erroneous note: erroneous constant used emitted for non-compiling format_args! usage

I tried this code:

format_args!("{:?}", { let [_a] = *(&[] as &[Vec<i32>]) else { return }; });

I expected to see this happen: an error about not being able to move out of a slice

Instead, this happened: the error and a bunch of note: erroneous constant used

error[E0508]: cannot move out of type `[Vec<i32>]`, a non-copy slice
 --> src/main.rs:2:35
  |
2 | format_args!("{:?}", { let [_a] = *(&[] as &[Vec<i32>]) else { return }; });
  |                             --    ^^^^^^^^^^^^^^^^^^^^^ cannot move out of here
  |                             |
  |                             data moved here
  |                             move occurs because `_a` has type `Vec<i32>`, which does not implement the `Copy` trait
  |
help: consider borrowing the pattern binding
  |
2 | format_args!("{:?}", { let [ref _a] = *(&[] as &[Vec<i32>]) else { return }; });
  |                             +++

note: erroneous constant used
 --> src/main.rs:2:14
  |
2 | format_args!("{:?}", { let [_a] = *(&[] as &[Vec<i32>]) else { return }; });
  |              ^^^^^^

note: erroneous constant used
 --> src/main.rs:2:37
  |
2 | format_args!("{:?}", { let [_a] = *(&[] as &[Vec<i32>]) else { return }; });
  |                                     ^^^

note: erroneous constant used
 --> src/main.rs:2:22
  |
2 | format_args!("{:?}", { let [_a] = *(&[] as &[Vec<i32>]) else { return }; });
  |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this note originates in the macro `format_args` (in Nightly builds, run with -Z macro-backtrace for more info)

Meta

Fails on 1.71.0-nightly (2023-04-25 458d4dae845ec155b285), works fine on 1.70.0-beta.1 (2023-04-19 1b7dd2252b99671ce5d1)

It also applies to way simpler code like: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=aeadbb4b94770923ec61da515e15e45c

    let mut x = String::from("hello");
    let z = &x;
    let y = &mut x;
    println!("{y}{z}");

error[[E0502]](https://doc.rust-lang.org/nightly/error_codes/E0502.html): cannot borrow `x` as mutable because it is also borrowed as immutable
 --> src/main.rs:4:13
  |
3 |     let z = &x;
  |             -- immutable borrow occurs here
4 |     let y = &mut x;
  |             ^^^^^^ mutable borrow occurs here
5 |     println!("{y}{z}");
  |                  --- immutable borrow later used here

note: erroneous constant used
 --> src/main.rs:5:14
  |
5 |     println!("{y}{z}");
  |              ^^^^^^^^

@cjgillot

searched nightlies: from nightly-2023-04-01 to nightly-2023-04-26
regressed nightly: nightly-2023-04-22
searched commit range: 8bdcc62...fec9adc
regressed commit: fa4cc63

bisected with cargo-bisect-rustc v0.6.6

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc 2023-04-01 --prompt --access github

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc 2023-04-01 --prompt --access github

#110107 @cjgillot

Intestingly, the same day also contained two format_args PRs but I have bisected this twice (with manual promt) and const prop lint really does seem to be the cause:

commit[0] 2023-04-20: Auto merge of #110616 - m-ou-se:fmt-lang-items, r=jyn514
commit[1] 2023-04-20: Auto merge of #109999 - m-ou-se:flatten-format-args, r=oli-obk

@rustbot

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

Rollup merge of rust-lang#111038 - tmiasko:untainted-promoteds, r=com…

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erroneous `note: erroneous constant used` emitted for non-compiling `format_args!` usage #110856

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erroneous note: erroneous constant used emitted for non-compiling format_args! usage #110856

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Noratrieb commented on Apr 26, 2023

Noratrieb commented on Apr 26, 2023

apiraino commented on Apr 27, 2023

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erroneous `note: erroneous constant used` emitted for non-compiling `format_args!` usage #110856