Bad diagnostic with `Cow` and `to_owned` where `into_owned` was meant

[edwloef]

Code

fn main() {
    _ = std::env::var_os("RUST_LOG").map_or("warn".into(), |x| x.to_string_lossy().to_owned());
}

Current output

error[E0515]: cannot return value referencing function parameter `x`
 --> src/main.rs:2:64
  |
2 |     _ = std::env::var_os("RUST_LOG").map_or("warn".into(), |x| x.to_string_lossy().to_owned());
  |                                                                -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                                                                |
  |                                                                returns a value referencing data owned by the current function
  |                                                                `x` is borrowed here

For more information about this error, try `rustc --explain E0515`.

Desired output

error[E0515]: cannot return value referencing function parameter `x`
 --> src/main.rs:2:64
  |
2 |     _ = std::env::var_os("RUST_LOG").map_or("warn".into(), |x| x.to_string_lossy().to_owned());
  |                                                                -^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                                                                |
  |                                                                returns a value referencing data owned by the current function
  |                                                                `x` is borrowed here
  |
help: Try using `.into_owned()` if you meant to convert a `Cow<'_, T>` to an owned `T`
  |
2 |     _ = std::env::var_os("RUST_LOG").map_or("warn".into(), |x| x.to_string_lossy().into_owned());
  |                                                                                    ++

For more information about this error, try `rustc --explain E0515`.

Rationale and extra context

The current error message isn't very helpful, and the two method names are very similar.

Other cases

Rust Version

rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.88.0
LLVM version: 20.1.5

Anything else?

No response

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