Suggest wrapping code into `fn main() {}` when encountering `let` within `main.rs`

Code

let hello = "xyz";

Current output

error: expected item, found keyword `let`
 --> src/main.rs:1:1
  |
1 | let hello = "xyz";
  | ^^^ consider using `const` or `static` instead of `let` for global variables

error: could not compile `thing` (bin "thing") due to 1 previous error

Desired output

error: expected item, found keyword `let`
 --> src/main.rs:1:1
  |
1 | let hello = "xyz";
  | ^^^^^^^^^^^^^ consider wrapping this code within `fn main() {}`

error: could not compile `thing` (bin "thing") due to 1 previous error

Rationale and extra context

Sometimes users copy code from a crate's readme or doctests and expect it to run as is. In some cases like reqwest ^1 or sqlx ^2 it does, but in others like ours ^3 it doesn't.

In our case we are thinking of adding fn main() {} to the readme, but this doesn't solve the case where e.g. the doctests don't have it, which thanks to how rustdoc works ^4 they don't in most cases.

If the file is main.rs, fn main() {} hasn't been declared yet and let is encountered, the compiler should be able to suggest wrapping the code into fn main() {}.

Other cases

No response

Rust Version

rustc 1.82.0-nightly (1f12b9b0f 2024-08-27)
binary: rustc
commit-hash: 1f12b9b0fdbe735968ac002792a720f0ba4faca6
commit-date: 2024-08-27
host: x86_64-unknown-linux-gnu
release: 1.82.0-nightly
LLVM version: 19.1.0

Anything else?

No response