Module path attribute override with nested macros doesnt work

@jyn514

I tried this code:

Repository with the reproduction

I expected to see this happen:

The code should compile without errors, because this feature was advertised in Rust 1.54 announcement and in #83366.

Instead, this happened:

Compile error file not found for module generated was issued

More info in README.md of the reproduction repo

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rustc --version --verbose:

rustc 1.54.0 (a178d0322 2021-07-26)
binary: rustc
commit-hash: a178d0322ce20e33eac124758e837cbd80a6f633
commit-date: 2021-07-26
host: x86_64-unknown-linux-gnu
release: 1.54.0
LLVM version: 12.0.1

cc @jyn514

I can confirm this doesn't work, and is a bit unfortunate it slipped through (I admit I didn't test it).

From what I can tell, the extended attr=expr expansion happens during ~~lowering to HIR~~ (no, I don't know how that works), but #[path] is needed during expansion before that happens. The check here silently ignores the attribute because the value is not a string (it is a macro expression).

I don't think there is any true support for eager expansion, so I don't think there is any existing support for something like this. At a bare minimum, the #[path] attribute should generate an error if the value is not a string literal.

I think we should maybe update the release post, since it is misleading.

@ehuss

@ehuss do you think adding support for this eager expansion is feasible / welcome? I'd be interested in hacking on it if it's not terribly difficult. I also really want this feature for another project I maintain.

Unfortunately I think it might be difficult to support. #[path] would likely need special handling because it is needed during expansion. I am not on the compiler or language teams, so I'm not the one to say definitively.

@petrochenkov

cc @petrochenkov @jyn514 -- I would have expected this to work based on discussions when landing this feature, so I'm a little surprised that it doesn't.

@Mark-Simulacrum

@Mark-Simulacrum

I would have expected this to work based on discussions when landing this feature, so I'm a little surprised that it doesn't.

Internally "mod m; -> mod m { /* contents */ }" effectively works as "#[include_it] mod m; -> mod m { /* contents */ }", i.e. it's an expansion of the primary item m, which is performed before any nested expansions (including expansions inside attributes).

At a bare minimum, the #[path] attribute should generate an error if the value is not a string literal.

+1

Note: It doesn't work at all, even if the macros aren't nested. For example #[path = concat!("file.rs")] doesn't work either.

There is also the issue #48250 which is pretty much the same as this. Also the RELEASES.md should be changed to reflect that this doesn't work, it currently states that it does:

You can also use this to include auto-generated modules:
#[path = concat!(env!("OUT_DIR"), "/generated.rs")]
mod generated;

Change bindings inclusion

Posted #88057 to update the RELEASES, thanks for catching that!

Rollup merge of rust-lang#88057 - ehuss:releases-doc-macros, r=Mark-S…

Posted #88680 to add errors when attempting to use macros with #[path].

Rust 1.54 release notes should be updated to say that this doesn't work. It currently says Read here for more details with here linking to #83366 which says that this is supported syntax.

@malaire

@malaire I have updated the description of #83366 to remove the mentions of #[path].

Should we consider this issue resolved then?

I think it should work, imo it's a bug that it doesn't.

Current example error message for searchability:

error: malformed `path` attribute input
 --> src/main.rs:1:1
  |
1 | #[path = concat!(env!("OUT_DIR"), "/zbus_xmlgen_output.rs")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: must be of the form: `#[path = "file"]`

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Module path attribute override with nested macros doesnt work #87681

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Module path attribute override with nested macros doesnt work #87681

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ehuss commented on Aug 1, 2021

danobi commented on Aug 2, 2021

ehuss commented on Aug 2, 2021

Mark-Simulacrum commented on Aug 2, 2021

petrochenkov commented on Aug 2, 2021

N3xed commented on Aug 15, 2021

ehuss commented on Aug 15, 2021

ehuss commented on Sep 6, 2021

malaire commented on Dec 4, 2021

ehuss commented on Dec 4, 2021

Veetaha commented on Mar 31, 2022

jyn514 commented on Mar 31, 2022

Nemo157 commented on Nov 26, 2023

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