Improved diagnostics for non-primitive cast on non-primitive types (Arc, Option)

[Kivooeo]

here is a small fix that improving error messaging when user is trying to do something like this

let _ = "x" as Arc<str>;
let _ = 2 as Option<i32>;

before it looks like this

error[E0605]: non-primitive cast: `&'static str` as `Arc<str>`
 --> src\main.rs:3:13
  |
3 |     let _ = "x" as Arc<str>;
  |             ^^^^^^^^^^^^^^^ help: consider using the `From` trait instead: `Arc<str>::from("x")`

error[E0605]: non-primitive cast: `i32` as `Option<i32>`
 --> src\main.rs:4:13
  |
4 |     let _ = 2 as Option<i32>;

which looks horrible to be honest
so i made a small fix that make errors looks like this

error[E0605]: non-primitive cast: `&'static str` as `Arc<str>`
  |
3 |     let _ = "x" as Arc<str>;
  |             ^^^^^^^^^^^^^^^
  |
  = note: an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
help: consider using the `From` trait instead
  |
3 -     let _ = "x" as Arc<str>;
3 +     let _ = Arc::<str>::from("x");
  |

error[E0605]: non-primitive cast: `i32` as `Option<i32>`
  |
4 |     let _ = 2 as Option<i32>;
  |             ^^^^^^^^^^^^^^^^
  |
  = note: an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
help: consider using the `From` trait instead
  |
4 -     let _ = 2 as Option<i32>;
4 +     let _ = Option::<i32>::from(2);

What improves?

1. Arc<str>::from("x") which makes no sense because of missing ::
2. readability

Related Issue
fixes #135412

[rustbot]

r? @wesleywiser

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[wesleywiser]

Thanks @Kivooeo!

@bors r+ rollup

[bors]

📌 Commit c9deaf6 has been approved by wesleywiser

It is now in the queue for this repository.