Unneeded Unit Expression Warning From Clippy

[jyhi]

Describe the Bug

wasm-bindgen 0.2.79 released not long ago triggered a workflow error in our repository, which doesn't exist before.

A local run of cargo +nightly clippy -- -Z macro-backtrace with clippy 0.1.60 (5e57faa 2022-01-19) prints the following:

    Checking client-wasm v0.0.1 (/home/user/repo/iota.rs/bindings/wasm/native)
warning: unneeded unit expression
  --> src/address_getter.rs:22:1
   |
22 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ in this procedural macro expansion
   |
  ::: /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/wasm-bindgen-macro-0.2.79/src/lib.rs:22:1
   |
22 | pub fn __wasm_bindgen_class_marker(attr: TokenStream, input: TokenStream) -> TokenStream {
   | ---------------------------------------------------------------------------------------- in this expansion of `#[wasm_bindgen::prelude::__wasm_bindgen_class_marker]`
   |
   = note: `#[warn(clippy::unused_unit)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: unneeded unit expression
  --> src/address_getter.rs:22:1
   |
22 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ in this procedural macro expansion
   |
  ::: /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/wasm-bindgen-macro-0.2.79/src/lib.rs:22:1
   |
22 | pub fn __wasm_bindgen_class_marker(attr: TokenStream, input: TokenStream) -> TokenStream {
   | ---------------------------------------------------------------------------------------- in this expansion of `#[wasm_bindgen::prelude::__wasm_bindgen_class_marker]`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: unneeded unit expression
  --> src/balance_getter.rs:20:1
   |
20 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ in this procedural macro expansion
   |
  ::: /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/wasm-bindgen-macro-0.2.79/src/lib.rs:22:1
   |
22 | pub fn __wasm_bindgen_class_marker(attr: TokenStream, input: TokenStream) -> TokenStream {
   | ---------------------------------------------------------------------------------------- in this expansion of `#[wasm_bindgen::prelude::__wasm_bindgen_class_marker]`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: unneeded unit expression
  --> src/client.rs:46:1
   |
46 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ in this procedural macro expansion
   |
  ::: /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/wasm-bindgen-macro-0.2.79/src/lib.rs:22:1
   |
22 | pub fn __wasm_bindgen_class_marker(attr: TokenStream, input: TokenStream) -> TokenStream {
   | ---------------------------------------------------------------------------------------- in this expansion of `#[wasm_bindgen::prelude::__wasm_bindgen_class_marker]`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: unneeded unit expression
  --> src/client_builder.rs:28:1
   |
28 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ in this procedural macro expansion
   |
  ::: /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/wasm-bindgen-macro-0.2.79/src/lib.rs:22:1
   |
22 | pub fn __wasm_bindgen_class_marker(attr: TokenStream, input: TokenStream) -> TokenStream {
   | ---------------------------------------------------------------------------------------- in this expansion of `#[wasm_bindgen::prelude::__wasm_bindgen_class_marker]`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: unneeded unit expression
  --> src/get_address.rs:16:1
   |
16 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ in this procedural macro expansion
   |
  ::: /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/wasm-bindgen-macro-0.2.79/src/lib.rs:22:1
   |
22 | pub fn __wasm_bindgen_class_marker(attr: TokenStream, input: TokenStream) -> TokenStream {
   | ---------------------------------------------------------------------------------------- in this expansion of `#[wasm_bindgen::prelude::__wasm_bindgen_class_marker]`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: unneeded unit expression
  --> src/message_builder.rs:36:1
   |
36 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ in this procedural macro expansion
   |
  ::: /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/wasm-bindgen-macro-0.2.79/src/lib.rs:22:1
   |
22 | pub fn __wasm_bindgen_class_marker(attr: TokenStream, input: TokenStream) -> TokenStream {
   | ---------------------------------------------------------------------------------------- in this expansion of `#[wasm_bindgen::prelude::__wasm_bindgen_class_marker]`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: unneeded unit expression
  --> src/message_getter.rs:17:1
   |
17 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ in this procedural macro expansion
   |
  ::: /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/wasm-bindgen-macro-0.2.79/src/lib.rs:22:1
   |
22 | pub fn __wasm_bindgen_class_marker(attr: TokenStream, input: TokenStream) -> TokenStream {
   | ---------------------------------------------------------------------------------------- in this expansion of `#[wasm_bindgen::prelude::__wasm_bindgen_class_marker]`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: unneeded unit expression
  --> src/unspent_address_getter.rs:19:1
   |
19 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ in this procedural macro expansion
   |
  ::: /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/wasm-bindgen-macro-0.2.79/src/lib.rs:22:1
   |
22 | pub fn __wasm_bindgen_class_marker(attr: TokenStream, input: TokenStream) -> TokenStream {
   | ---------------------------------------------------------------------------------------- in this expansion of `#[wasm_bindgen::prelude::__wasm_bindgen_class_marker]`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: unneeded unit expression
  --> src/lib.rs:27:1
   |
27 | #[wasm_bindgen(start)]
   | ^^^^^^^^^^^^^^^^^^^^^^ in this procedural macro expansion
   |
  ::: /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/wasm-bindgen-macro-0.2.79/src/lib.rs:9:1
   |
9  | pub fn wasm_bindgen(attr: TokenStream, input: TokenStream) -> TokenStream {
   | ------------------------------------------------------------------------- in this expansion of `#[wasm_bindgen]`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: `client-wasm` (lib) generated 100 warnings (90 duplicates)
    Finished dev [unoptimized + debuginfo] target(s) in 0.77s

