function-like procedural macros: no dead_code warning on created code (e.g. "function is never used" warning) · Issue #73556 · rust-lang/rust (original) (raw)

Problem: I got no "function is never used" warning (dead_code) on code generated by function-like procedural macros

I dare say this is a bug but I'm fairly new to procedural macros and maybe I'm doing something wrong. After quite a lot of searching I was unable to figure out if that was really the case so I decided to open an issue.

Ref: https://doc.rust-lang.org/reference/procedural-macros.html

Cargo.toml:

$ cat Cargo.toml [package] name = "aux" version = "0.1.0" edition = "2018"

[lib] proc-macro = true

src/lib.rs:

$ cat src/lib.rs use proc_macro::TokenStream; #[proc_macro] pub fn make_dummy(_item: TokenStream) -> TokenStream { "fn dummy() {}".parse().unwrap() }

src/main.rs:

$ cat src/main.rs aux::make_dummy!(); fn dummy2() {} fn main() { println!("hello"); }

Expected: with the previous code I expected two function is never used warnings:

The first one for the generated fn dummy() {} code
The second one for fn dummy2() {} code

Instead: I just got a warning on dummy2() but nothing on dummy():

$ cargo build Compiling aux v0.1.0 (/var/tmp/aux) warning: function is never used: dummy2 --> src/main.rs:2:4 | 2 | fn dummy2() {} | ^^^^^^ | = note: #[warn(dead_code)] on by default

warning: 1 warning emitted

Finished dev [unoptimized + debuginfo] target(s) in 0.23s

Expanded code: both dummy() and dummy2() are there, but there's only a build warning on dummy2():

$ cargo expand --bin aux Checking aux v0.1.0 (/var/tmp/aux) Finished check [unoptimized + debuginfo] target(s) in 0.06s

#![feature(prelude_import)] #[prelude_import] use std::prelude::v1::*; #[macro_use] extern crate std; fn dummy() {} fn dummy2() {} fn main() { { ::std::io::_print(::core::fmt::Arguments::new_v1( &["hello\n"], &match () { () => [], }, )); }; }

rustc version: I checked with the following versions of the compiler with same result:

rustc 1.44.1:

$ rustc --version --verbose
rustc 1.44.1 (c7087fe00 2020-06-17)
binary: rustc
commit-hash: c7087fe00d2ba919df1d813c040a5d47e43b0fe7
commit-date: 2020-06-17
host: x86_64-unknown-linux-gnu
release: 1.44.1
LLVM version: 9.0

rustc 1.46.0-nightly:

$ rustc --version --verbose
rustc 1.46.0-nightly (feb3536eb 2020-06-09)
binary: rustc
commit-hash: feb3536eba10c2e4585d066629598f03d5ddc7c6
commit-date: 2020-06-09
host: x86_64-unknown-linux-gnu
release: 1.46.0-nightly
LLVM version: 10.0