Weird output about spills and variables

[matthiaskrgr]

I tried this code:

const SZ: usize = 64_000_000;
type BigDrop = [String; SZ];

fn f(from_fn: BigDrop) {}

fn f2(_moveme: BigDrop) -> String {
    let [a, ..] = _moveme;
    a
}

fn main() {
    f(std::array::from_fn(|_| String::new()));
    f2(std::array::from_fn(|_| String::new()));
}

I expected to see this happen: explanation

Instead, this happened: explanation

Meta

rustc --version --verbose:

rustc 1.71.0-nightly (b628260df 2023-04-22)
binary: rustc
commit-hash: b628260df0587ae559253d8640ecb8738d3de613
commit-date: 2023-04-22
host: x86_64-unknown-linux-gnu
release: 1.71.0-nightly
LLVM version: 16.0.2

If you compile this you will get very interesting output

warning: unused variable: `from_fn`
 --> B3A5F4060827522A3328FC59CAE6348272F5F8B0627152DED589D0B44CBBDBB5.rs:6:6
  |
6 | fn f(from_fn: BigDrop) {}
  |      ^^^^^^^ help: if this is intentional, prefix it with an underscore: `_from_fn`
  |
  = note: `#[warn(unused_variables)]` on by default

80/9216000120 (0.00%) spills, 9216000040/9216000120 (100.00%) variables
80/9216000120 (0.00%) spills, 9216000040/9216000120 (100.00%) variables
warning: 1 warning emitted

I could not find any traces of this inside the rust source code, it seems to be coming from llvm directly?! Some leftover debug print maybe?

(llvm git: llvm/lib/CodeGen/PrologEpilogInserter.cpp:316: dbgs() << formatv("{0}/{1} ({3:P}) spills, {2}/{1} ({4:P}) variables",)

[Ezrashaw]

Shall I add a UI test for this so we can track it?

How on earth did this get through LLVM's review process? LLVM isn't meant to print output at all when invoked by rustc. 🤣

@rustbot claim
(I'll see if I can get a fix upstream)

[rustbot]

Error: Parsing assign command in comment failed: ...' claim' | error: expected end of command at >| ' (I'll see'...

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[matthiaskrgr]

another one:

#![allow(invalid_value)]

fn main() {
    // The array avoids a `Scalar` layout which detects uninit without even doing validation.
    let _val = unsafe { std::mem::MaybeUninit::<[*const u8; 0xABCDABCD]>::uninit().assume_init() };
    //~^ ERROR: uninitialized
}

[Ezrashaw]

This is fixed upstream in LLVM now, so at some point this bug will just disappear.

[Ezrashaw]

@matthiaskrgr This is now fixed in nightly and stable. This issue can be closed.