |
4 |     Foo::new();        
  |         +

error[E0599]: no function or associated item named `new` found for struct `Foo` in the current scope
 --> src/main.rs:4:9
  |
1 | struct Foo { a: () }
  | ---------- function or associated item `new` not found for this struct
...
4 |     Foo:new();        
  |         ^^^ function or associated item not found in `Foo`

Which I deem perfectly acceptable.

Similarly for #84566 (comment):

error: path separator must be a double colon
 --> src/main.rs:7:32
  |
7 |     tokio::time::sleep(Duration:from_millis(200))
  |                                ^
  |
help: use a double colon instead
  |
7 |     tokio::time::sleep(Duration::from_millis(200))
  |                                 +

error[E0308]: mismatched types
 --> src/main.rs:7:5
  |
6 | fn main() {
  |          - expected `()` because of default return type
7 |     tokio::time::sleep(Duration:from_millis(200))
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- help: consider using a semicolon here: `;`
  |     |
  |     expected `()`, found `Sleep`

[fmease]

Since we have a (regression) test for this already like tests/ui/suggestions/type-ascription-instead-of-path.rs I'll just close this as completed!