RS.CLIPPY.EXPECT_FUN_CALL

Using any `expect` method with a function call

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: expect_fun_call. Copyright ©2025 The Rust Team. All rights reserved.

What it does

Checks for calls to .expect(&format!(...)), .expect(foo(..)), etc., and suggests to use unwrap_or_else instead

Why is this bad?

The function will always be called.

Known problems

If the function has side-effects, not calling it will change the semantics of the program, but you shouldn't rely on that anyway.

Example

foo.expect(&format!("Err {}: {}", err_code, err_msg));

// or

foo.expect(format!("Err {}: {}", err_code, err_msg).as_str());

Use instead:

foo.unwrap_or_else(|| panic!("Err {}: {}", err_code, err_msg));