len_without_is_empty fails on Surf

[Fishrock123]

Lint name:

I tried this code:

    pub fn len(&self) -> Option<usize> {
        self.req.len()
    }

    pub fn is_empty(&self) -> Option<bool> {
        self.req.is_empty()
    }

https://github.com/http-rs/surf/blob/fb228d8536b041f26f53f7b6eaa3ff56238d9f2c/src/request.rs#L272-L286

I expected to see this happen:

No warning. We implement a is_empty() but since the request body is a stream (AsyncRead), we may not know the length for e.g. chunked request bodies. The natural API for expressing that is Option<>.

Instead, this happened:

https://github.com/http-rs/surf/pull/294/checks?check_run_id=2177417044

warning: struct `Request` has a public `len` method, but the `is_empty` method has an unexpected signature
   --> src/request.rs:278:5
    |
278 |     pub fn len(&self) -> Option<usize> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(clippy::len_without_is_empty)]` on by default
note: `is_empty` defined here
   --> src/request.rs:284:5
    |
284 |     pub fn is_empty(&self) -> Option<bool> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: expected signature: `(&self) -> bool`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#len_without_is_empty

error: struct `Request` has a public `len` method, but the `is_empty` method has an unexpected signature
   --> src/request.rs:278:5
    |
278 |     pub fn len(&self) -> Option<usize> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
note: the lint level is defined here
   --> src/lib.rs:78:24
    |
78  | #![cfg_attr(test, deny(warnings))]
    |                        ^^^^^^^^
    = note: `#[deny(clippy::len_without_is_empty)]` implied by `#[deny(warnings)]`
note: `is_empty` defined here
   --> src/request.rs:284:5
    |
284 |     pub fn is_empty(&self) -> Option<bool> {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: expected signature: `(&self) -> bool`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#len_without_is_empty

Meta

• cargo clippy -V: clippy 0.1.52 (f5d8117 2021-03-16)
• rustc -Vv:

rustc 1.52.0-nightly (f5d8117c3 2021-03-16)
binary: rustc
commit-hash: f5d8117c338a788bd24abec733fd143dfceb25a0
commit-date: 2021-03-16
host: x86_64-unknown-linux-gnu
release: 1.52.0-nightly
LLVM version: 12.0.0

[llogiq]

So I guess the lint should at least check if len(&self) returns usize and not fire otherwise.