Improve error messages involving type aliases

[nodakai]

extern crate libc;
fn main() {
    let a: libc::ssize_t = 0;
    let () = a;
}

$ rustc show-typedef.rs
show-typedef.rs:4:17: 4:19 error: mismatched types: expected `i64`, found `()` (expected i64, found ())
show-typedef.rs:4             let () = a;
                                  ^~
error: aborting due to previous error

It would be great if rustc could show libc::ssize_t as well as i64 in the error message. Compare it with GCC 4.9

$ g++49 show-typedef.cpp
show-typedef.cpp: In function 'int main()':
show-typedef.cpp:3:28: error: invalid operands of types 'void' and 'const size_type {aka const long unsigned int}' to binary 'operator+'
     (void)0 + std::string::npos;
                            ^

#include <string>
int main() {
    (void)0 + std::string::npos;
}

[steveklabnik]

This doesn't need libc to reproduce, it happens with any type synonym:

type Lol = i32;
fn main() {
    let a: Lol = 0;
    let () = a;
}

[nodakai]

Right, libc was just for presenting a motivating example.

[steveklabnik]

Triage: no change

[estebank]

This is fundamentally the same as #21934, only more specific.

[camelid]

Rustdoc would benefit from work on this; see https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/To.20alias.20or.20not.20to.20alias.

[TimJentzsch]

Current output with 1.65.0-nightly (2022-08-10 29e4a9ee0253cd39e552):

type Lol = i32;
fn main() {
    let a: Lol = 0;
    let () = a;
}

error[[E0308]](https://doc.rust-lang.org/nightly/error-index.html#E0308): mismatched types
 --> src/main.rs:4:9
  |
4 |     let () = a;
  |         ^^   - this expression has type `i32`
  |         |
  |         expected `i32`, found `()`

For more information about this error, try `rustc --explain E0308`.