Description
What it does
Lint should detect if we create a temporary variable that is dropped without usage (or side-effects).
See example below for creating a String which should have been assigned to the shadowed name, but instead does nothing.
Categories (optional)
- Kind:
clippy::correctness
What is the advantage of the recommended code over the original code
Original code creates a temporary String, which has the correct transformation applied but is then dropped, and the .len() is called on the original &str. This is not the intended behaviour but there are no warnings or errors.
The new code correctly assigns the temporary variable (to shadow the same username variable), which is then used.
The lint could just check the that variable is assigned and used later in the code.
Drawbacks
How can we distinguish if a method called on the temporary variable has side effects, and that is what was intended? Perhaps only apply for standard library types and methods.
Example
validate_with(|username: &str| -> Result<(), &str> {
// Here String is immediately dropped
username.to_string().retain(|c| !c.is_whitespace() && c.is_digit(10));
if username.len() != 16 { // This erroneously checks against original &str
return Err("Mullvad account number should be 16 digits!");
}
Ok(())
})Could be written as:
validate_with(|username: &str| -> Result<(), &str> {
// Now the temporary String is assigned and used
let username = username.to_string().retain(|c| !c.is_whitespace() && c.is_digit(10));
if username.len() != 16 { // Checks new String as intended
return Err("Mullvad account number should be 16 digits!");
}
Ok(())
})