Compiler suggestion "try using a conversion method: .to_string"

[Deleplace]

I tried this code (playground):

fn main() {
    let mut path: String = "/usr".to_string();
    let folder: String = "lib".to_string();
    
    path = format!("{}/{}", path, folder).as_str();
    
    println!("{}", &path);
}

I expected to see this happen:

error[[E0308]](https://doc.rust-lang.org/stable/error-index.html#E0308): mismatched types
 --> src/main.rs:6:12
  |
2 |     let mut path: String = "/usr".to_string();
  |                   ------ expected due to this type
...
5 |     path = format!("{}/{}", path, folder).as_str();
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- help: try removing the call to method: `.as_str()`
  |            |
  |            expected struct `String`, found `&str`

Instead, this happened:

error[[E0308]](https://doc.rust-lang.org/stable/error-index.html#E0308): mismatched types
 --> src/main.rs:6:12
  |
2 |     let mut path: String = "/usr".to_string();
  |                   ------ expected due to this type
...
5 |     path = format!("{}/{}", path, folder).as_str();
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- help: try using a conversion method: `.to_string()`
  |            |
  |            expected struct `String`, found `&str`

This is just a very specific instance where the suggestion to add some code, while "technically correct", is less useful than the better suggestion to remove some code.

Meta

rustc --version --verbose:

rustc 1.65.0 (897e37553 2022-11-02)
binary: rustc
commit-hash: 897e37553bba8b42751c67658967889d11ecd120
commit-date: 2022-11-02
host: aarch64-apple-darwin
release: 1.65.0
LLVM version: 15.0.0

Backtrace

<backtrace>

[chenyukang]

@rustbot claim

[jruderman]

@rustbot label +A-diagnostics

[estebank]

@chenyukang, it seems that given an expression, you can generically check if removing the last method call if present would give you the right type, so this could be implemented in a very generic way instead of special casing &str/String.

[Deleplace]

This looks awesome, thank you so much!

[Deleplace]

This is how it looks now in the nightly playground. Very cool.

error[[E0308]](https://doc.rust-lang.org/nightly/error-index.html#E0308): mismatched types
 --> src/main.rs:5:12
  |
2 |     let mut path: String = "/usr".to_string();
  |                   ------ expected due to this type
...
5 |     path = format!("{}/{}", path, folder).as_str();
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `String`, found `&str`
  |
help: try removing the method call
  |
5 -     path = format!("{}/{}", path, folder).as_str();
5 +     path = format!("{}/{}", path, folder);
  |

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error