error message found by NLL in librustc_codegen_llvm

[nikomatsakis]

Trying to port librustc_codegen_llvm to use NLL (cc #53172) yields these errors:

https://gist.github.com/memoryruins/14a2aad7fc85d0429ae9e4240ec0dacb

The error seem legitimate. I reduced the pattern in question to this test case:

https://play.rust-lang.org/?gist=6e22b15051f9a108d09f28a0b657f717&version=nightly&mode=debug&edition=2015

As far as I can tell, the Some(child.raw) is interpreted as a Some(&mut *child.raw), which borrows from Child -- but that borrow outlists the variable child. And child has a dtor that frees memory. So something may be legitimately wrong here?

You can solve this by adding a take method that clears the raw field afterwards:

https://play.rust-lang.org/?gist=64c20d529ad49d5871417c391a8dbcde&version=nightly&mode=debug&edition=2015

But I'm not sure what is the intention.

cc @eddyb @irinagpopa

[nikomatsakis]

Putting on the 2018 RC since this blocks #53172.

[nikomatsakis]

Here is the code that is failing to compile:

rust/src/librustc_codegen_llvm/back/archive.rs

Lines 239 to 254 in fbb6275

	for child in archive.iter() {
	let child = child.map_err(string_to_io_error)?;
	let child_name = match child.name() {
	Some(s) => s,
	None => continue,
	};
	if removals.iter().any(|r| r == child_name) {
	continue
	}
	let name = CString::new(child_name)?;
	members.push(llvm::LLVMRustArchiveMemberNew(ptr::null(),
	name.as_ptr(),
	Some(child.raw)));
	strings.push(name);
	}

[eddyb]

What happens if this line is changed from &'a ArchiveChild to &ArchiveChild<'a>?

rust/src/librustc_codegen_llvm/llvm/ffi.rs

Line 1567 in b73535f

Child: Option<&'a ArchiveChild>)

An &ArchiveChild doesn't point into the archive itself, it points to an owned object that itself points to the archive, and LLVMRustArchiveMemberNew copies the ArchiveChild (whereas the current signature suggests it keeps the &ArchiveChild).

[memoryruins]

@eddyb your suggestion was on point :)
fixed in commit: 085535b
merged in rollup: #53297

[nikomatsakis]

I'll close this then.