the advice for unused return value of `std::option::Option::<T>::map_or` is wrong

Summary

before:

error: unused return value of `std::option::Option::<T>::map_or` that must be used
   --> src/xxx.rs:119:9
    |
119 | /         self.m.get_mut(&key).map_or((), |series| {
120 | |             if let Some(last_elem) = series.last() {
121 | |                 if last_elem.timestamp == timestamp {
122 | |                     series.pop();
...   |
125 | |             series.push(Value { value, timestamp });
126 | |         });
    | |__________^
    |
    = note: if you don't need the returned value, use `if let` instead
    = note: `-D unused-must-use` implied by `-D warnings`
help: use `let _ = ...` to ignore the resulting value
    |
119 |         let _ = self.m.get_mut(&key).map_or((), |series| {
    |         +++++++

after:

error: this let-binding has unit value
   --> src/xxx.rs:119:9
    |
119 | /         let _ = self.m.get_mut(&key).map_or((), |series| {
120 | |             if let Some(last_elem) = series.last() {
121 | |                 if last_elem.timestamp == timestamp {
122 | |                     series.pop();
...   |
125 | |             series.push(Value { value, timestamp });
126 | |         });
    | |___________^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value
    = note: `-D clippy::let-unit-value` implied by `-D warnings`
help: omit the `let` binding
    |
119 ~         self.m.get_mut(&key).map_or((), |series| {
120 +             if let Some(last_elem) = series.last() {
121 +                 if last_elem.timestamp == timestamp {
122 +                     series.pop();
123 +                 }
124 +             }
125 +             series.push(Value { value, timestamp });
126 +         });

Reproducer

I tried this code:

pub struct TimeMap2 {
    m: HashMap<String, Vec<Value>>,
}

impl TimeMap2 {
    pub fn new() -> Self {
        Self { m: HashMap::new() }
    }

    pub fn set(&mut self, key: String, value: String, timestamp: i32) {
        if !self.m.contains_key(&key) {
            self.m.insert(key.clone(), Vec::new());
        }

        self.m.get_mut(&key).map_or((), |series| {
            if let Some(last_elem) = series.last() {
                if last_elem.timestamp == timestamp {
                    series.pop();
                }
            }
            series.push(Value { value, timestamp });
        });
    }

    pub fn get(&self, key: String, timestamp: i32) -> String {
        if let Some(series) = self.m.get(&key) {
            if series.is_empty() {
                return String::new();
            }
            return match series.binary_search_by_key(&timestamp, |v| v.timestamp) {
                Ok(idx) => series[idx].value.clone(),
                Err(idx) => {
                    if let Some(val) = series.get(idx - 1) {
                        return val.value.clone();
                    }
                    String::new()
                }
            };
        }
        String::new()
    }
}

I expected to see this happen:

Instead, this happened:

Version

rustc 1.71.0 (8ede3aae2 2023-07-12)
binary: rustc
commit-hash: 8ede3aae28fe6e4d52b38157d7bfe0d3bceef225
commit-date: 2023-07-12
host: x86_64-unknown-linux-gnu
release: 1.71.0
LLVM version: 16.0.5

Additional Labels

No response

Start with the reproducer and compare the two diagnostic outputs for the map_or call. Trace the lint that emits the unused-return-value suggestion and its interaction with let_unit_value; done means the advice no longer recommends a change that triggers the conflicting lint, with coverage for this example.