Add inline attr to CString::into_inner so it can optimize out NonNull checks

[elichai]

It seems that currently if you convert any of the standard library's container to a pointer and then to a NonNull pointer, all will optimize out the NULL check except CString(https://godbolt.org/z/YPKW9G5xn),
because for some reason CString::into_inner isn't inlined even though it's a private function that should compile into a simple mov instruction.

Adding a simple #[inline] attribute solves this, code example:

use std::ffi::CString;
use std::ptr::NonNull;

pub fn cstring_nonull(mut n: CString) -> NonNull<i8> {
    NonNull::new(CString::into_raw(n)).unwrap()
}

assembly before:

__ZN3wat14cstring_nonull17h371c755bcad76294E:
	.cfi_startproc
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset %rbp, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register %rbp
	callq	__ZN3std3ffi5c_str7CString10into_inner17h28ece07b276e2878E
	testq	%rax, %rax
	je	LBB0_2
	popq	%rbp
	retq
LBB0_2:
	leaq	l___unnamed_1(%rip), %rdi
	leaq	l___unnamed_2(%rip), %rdx
	movl	$43, %esi
	callq	__ZN4core9panicking5panic17h92a83fa9085a8f73E
	.cfi_endproc

	.section	__TEXT,__const
l___unnamed_1:
	.ascii	"called `Option::unwrap()` on a `None` value"

l___unnamed_3:
	.ascii	"wat.rs"

	.section	__DATA,__const
	.p2align	3
l___unnamed_2:
	.quad	l___unnamed_3
	.asciz	"\006\000\000\000\000\000\000\000\006\000\000\000(\000\000"

Assembly after:

__ZN3wat14cstring_nonull17h9645eb9341fb25d7E:
	.cfi_startproc
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset %rbp, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register %rbp
	movq	%rdi, %rax
	popq	%rbp
	retq
	.cfi_endproc

(Related discussion on zulip: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/NonNull.20From.3CBox.3CT.3E.3E)

[rust-highfive]

r? @kennytm

(rust-highfive has picked a reviewer for you, use r? to override)

[m-ou-se]

Thanks!

@bors r+

[bors]

📌 Commit 45099e6 has been approved by m-ou-se