False positive dead code warning

[alexkazik]

When code is generated via a proc macro there is a case which creates a false positive for the dead code warning.

Proc macro (src/lib.rs):

use proc_macro2::Span;
use quote::quote;
use syn::spanned::Spanned;
use syn::{parse_macro_input, DeriveInput, Ident};

#[proc_macro_derive(Test)]
pub fn proc_dead_code_test(tokens: proc_macro::TokenStream) -> proc_macro::TokenStream {
    let input = parse_macro_input!(tokens as DeriveInput);
    let repr = Ident::new("i8", input.span());
    let repr_call_size = Ident::new("i8", Span::call_site());
    let vis = input.vis;
    quote! {
        impl MyEnum {
            #vis fn false_positive(self) -> #repr {
                self as i8
            }
            pub(crate) fn ok_1(self) -> #repr {
                self as i8
            }
            #vis fn ok_2(self) -> i8 {
                self as i8
            }
            #vis fn ok_3(self) -> #repr_call_size {
                self as i8
            }
        }
    }
    .into()
}

Test (tests/all.rs):

use proc_dead_code_test::Test;

#[allow(dead_code)]
#[derive(Test)]
#[repr(i8)]
pub(crate) enum MyEnum {
    MyValue,
}

The current output is:

warning: associated function `false_positive` is never used
 --> tests/all.rs:4:10
  |
4 | #[derive(Test)]
  |          ^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `proc_dead_code_test` (test "all") generated 1 warning

The generated code for all four functions are identical (except the name, obviusly), tested with cargo expand.

One is reported as dead code, though no report should be given since #[allow(dead_code)] is used.

On 1.63.0 this is not a warning, but with 1.65.0-beta.1 it is.

I'm aware that this is a very special case and I don't mind if it doesn't get fixed but I thought I'll report it anyway.

For a quick copy-and-paste run here the Cargo.toml:

[package]
name = "proc_dead_code_test"
version = "0.1.0"
edition = "2021"

[lib]
proc-macro = true

[dependencies]
syn = { version = "1.0.99", features = ["derive", "parsing", "extra-traits"] }
quote = "1.0.21"
proc-macro2 = "1.0.43"

(I don't know how to setup play.rust-lang.org to create a proc-macro and a real test, thats why it's missing.)

[vincenzopalazzo]

We discuss this in the wg-macros triage meeting and the conclusion was that we would like to fix this warning, but there are no enough people interested currently in this area.

So if someone is interested in working on it, please open a discussion on our Zulip stream #wg-macros

[mu001999]

I tested this locally and found this has been fixed, cargo-bisect-rustc shows:

********************************************************************************
Regression in nightly-2022-10-03
********************************************************************************

fetching https://static.rust-lang.org/dist/2022-10-02/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2022-10-02: 40 B / 40 B [==========================================================] 100.00 % 215.77 KB/s converted 2022-10-02 to 57f097ea25f2c05f424fc9b9dc50dbd6d399845c
fetching https://static.rust-lang.org/dist/2022-10-03/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2022-10-03: 40 B / 40 B [==========================================================] 100.00 % 339.06 KB/s converted 2022-10-03 to 092255976862c1f0121bb4446acd38336a860c4f
looking for regression commit between 2022-10-02 and 2022-10-03
fetching (via remote github) commits from max(57f097ea25f2c05f424fc9b9dc50dbd6d399845c, 2022-09-30) to 092255976862c1f0121bb4446acd38336a860c4f
ending github query because we found starting sha: 57f097ea25f2c05f424fc9b9dc50dbd6d399845c
get_commits_between returning commits, len: 17
  commit[0] 2022-10-01: Auto merge of #102236 - cjgillot:compute_lint_levels_by_def, r=oli-obk
  commit[1] 2022-10-01: Auto merge of #102193 - ferrocene:pa-remote-test-server-improvements, r=Mark-Simulacrum
  commit[2] 2022-10-02: Auto merge of #102558 - matthiaskrgr:rollup-0odec1c, r=matthiaskrgr
  commit[3] 2022-10-02: Auto merge of #102424 - sunfishcode:sunfishcode/hidden-main, r=nagisa
  commit[4] 2022-10-02: Auto merge of #102535 - scottmcm:optimize-split-at-partition-point, r=thomcc
  commit[5] 2022-10-02: Auto merge of #102548 - nikic:inline-cell-replace, r=scottmcm
  commit[6] 2022-10-02: Auto merge of #98354 - camsteffen:is-some-and-by-value, r=m-ou-se
  commit[7] 2022-10-02: Auto merge of #102586 - Dylan-DPC:rollup-g107h6z, r=Dylan-DPC
  commit[8] 2022-10-02: Auto merge of #102530 - cuviper:upgrade-i586-gnu, r=pietroalbini
  commit[9] 2022-10-02: Auto merge of #100557 - dawnofmidnight:tarball-commit-info, r=Mark-Simulacrum
  commit[10] 2022-10-02: Auto merge of #102305 - flba-eb:remove_exclude_list, r=Mark-Simulacrum
  commit[11] 2022-10-03: Auto merge of #102503 - cuviper:x86-stack-probes, r=nagisa
  commit[12] 2022-10-03: Auto merge of #102508 - nnethercote:even-more-lexer-improvements, r=matklad
  commit[13] 2022-10-03: Auto merge of #102614 - matthiaskrgr:rollup-fjyu7oo, r=matthiaskrgr
  commit[14] 2022-10-03: Auto merge of #102551 - bjorn3:cg_ssa_cleanup, r=davidtwco
  commit[15] 2022-10-03: Auto merge of #102610 - dawnofmidnight:git-commit-hash, r=Mark-Simulacrum
  commit[16] 2022-10-03: Auto merge of #102627 - matthiaskrgr:rollup-2xtrqkw, r=matthiaskrgr

I think it might be #102538.