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.