Tighten "introduce new binding" suggestion

[Rageking8]

Given the following code: link

fn main() {
    x = x = x;
}

The current output is:

Compiling playground v0.0.1 (/playground)
error[[E0425]](https://doc.rust-lang.org/nightly/error-index.html#E0425): cannot find value `x` in this scope
 --> src/main.rs:2:5
  |
2 |     x = x = x;
  |     ^
  |
help: you might have meant to introduce a new binding
  |
2 |     let x = x = x;
  |     +++

error[[E0425]](https://doc.rust-lang.org/nightly/error-index.html#E0425): cannot find value `x` in this scope
 --> src/main.rs:2:9
  |
2 |     x = x = x;
  |         ^
  |
help: you might have meant to introduce a new binding
  |
2 |     x = let x = x;
  |         +++

error[[E0425]](https://doc.rust-lang.org/nightly/error-index.html#E0425): cannot find value `x` in this scope
 --> src/main.rs:2:13
  |
2 |     x = x = x;
  |             ^
  |
help: you might have meant to introduce a new binding
  |
2 |     x = x = let x;
  |             +++

For more information about this error, try `rustc --explain E0425`.
error: could not compile `playground` due to 3 previous errors

Ideally the output should first check the RHS, before suggest adding let. It should also avoid suggesting when code is invalid. Thanks.

Something like this should also not suggest adding let: link

fn main() {
    x = i32
}

Compiling playground v0.0.1 (/playground)
error[[E0425]](https://doc.rust-lang.org/nightly/error-index.html#E0425): cannot find value `x` in this scope
 --> src/main.rs:2:5
  |
2 |     x = i32
  |     ^
  |
help: you might have meant to introduce a new binding
  |
2 |     let x = i32
  |     +++

error[[E0423]](https://doc.rust-lang.org/nightly/error-index.html#E0423): expected value, found builtin type `i32`
 --> src/main.rs:2:9
  |
2 |     x = i32
  |         ^^^ not a value
  |
help: consider importing one of these items instead
  |
1 | [use fastrand::i32;](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)
  |
1 | [use nom::character::complete::i32;](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)
  |
1 | [use nom::character::streaming::i32;](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)
  |
1 | [use nom::number::complete::i32;](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)
  |
    and 1 other candidate

Some errors have detailed explanations: E0423, E0425.
For more information about an error, try `rustc --explain E0423`.
error: could not compile `playground` due to 2 previous errors

cc @chenyukang

[compiler-errors]

I'm not convinced the second example should be something we need to fix, but the first example should be very easy to fix.

[chenyukang]

@rustbot claim