Consider the following code (playground):

fn main() {
    let err = Err(());
    let _: usize = err.unwrap_or_else(|err| err_exit(err));
    unreachable!();
}

fn err_exit(_: ()) -> ! {
    std::process::exit(1);
}

When compiled with rustc 1.36, it gives the following assembly:

core::result::Result<T,E>::unwrap_or_else:
	pushq	%rax
	callq	playground::main::{{closure}}
	ud2

playground::main:
	pushq	%rax
	callq	core::result::Result<T,E>::unwrap_or_else
	ud2

playground::main::{{closure}}:
	pushq	%rax
	callq	playground::err_exit
	ud2

playground::err_exit:
	pushq	%rax
	movl	$1, %edi
	callq	*std::process::exit@GOTPCREL(%rip)
	ud2

Note how the closure is not inlined, even though it would be trivial to do so (replace callq playground::main::{{closure}} with callq playground::err_exit).

There was already a bug report at https://bugs.llvm.org/show_bug.cgi?id=26495

I commented there with what I found out so far.