Steps to Reproduce

1. git clone https://github.com/iotaledger/iota.rs
2. cd iota.rs/bindings/wasm/native
3. cargo clippy (version of toolchain doesn't actually matter)
4. See warnings

Expected Behavior

No warning should be given.

Actual Behavior

client-wasm (lib) generated 100 warnings (90 duplicates).

Additional Context

We haven't tried to identify whether this is a problem from our side yet.

[jetli]

same here, this breaks my CIs

[damianwadley]

Example input:

    #[wasm_bindgen(js_name = "setTimeout")]
    pub fn set_timeout(&mut self, timeout: u16) -> u16 {
        let old = self.timeout;
        self.timeout = timeout;
        old
    }

Expanded output:

        pub fn set_timeout(&mut self, timeout: u16) -> u16 {
            #[automatically_derived]
            const __wasm_bindgen_generated_StructName_setTimeout__const: () = {
                pub extern "C" fn __wasm_bindgen_generated_StructName_setTimeout(
                    me: u32,
                    arg1: <u16 as wasm_bindgen::convert::FromWasmAbi>::Abi,
                ) -> <u16 as wasm_bindgen::convert::ReturnWasmAbi>::Abi {
                    let _ret = {
                        let mut me = unsafe {
                            < WasmStructName as wasm_bindgen :: convert :: RefMutFromWasmAbi > :: ref_mut_from_abi (me)
                        };
                        let me = &mut *me;
                        let arg1 =
                            unsafe { <u16 as wasm_bindgen::convert::FromWasmAbi>::from_abi(arg1) };
                        me.set_timeout(arg1)
                    };
                    <u16 as wasm_bindgen::convert::ReturnWasmAbi>::return_abi(_ret)
                }
                ()
            };
            let old = self.timeout;
            self.timeout = timeout;
            old
        }

That () line is triggering the warning.

[Razican]

I can confirm that this error comes with the update from 0.2.78 to 0.2.79.

[Razican]

@alexcrichton would it be possible to release a new version with the fix?

[alexcrichton]

Sorry but no I would prefer to not make a release simply for clippy.

[anonrig]

I'm facing a similar issue. As a new developer working on Rust, I thought this was caused by a bug on my side, and sad to see that even though the maintainer acknowledges the issue, he does not prefer to "make a release simply for clippy".

Can you at least point us to a solution/explanation for this?

[tgecho]

Can you at least point us to a solution/explanation for this?

As I understand these things (being somewhat new to Rust as well), normally you wouldn't see a warning for code that is in another crate. However in this case it's being caused by something inside macro generated code... which is in a sense actually in your crate. Still not your fault, and still an annoying bummer.

If you want to suppress the warning while waiting for the next release, you should be able to add this to the top of the affected file: #![allow(clippy::unused_unit)]

See https://github.com/rust-lang/rust-clippy#allowingdenying-lints

[arnauorriols]

Is there any ETA of the release? Or any argument why it's not being considered significant? Changing the linting (or update) policies because of a misbehaving 3rd party macro is quite a red flag to us. I'm more concerned about this than the bug itself.

[mimoo]

seeing this in our CI as well