Rust: Running rustdoc on code using quote generates false lint warnings for unused_braces

[shepmaster]

I tried this code:

[package]
name = "repro"
version = "0.1.0"
edition = "2018"

[dependencies]
quote = "=1.0.3"

use quote::quote;

pub fn repro() {
    let many = [1, 2, 3];

    let _together = quote! {
        #(#many),*
    };
}

I think this is a basic and common usage of the quote crate.

I expected to see no warnings:

% RUSTFLAGS=-Dwarnings cargo +nightly-2020-04-04 build
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s

% RUSTFLAGS=-Dwarnings cargo +nightly-2020-04-04 doc
 Documenting repro v0.1.0 (/private/tmp/repro)
    Finished dev [unoptimized + debuginfo] target(s) in 0.82s

Instead, warnings are generated (and my CI build fails due to the deny):

% RUSTFLAGS=-Dwarnings cargo +nightly-2020-04-04 build
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s

% RUSTFLAGS=-Dwarnings cargo +nightly-2020-04-04 doc
 Documenting repro v0.1.0 (/private/tmp/repro)
warning: unnecessary braces around block return value
  |
  = note: `#[warn(unused_braces)]` on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.82s

Note that the warning only appears when running rustdoc.

Meta

rustc +nightly-2020-04-04 --version --verbose:

rustc 1.44.0-nightly (74bd074ee 2020-04-03)
binary: rustc
commit-hash: 74bd074eefcf4915c73d1ab91bc90859664729e6
commit-date: 2020-04-03
host: x86_64-apple-darwin
release: 1.44.0-nightly
LLVM version: 9.0

[shepmaster]

/cc @lcnr

[eddyb]

Note that the warning only appears when running rustdoc.

Hang on, could this be because of the transformation rustdoc uses (AFAIK) that replaces fn bodies with loop {}?

[Nemo157]

Simplified repro source, both inner blocks are necessary for some reason:

pub fn repro() {
    {
        {
            use std;
        }
    }
}

[eddyb]

@Nemo157 My guess it's because {{}} is basically {()} - the outer block has the inner block as its final expression. Curious that this is rustdoc-specific.

[tmandry]

I just hit a similar error message in rustc (not rustdoc). Unused braces lint with no span or other information. I don't have a reproducer currently, since it's from a big crate and I don't have much to go by.

[grantperry]

I'm getting unnescessary braces around block return value from check/clippy

warning: unnecessary braces around block return value
   --> src/models/user.rs:137:63
    |
137 |     fn ticket(&self, context : &SharedContext) -> Option<Ticket> { self.get_ticket(context) }
    |                                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove these braces

Is this related or should I create a new issue?

Using Nightly 1.44.0 2020-04-16

[lcnr]

@grantperry is this inside a macro?

[grantperry]

@lcnr yes it is. Sounds like it's something you already know about?

[lcnr]

If you are using a proc macro using quote then this should be the same issue afaict

[lcnr]

I tried to fix this by checking if the surrounding expression comes from an expansion, but this didn't actually fix this. :thinking: @matklad do you have an idea/can this be fixed in quote?

My approach stilled warned for the following, which seems to be the same issue afaict.

https://github.com/rust-lang/rust/blob/4ca5fd2d7b6b1d75b6cb8f679e8523fb3e7b19e2/src/librustc_macros/src/query.rs#L362-L370

[matklad]

I don't think I am familiar with the code in question @lcnr

[lcnr]

Sorry, mb :sweat: meant @dtolnay as he is the owner of quote.

[dtolnay]

Labeling regression-from-stable-to-nightly. I would consider accepting a workaround in quote if somebody who is not me sends a PR and it is a very simple fix, but really this needs to be fixed in rustdoc / rustc.

[LeSeulArtichaut]

Let’s try to find the cause of the regression
@rustbot ping cleanup

[rustbot]

Error: Only Rust team members can ping teams.

Please let @rust-lang/release know if you're having trouble with this bot.

[Dylan-DPC]

@rustbot ping cleanup

[rustbot]

Hey Cleanup Crew ICE-breakers! This bug has been identified as a good
"Cleanup ICE-breaking candidate". In case it's useful, here are some
[instructions] for tackling these sorts of bugs. Maybe take a look?
Thanks! <3

cc @AminArria @chrissimpkins @contrun @DutchGhost @elshize @ethanboxx @h-michael @HallerPatrick @hdhoang @hellow554 @imtsuki @jakevossen5 @kanru @KarlK90 @LeSeulArtichaut @MAdrianMattocks @matheus-consoli @mental32 @nmccarty @Noah-Kennedy @pard68 @PeytonT @pierreN @Redblueflame @RobbieClarken @RobertoSnap @robjtede @SarthakSingh31 @senden9 @shekohex @sinato @spastorino @turboladen @woshilapin @yerke

[AminArria]

searched nightlies: from nightly-2020-03-29 to nightly-2020-04-05
regressed nightly: nightly-2020-04-02
searched commits: from https://github.com/rust-lang/rust/commit/a5b09d35473615e7142f5570f5c5fad0caf68bd2 to https://github.com/rust-lang/rust/commit/76b11980ad416c3ad6143504c2277757ecacf9b5
regressed commit: https://github.com/rust-lang/rust/commit/58dd1ce8383aaebcad9b6027b89a316fd868b35c

I guess the PR would be #70081

[shepmaster]

I'm sorry, that was my bad for not including that information in the original comment. We already knew the cause of the regression (found in #70717), and the author of the regression is already aware (@lcnr) of the issue, but not yet a fix.

[spastorino]

Assigning P-medium as discussed as part of the Prioritization Working Group process and removing I-prioritize.