map_flatten does not cover Result.map

[Dionysusnu]

Lint name: map_flatten

I tried this code:

let a: Result<&str, ParseIntError> = Ok("1");
let c: i32 = a.map(|b| b.parse()).flatten().unwrap();

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=3c1eced0e47dac7f563945134ddc3bc8

I expected to see this happen:

This should fall under lint rule map_flatten.
The lint should advise the user to use andThen, instead of the .map(..).flatten()

Instead, this happened:

Result is not currently covered by the lint rule.
However, it is nearly the same as Option's map_flatten, and therefore makes sense to be included.

Meta

• cargo clippy -V: e.g. clippy 0.1.53 (53cb7b09 2021-06-17)
  -- Although I did run rustup update, I'm not sure if this is the latest dev version, but the lint rule doesn't seem to have changed in any recent versions.
• rustc -Vv:

rustc 1.53.0 (53cb7b09b 2021-06-17)
binary: rustc
commit-hash: 53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b
commit-date: 2021-06-17
host: x86_64-pc-windows-msvc
release: 1.53.0
LLVM version: 12.0.1

[Dionysusnu]

First issue here, still relatively new to Rust. Sorry if this is bad/wrong, please tell me if so.

[xFrednet]

This is the right place to report this. It was probably not implemented as flatten for results is still behind a feature flag. I've created a second playground that doesn't use the parse method: playground. Thank you for providing an example, that was very helpful! 🙃

The implementation has to check that the wrapped error types are the same and that the feature result_flattening is enabled.

[camsteffen]

The implementation has to check that the wrapped error types are the same and that the feature result_flattening is enabled.

We already know those things are true if flatten compiled!