Add a higher-level API for parsing attributes by scrabsha · Pull Request #155696 · rust-lang/rust

scrabsha · 2026-04-23T18:31:39Z

I got quite frustrated that a lot of the attribute parsing API repeats the same pattern over and over:

let Some(list) = args.list() else {
    cx.expected_list(args, span);
    return None;
}

(Example uses list, but this happens to most of the methods in rustc_attr_parsing::parser actually)

Besides the repetitions, here's why I don't like it:

It separates the checks on the attribute arguments with the error emission
Early returns are explicit and take a lot of space
It is quite easy to accidentally generate an error message that has little to do with the check that has just been done (I already fixed things in 1, 2)
I am convinced that APIs that require too much boilerplate and repetitions give people an incentive to use Automated Code Generation Tools ™️ and I would like to avoid giving this incentive.

This PR adds an AcceptContext::expect_list method, allowing to replace the code in the previous snippet with:

let list = cx.expect_list(args, span)?;

which I believe solves the concerns I wrote earlier.

I already introduced an AcceptContext::expect_single_element_list method that follows similar style in #154827, and ideally I would like this PR to generalize this idea.

I do not expect this higher-level API to replace the methods in rustc_attr_parsing::parser, but rather to provide a higher-level version that can be used in most simple situations. This is explained both in the documentation added in this PR and in #154827 (comment).

In the long term, I also hope we could make AcceptContext::expect_list and friends push custom suggestions to the diagnostics and hopefully stop relying on the attribute template in future.

The first commit introduces AcceptContext::expect_list and AcceptContext::expect_single and uses it where it makes sense to. I added only these two because I figured it would be a nice way to ask for feedback without investing too much time. The second commit adds documentation for the newly added methods.

@JonathanBrouwer I think you're the best person to ask feedback from, let me know what you think!

I also tried to make the parsing functions return a Result<Self, ErrorGuaranteed>, but I don't think it is possible due to Combine::finalize

rustbot · 2026-04-23T18:39:55Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rustbot · 2026-04-24T15:56:32Z

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred to diagnostic attributes.

cc @mejrs

scrabsha · 2026-04-24T16:00:59Z

+// FIXME(scrabsha): once #155696 is merged, update this and mention the higher-level APIs.
 /// Utility that deconstructs a MetaItem into usable parts.
 ///
 /// MetaItems are syntactically extremely flexible, but specific attributes want to parse
 /// them in custom, more restricted ways. This can be done using this struct.
 ///
 /// MetaItems consist of some path, and some args. The args could be empty. In other words:
 ///
 /// - `name` -> args are empty
-/// - `name(...)` -> args are a [`list`](ArgParser::list), which is the bit between the parentheses
+/// - `name(...)` -> args are a [`list`](ArgParser::as_list), which is the bit between the parentheses
 /// - `name = value`-> arg is [`name_value`](ArgParser::name_value), where the argument is the
 ///   `= value` part
 ///
 /// The syntax of MetaItems can be found at <https://doc.rust-lang.org/reference/attributes.html>


It felt wrong to mention AcceptContext::expect_list/AcceptContext::expect_single here given that the other functions don't exist yet. If that's good for you, I'll update this doc comment once all the AcceptContext::expect_* functions are implemented.

scrabsha · 2026-04-24T16:26:01Z

@JonathanBrouwer so this PR renames list -> as_list and single -> as_single. should I immediately rename every method in parser.rs or is it ok if I only rename the other methods as I implement their replacement?

(I don't really have an opinion on what is better, it's up to you ^^)

JonathanBrouwer

@bors r+ rollup

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rust-bors · 2026-04-25T11:00:44Z

📌 Commit 067ef3d has been approved by JonathanBrouwer

It is now in the queue for this repository.

