#![crate_name = EXPR] semantically allows EXPR to be a macro call but otherwise mostly ignores it

[fmease]

Contrary to #![crate_type = EXPR], #![crate_name = EXPR] does not semantically reject macro calls inside EXPR.
Instead, it eagerly expands them but otherwise ignores the result (apart from errors).

I presume this regressed when #78835 ( F-extended_key_value_attributes `#![feature(extended_key_value_attributes)] ) was stabilized.
If so, this is a 1.53→1.54 stable-to-stable regression.
Update: It's a 1.75→1.76 stable-to-stable regression. Regressing PR: #117584.
Thanks ehuss, for the investigation! The result of the expansion used to be used!

Examples

The following examples all pass compilation and rustc completely ignores the crate name that comes from the expansion. This can be observed by for example running rustc file.rs --print=crate-name prints file (the file name is assumed to be file.rs).

(A)

#![crate_name = concat!("alia", "s")] // ignored, crate name is `file`, not `alias`
fn main() {}

(B)

#![crate_name = include_str!("crate_name.txt")] // ignored, crate name is `file`, not `alias`
fn main() {}

where crate_name.txt exists and consists of alias.

(C)

#![crate_name = dep::generate!()] // ignored, crate name is `file`, not `alias`
fn main() {}

where we compile file.rs with --extern=dep -L. --edition=2021 and where dep.rs is:

#[macro_export]
macro_rules! generate { () => { "alias" } }

[fmease]

Needs a future-incompat warn-by-default lint as a first step.

[ehuss]

😦 Not sure how I missed this one in #88680.

[fmease]

@ehuss Don't beat yourself up, nobody else noticed it either ^^.

Oh, I've just double-checked the dates, your validation PR was merged Sep 26, 2021 which is after the aforementioned stabilization PR, merged May 19, 2021. I've only skimmed the thread but it seems to have been FCP'ed because it was a breaking change?

Maybe we can even get away with making macro calls inside #![crate_name] a hard error after an FCP? Well, almost three years have passed since EKVA's stab PR but on the other hand, I presume that very few people actually use #![crate_name] since the majority just uses Cargo.

AFAIK, crater only supports Cargo projects (makes sense), so we can't use it to look for breakages.
However, GitHub Code Search returns zero matches given the regex #!\[crate_name\s*=\s*[\w_:]+ ... so there's hope.

[ehuss]

Oh wait, this used to work. It regressed relatively recently in #117584. cc @bjorn3

We could hard error it if nobody is using it. Before doing that, I think we'd need a good motivation to not support it (such as, is the crate_name needed before expansion?).

[fmease]

We could hard error it if nobody is using it. Before doing that, I think we'd need a good motivation to not support it (such as, is the crate_name needed before expansion?).

I haven't checked the code yet but I don't know if rustc's current architecture permits macro expansion for --print=crate-name. This relates to the deprecation of #![cfg_attr(SPEC, crate_name = "...")] and #![cfg_attr(SPEC, crate_type = "...")] (#91632), I think.

Well, we could theoretically expand macros for --print=crate-name but that would also be a breaking change since #![crate_name = compile_error!("...")] currently compiles successfully under --print=crate-name because it only parses the crates I believe and if we were to start expanding, this would obviously regress. Note that contrary to --print=crate-name, --print=link-args for example does parse & expand (& typeck & compile) the crate but that's an outlier under all print request flags.

Edit: Clarification: The CLI flag --print=crate-name is just a representative for rustc's internal understanding of the name of the local crate. Oh well, in 1.75 all examples used to work “as expected” while --print=crate-name didn't, so the struck-through sentence isn't grounded in reality.

[ehuss]

Error for example (A) in Rust 1.75:

Hm, I'm not getting the same result. When I compile A with 1.75, it generates a binary alias. With 1.76, it generates a binary named based on the source filename.

rustc's current architecture permits macro expansion for --print=crate-name

Oh, that's interesting. Yea, --print=crate-name does not seem to work for eager expansion in any version.

[fmease]

Hm, I'm not getting the same result. When I compile A with 1.75, it generates a binary alias. With 1.76, it generates a binary named based on the source filename.

Oh 🤦, I tested this with #![crate_name = concat!()]. I can confirm that with 1.75 the result of the expansion used to get used.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high

[fmease]

I nominate this for T-compiler discussion since I wonder if it was ever intentional to (correctly) support #![crate_name = MACRO_CALL] (i.e., use the result of the expansion) between versions 1.54 (stabilization of EKVA¹) and 1.75 inclusive (the expansion gets ignored since PR #117584 / 1.76 which is definitely a bug) or if #![crate_name = MACRO_CALL] should've always lead to a semantic error similar to #![crate_type = MACRO_CALL].

Possibly relevant to the discussion:

• Tracking issue for future-incompatibility lint deprecated_cfg_attr_crate_type_name #91632
• CLI option --print=crate-name

This might be a T-lang question at its core but I figure it makes sense for T-compiler to chime in first since its members might have more background information on this (compiler stages / passes, architectural limitations, when to expand when not to for simplicity of implementation).

Footnotes

1. #![feature(extended_key_value_attributes)]. ↩

[pnkfelix]

@rustbot label: +I-lang-nominated

I think the question of whether we ever intended to support #![crate_name = MACRO_CALL] belongs under T-lang rather than T-compiler.

[petrochenkov]

There's a list of attributes that need to be used very early, before macro expansion - #108221 (comment) (also see uses of pre_configured_attrs in the compiler).
In such attributes #![attr = mac_call!()] cannot be supported.

There are also some proposals that may reduce the number of attributes in this list, like rust-lang/compiler-team#726.

[scottmcm]

I think that intentionally-breaking this, so that it works like crate_type would make sense. Let's see if lang agrees:

@rfcbot fcp merge

[rfcbot]

Team member @scottmcm has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[nikomatsakis]

@rfcbot reviewed

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[tmandry]

@rfcbot reviewed

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.