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tracing: fix macro hygiene for concat! (#1842)
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## Motivation

In my library I define a `macro_rules! concat` macro, i.e.
[`callbag::concat`](https://docs.rs/callbag/latest/callbag/macro.concat.html).

When I try to call `tracing::info!(...)`, I get error output such as
this:

<details>
<summary>error output</summary>

<!-- leave a blank line above -->
```
> RUSTFLAGS='-Z macro-backtrace' cargo +nightly clippy --features trace
    Checking callbag v0.14.0 (/home/teohhanhui/projects/teohhanhui/callbag-rs)
error[E0308]: mismatched types
  --> src/concat.rs:89:9
   |
89 |         info!("from sink: {message:?}");
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `&str`, found `u32`

error[E0277]: the trait bound `std::sync::Arc<core::Callbag<never::Never, _>>: std::convert::From<&str>` is not satisfied
    --> src/concat.rs:58:9
     |
56   | / macro_rules! concat {
57   | |     ($($s:expr),* $(,)?) => {
58   | |         $crate::concat(::std::vec![$($s),*].into_boxed_slice())
     | |         ^^^^^^^^^^^^^^ the trait `std::convert::From<&str>` is not implemented for `std::sync::Arc<core::Callbag<never::Never, _>>`
59   | |     };
60   | | }
     | |_- in this expansion of `concat!` (#5)
...
89   |           info!("from sink: {message:?}");
     |           ------------------------------- in this macro invocation (#1)
     |
    ::: src/utils/tracing.rs:47:1
     |
47   | / macro_rules! info {
48   | |     ($($arg:tt)+) => {
49   | |         ::cfg_if::cfg_if! {
50   | |             if #[cfg(feature = "trace")] {
51   | |                 ::tracing::info!($($arg)+)
     | |                 -------------------------- in this macro invocation (#2)
...    |
54   | |     };
55   | | }
     | |_- in this expansion of `info!` (#1)
     |
    ::: /home/teohhanhui/.cargo/registry/src/github.com-1ecc6299db9ec823/tracing-0.1.29/src/macros.rs:586:1
     |
586  |   macro_rules! event {
     |  _-
     | |_|
     | |
587  | |     (target: $target:expr, parent: $parent:expr, $lvl:expr, { $($fields:tt)* } )=> (
588  | |         $crate::__tracing_log!(
589  | |             target: $target,
...    |
644  |                   name: concat!(
     |  _______________________-
645  |                       "event ",
646  |                       file!(),
647  |                       ":",
648  |                       line!()
649  | |                 ),
     | |_________________- in this macro invocation (#5)
...
667  | /         $crate::event!(
668  |               target: $target,
669  |               $lvl,
670  |               { message = format_args!($($arg)+), $($fields)* }
671  | |         )
     | |_________- in this macro invocation (#4)
...
791  | |     );
792  | | }
     | | -
     | |_|
     | |_in this expansion of `$crate::event!` (#3)
     |   in this expansion of `$crate::event!` (#4)
...
1229 | / macro_rules! info {
1230 |        (target: $target:expr, parent: $parent:expr, { $($field:tt)* }, $($arg:tt)* ) => (
1231 |           $crate::event!(target: $target, parent: $parent, $crate::Level::INFO, { $($field)* }, $($arg)*)
1232 |       );
...
1398 | /         $crate::event!(
1399 | |             target: module_path!(),
1400 | |             $crate::Level::INFO,
1401 | |             {},
1402 | |             $($arg)+
1403 | |         )
     | |_________- in this macro invocation (#3)
1404 |       );
1405 | | }
     | |_- in this expansion of `::tracing::info!` (#2)
     |
     = help: the following implementations were found:
               <std::sync::Arc<B> as std::convert::From<std::borrow::Cow<'a, B>>>
               <std::sync::Arc<T> as std::convert::From<T>>
               <std::sync::Arc<T> as std::convert::From<std::boxed::Box<T>>>
               <std::sync::Arc<[T]> as std::convert::From<&[T]>>
             and 9 others
     = note: required because of the requirements on the impl of `std::convert::Into<std::sync::Arc<core::Callbag<never::Never, _>>>` for `&str`
note: required by a bound in `concat::concat`
    --> src/concat.rs:81:8
     |
72   | pub fn concat<
     |        ------ required by a bound in this
...
81   |     S: Into<Arc<Source<T>>> + Send + Sync,