…thanBrouwer Add a higher-level API for parsing attributes

…uwer Rollup of 6 pull requests Successful merges: - #154803 (Fix ICE from cfg_attr_trace ) - #155485 (Add an edge-case test for `--remap-path-prefix` for `rustc` & `rustdoc`) - #155659 (cleanup, restructure and merge `tests/ui/deriving` into `tests/ui/derives`) - #155696 (Add a higher-level API for parsing attributes) - #155734 (Lint doc comments in cfg_select branches) - #155769 (triagebot.toml: Ping Enselic when tests/debuginfo/basic-stepping.rs changes)

…thanBrouwer Add a higher-level API for parsing attributes

…uwer Rollup of 5 pull requests Successful merges: - #154803 (Fix ICE from cfg_attr_trace ) - #155485 (Add an edge-case test for `--remap-path-prefix` for `rustc` & `rustdoc`) - #155659 (cleanup, restructure and merge `tests/ui/deriving` into `tests/ui/derives`) - #155696 (Add a higher-level API for parsing attributes) - #155769 (triagebot.toml: Ping Enselic when tests/debuginfo/basic-stepping.rs changes)

…uwer Rollup of 9 pull requests Successful merges: - #146181 (Add intrinsic for launch-sized workgroup memory on GPUs) - #154803 (Fix ICE from cfg_attr_trace ) - #155065 (Error on invalid macho section specifier) - #155485 (Add an edge-case test for `--remap-path-prefix` for `rustc` & `rustdoc`) - #155659 (cleanup, restructure and merge `tests/ui/deriving` into `tests/ui/derives`) - #155676 ( Reject implementing const Drop for types that are not const `Destruct` already) - #155696 (Add a higher-level API for parsing attributes) - #155769 (triagebot.toml: Ping Enselic when tests/debuginfo/basic-stepping.rs changes) - #155783 (Do not suggest internal cfg trace attributes)

Rollup merge of #155696 - scrabsha:push-kxqstpltlwzn, r=JonathanBrouwer Add a higher-level API for parsing attributes

rustbot added A-attributes Area: Attributes (`#[…]`, `#![…]`) S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 23, 2026

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Comment thread compiler/rustc_attr_parsing/src/context.rs Outdated

Comment thread compiler/rustc_attr_parsing/src/context.rs

Comment thread compiler/rustc_attr_parsing/src/context.rs Outdated

Comment thread compiler/rustc_attr_parsing/src/context.rs Outdated

scrabsha added 2 commits April 24, 2026 15:33

Add a simpler, harder to misuse, attribute parsing API

9aa431c

Add documentation for higher-level attribute parsing API

067ef3d

scrabsha force-pushed the push-kxqstpltlwzn branch from 5c92c1e to 067ef3d Compare April 24, 2026 15:55

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 24, 2026

scrabsha marked this pull request as ready for review April 24, 2026 15:56

scrabsha commented Apr 24, 2026

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JonathanBrouwer approved these changes Apr 25, 2026

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rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 25, 2026

JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request Apr 25, 2026

Rollup merge of rust-lang#155696 - scrabsha:push-kxqstpltlwzn, r=Jona…

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…thanBrouwer Add a higher-level API for parsing attributes

JonathanBrouwer mentioned this pull request Apr 25, 2026

Rollup of 6 pull requests #155773

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JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request Apr 25, 2026

Rollup merge of rust-lang#155696 - scrabsha:push-kxqstpltlwzn, r=Jona…

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…thanBrouwer Add a higher-level API for parsing attributes

JonathanBrouwer mentioned this pull request Apr 25, 2026

Rollup of 5 pull requests #155786

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scrabsha mentioned this pull request Apr 25, 2026

Error when an arg to #[allow_internal_unstable] is a key-value #155787

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JonathanBrouwer mentioned this pull request Apr 25, 2026

Rollup of 9 pull requests #155796

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rust-bors Bot merged commit 97dd613 into rust-lang:main Apr 26, 2026
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rustbot added this to the 1.97.0 milestone Apr 26, 2026

rust-timer added a commit that referenced this pull request Apr 26, 2026

Unrolled build for #155696

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Rollup merge of #155696 - scrabsha:push-kxqstpltlwzn, r=JonathanBrouwer Add a higher-level API for parsing attributes

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