Lint: function calls when using .expect on Result should be unwrap_or_else

What it does

Catches use of non-const function calls (like the format macro) within a expect method call on Result.

This should be replaced by unwrap_or_else, in order to avoid work being performed on the happy path.

Lint Name

function_call_in_expect

Category

perf

Advantage

• The function called in the .expect call will be evaluated even when the method will not panic
• Using unwrap_or_else avoids evaluation unless the method is called

This is particularly relevant when working with allocating functions / macros, like format!

Drawbacks

This is a performance micro-optimization, and makes your code slightly uglier.

Detecting whether or not a function is being used in a const way (as opposed to "if it could be const") may not be trivial. If this can't be done, then there are false positives when const functions being called within the .expect.

Example

        .expect(bevy_manifest.get_path(crate::BEVY_CREVICE))

Could be written as:

        .unwrap_or_else(|| bevy_manifest.get_path(crate::BEVY_CREVICE))

Start by searching the rust-clippy repository for function_call_in_expect and related Result or .expect lint tests. Use the supplied .expect and .unwrap_or_else examples to define the expected diagnostic, and investigate how to distinguish non-const calls from const calls. Done means the lint handles the intended performance case without false positives from const calls.