deprecated proc_macro trigger warning on build library

[la10736]

Use deprecated attribute on a procedural macro function doesn't work like expected: it triggers a warning message also when library is building and not just in call place.

I know that the deprecate attribute support is not landed in stable but this warning is present in all channels (stable, beta and nightly).

  stable-x86_64-unknown-linux-gnu unchanged - rustc 1.38.0 (625451e37 2019-09-23)
    beta-x86_64-unknown-linux-gnu unchanged - rustc 1.39.0-beta.5 (fa5c2f3e5 2019-10-02)
   nightly-x86_64-unknown-linux-gnu updated - rustc 1.40.0-nightly (421bd77f4 2019-10-06)

Compiling lib simple by cargo +beta build raise follow warning:

   Compiling deprecete_proc_macro v0.1.0 (/home/michele/temp/ppp/deprecete_proc_macro)
warning: use of deprecated item 'deprecable_fn'
 --> src/lib.rs:6:1
  |
6 | pub fn deprecable_fn(input: proc_macro::TokenStream) -> proc_macro::TokenStream { input }
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[warn(deprecated)]` on by default

warning: use of deprecated item 'deprecable_derive'
  --> src/lib.rs:10:1
   |
10 | pub fn deprecable_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream { input }
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: use of deprecated item 'deprecable_attr'
  --> src/lib.rs:14:1
   |
14 | pub fn deprecable_attr(_args: proc_macro::TokenStream, input: proc_macro::TokenStream) -> proc_macro::TokenStream { input }
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    Finished dev [unoptimized + debuginfo] target(s) in 0.70s

Sadly this issue make deprecated attribute not usable at all for procedural macro.

Follow the code to reproduce this issue:

Cargo.toml

[package]
name = "deprecete_proc_macro"
version = "0.1.0"
authors = ["me <myself@gmail.com>"]
edition = "2018"

[lib]
proc_macro=true

src/lib.rs

extern crate proc_macro;

#[deprecated]
#[proc_macro]
pub fn deprecable_fn(input: proc_macro::TokenStream) -> proc_macro::TokenStream { input }

#[deprecated]
#[proc_macro_derive(Deprecable)]
pub fn deprecable_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream { input }

#[deprecated]
#[proc_macro_attribute]
pub fn deprecable_attr(_args: proc_macro::TokenStream, input: proc_macro::TokenStream) -> proc_macro::TokenStream { input }

This issue has been assigned to @XiangQingW via this comment.

[petrochenkov]

The most likely reason is that deprecable_fn and friends are used from the proc macro harness automatically generated for proc macro crates.

So either

• the harness should generate #[allow(deprecated)] (there are precedents for this, e.g. test harness generates #[allow(dead_code)]), or
• deprecated should not be reported from "externally generated code", which usually includes macros from other crates, but also code generated by the compiler.

[petrochenkov]

Adding #[allow(deprecated)] should be a very simple fix - the static item referring to the proc macro definition functions is generated in fn mk_decls in libsyntax_ext/proc_macro_harness.rs.

[XiangQingW]

@rustbot claim