rust-lang / rust-lang/rust-clippy

`let_unit_value` targeting entire function

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C-bug I-false-positive
Dominant language
Rust
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Description

Lint name: let_unit_value

This is one of the functions being targeted:

    #[cfg_attr(feature = "tracing", tracing::instrument(level = "trace"))]
    pub fn bytes(&self, bytes: &[u8]) {
        if self.wants(EventTypeFlags::SHARD_PAYLOAD) {
            self.send(Event::ShardPayload(Payload {
                bytes: bytes.to_vec(),
            }))
        }
    }

I expected to see this happen: the lint to not be triggered

Instead, this happened:

clippy output
Posting clippy checks here instead.
error: this let-binding has unit value
   --> gateway/src/shard/emitter.rs:94:39
    |
94  |       pub fn bytes(&self, bytes: &[u8]) {
    |  _______________________________________^
95  | |         if self.wants(EventTypeFlags::SHARD_PAYLOAD) {
96  | |             self.send(Event::ShardPayload(Payload {
97  | |                 bytes: bytes.to_vec(),
98  | |             }))
99  | |         }
100 | |     }
    | |_____^
    |
note: the lint level is defined here
   --> gateway/src/lib.rs:122:5
    |
122 |     clippy::pedantic,
    |     ^^^^^^^^^^^^^^^^
    = note: `#[deny(clippy::let_unit_value)]` implied by `#[deny(clippy::pedantic)]`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value
help: omit the `let` binding
    |
94  |     pub fn bytes(&self, bytes: &[u8]) {
95  |         if self.wants(EventTypeFlags::SHARD_PAYLOAD) {
96  |             self.send(Event::ShardPayload(Payload {
97  |                 bytes: bytes.to_vec(),
98  |             }))
99  |         }
  ...


error: this let-binding has unit value
   --> gateway/src/shard/emitter.rs:104:39
    |
104 |       pub fn event(&self, event: Event) {
    |  _______________________________________^
105 | |         let event_type = EventTypeFlags::from(event.kind());
106 | |
107 | |         if self.wants(event_type) {
108 | |             self.send(event);
109 | |         }
110 | |     }
    | |_____^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value
help: omit the `let` binding
    |
104 |     pub fn event(&self, event: Event) {
105 |         let event_type = EventTypeFlags::from(event.kind());
106 | 
107 |         if self.wants(event_type) {
108 |             self.send(event);
109 |         }
  ...


error: this let-binding has unit value
   --> gateway/src/shard/processor/compression/inflater.rs:50:73
    |
50  |       pub fn msg(&mut self) -> Result<Option<&mut [u8]>, DecompressError> {
    |  _________________________________________________________________________^
51  | |         let length = self.compressed.len();
52  | |
53  | |         // Check if a partial payload was received. If it was, we can just
...   |
120 | |         Ok(Some(&mut self.buffer))
121 | |     }
    | |_____^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value
help: omit the `let` binding
    |
50  |     pub fn msg(&mut self) -> Result<Option<&mut [u8]>, DecompressError> {
51  |         let length = self.compressed.len();
52  | 
53  |         // Check if a partial payload was received. If it was, we can just
54  |         // return that no decompressed message is available.
55  |         if length < 4 || self.compressed[(length - 4)..] != ZLIB_SUFFIX {
  ...


error: this let-binding has unit value
   --> gateway/src/shard/processor/compression/inflater.rs:128:29
    |
128 |       pub fn clear(&mut self) {
    |  _____________________________^
129 | |         self.shrink();
130 | |
131 | |         self.compressed.clear();
132 | |         self.internal_buffer.clear();
133 | |         self.buffer.clear();
134 | |     }
    | |_____^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value
help: omit the `let` binding
    |
128 |     pub fn clear(&mut self) {
129 |         self.shrink();
130 | 
131 |         self.compressed.clear();
132 |         self.internal_buffer.clear();
133 |         self.buffer.clear();
  ...


##[error]Exiting due to clippy errors
Post job cleanup.

This only happens in our CI run, in one crate, on functions that are behind a feature flag. I don't know yet if it's due to run caching (it happens on separate cache keys, on separate repos, so maybe not) or a strangeness in clippy.

Link to an action run: https://github.com/twilight-rs/twilight/pull/1176/checks?check_run_id=3782693317

Meta

Rust versIon: Happens on both stable-x86_64-unknown-linux-gnu unchanged - rustc 1.55.0 (c8dfcfe04 2021-09-06) and the preinstalled "Rust 1.55, Clippy 1.55" that the actions environment gives us
https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-README.md#rust-tools

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the reported diagnostics using gateway/src/shard/emitter.rs and gateway/src/shard/processor/compression/inflater.rs, with the feature-gated code and clippy::pedantic enabled in gateway/src/lib.rs. Compare stable Rust 1.55 and the CI run linked in the issue; done means these functions no longer receive erroneous let_unit_value diagnostics while the lint still reports genuine cases.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
tooling
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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