Wrong type mismatch with `cfg_if!` in function body

[jonas-schievink]

This code reports an incorrect "expected MemoryUsage, found ()" type mismatch error:

rust-analyzer/crates/profile/src/memory_usage.rs

Lines 28 to 55 in 0fe3bcf

	cfg_if! {
	if #[cfg(all(feature = "jemalloc", not(target_env = "msvc")))] {
	jemalloc_ctl::epoch::advance().unwrap();
	MemoryUsage {
	allocated: Bytes(jemalloc_ctl::stats::allocated::read().unwrap() as isize),
	}
	} else if #[cfg(all(target_os = "linux", target_env = "gnu"))] {
	memusage_linux()
	} else if #[cfg(windows)] {
	// There doesn't seem to be an API for determining heap usage, so we try to
	// approximate that by using the Commit Charge value.
	use winapi::um::processthreadsapi::*;
	use winapi::um::psapi::*;
	use std::mem::{MaybeUninit, size_of};
	let proc = unsafe { GetCurrentProcess() };
	let mut mem_counters = MaybeUninit::uninit();
	let cb = size_of::<PROCESS_MEMORY_COUNTERS>();
	let ret = unsafe { GetProcessMemoryInfo(proc, mem_counters.as_mut_ptr(), cb as u32) };
	assert!(ret != 0);
	let usage = unsafe { mem_counters.assume_init().PagefileUsage };
	MemoryUsage { allocated: Bytes(usage as isize) }
	} else {
	MemoryUsage { allocated: Bytes(0) }
	}
	}

Might be due to incorrect handling of macros in block tail position.

[jonas-schievink]

Reduced to:

macro_rules! m2 {
    ($($t:tt)*) => {$($t)*};
}

pub fn macrostmts() -> u8 {
    m2! { 0 }

    m2! {}
}

The HIR looks like this:

fn macrostmts(…) {
    { // macro statements
        0
    }
    { // macro statements
    
    }
}

so it seems that an empty MacroStmts expression is treated like an empty block, which is wrong.

[Veykril]

Might've regressed in #12668