     |        ^^^^^^^^^^^^^^^^^^^^ required by this bound in `concat::concat`

error[E0308]: mismatched types
    --> src/concat.rs:58:9
     |
56   | / macro_rules! concat {
57   | |     ($($s:expr),* $(,)?) => {
58   | |         $crate::concat(::std::vec![$($s),*].into_boxed_slice())
     | |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `&str`, found struct `core::Callbag`
59   | |     };
60   | | }
     | |_- in this expansion of `concat!` (#5)
...
89   |           info!("from sink: {message:?}");
     |           ------------------------------- in this macro invocation (#1)
     |
    ::: src/utils/tracing.rs:47:1
     |
47   | / macro_rules! info {
48   | |     ($($arg:tt)+) => {
49   | |         ::cfg_if::cfg_if! {
50   | |             if #[cfg(feature = "trace")] {
51   | |                 ::tracing::info!($($arg)+)
     | |                 -------------------------- in this macro invocation (#2)
...    |
54   | |     };
55   | | }
     | |_- in this expansion of `info!` (#1)
     |
    ::: /home/teohhanhui/.cargo/registry/src/github.com-1ecc6299db9ec823/tracing-0.1.29/src/macros.rs:586:1
     |
586  |   macro_rules! event {
     |  _-
     | |_|
     | |
587  | |     (target: $target:expr, parent: $parent:expr, $lvl:expr, { $($fields:tt)* } )=> (
588  | |         $crate::__tracing_log!(
589  | |             target: $target,
...    |
644  |                   name: concat!(
     |  _______________________-
645  |                       "event ",
646  |                       file!(),
647  |                       ":",
648  |                       line!()
649  | |                 ),
     | |_________________- in this macro invocation (#5)
...
667  | /         $crate::event!(
668  |               target: $target,
669  |               $lvl,
670  |               { message = format_args!($($arg)+), $($fields)* }
671  | |         )
     | |_________- in this macro invocation (#4)
...
791  | |     );
792  | | }
     | | -
     | |_|
     | |_in this expansion of `$crate::event!` (#3)
     |   in this expansion of `$crate::event!` (#4)
...
1229 | / macro_rules! info {
1230 |        (target: $target:expr, parent: $parent:expr, { $($field:tt)* }, $($arg:tt)* ) => (
1231 |           $crate::event!(target: $target, parent: $parent, $crate::Level::INFO, { $($field)* }, $($arg)*)
1232 |       );
...
1398 | /         $crate::event!(
1399 | |             target: module_path!(),
1400 | |             $crate::Level::INFO,
1401 | |             {},
1402 | |             $($arg)+
1403 | |         )
     | |_________- in this macro invocation (#3)
1404 |       );
1405 | | }
     | |_- in this expansion of `::tracing::info!` (#2)
     |
     = note: expected reference `&'static str`
                   found struct `core::Callbag<never::Never, _>`

Some errors have detailed explanations: E0277, E0308.
For more information about an error, try `rustc --explain E0277`.
error: could not compile `callbag` due to 3 previous errors
```
</details>

This is because of my `concat` macro being in scope.

## Solution

This branch adds a re-export of `core::concat!` in the
`__macro_support` module, and changes all the `tracing` macros to use
that, rather than using an un-namespaced `concat!`. The re-export
ensures that everything still works even in a crate that redefines the
`core` name to something else.

Co-authored-by: Eliza Weisman <eliza@buoyant.io>
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@teohhanhui @hawkw
teohhanhui and hawkw committed Jan 28, 2022
1 parent bcd0972 commit 7cf4854
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Showing 2 changed files with 4 additions and 3 deletions.
1 change: 1 addition & 0 deletions tracing/src/lib.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -936,6 +936,7 @@ pub mod __macro_support {
sync::atomic::{AtomicUsize, Ordering},
};
use crate::{subscriber::Interest, Metadata};
pub use core::concat;
use tracing_core::Once;

/// Callsite implementation used by macro-generated code.
Expand Down
6 changes: 3 additions & 3 deletions tracing/src/macros.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -592,7 +592,7 @@ macro_rules! event {
if $crate::level_enabled!($lvl) {
use $crate::__macro_support::*;
static CALLSITE: $crate::__macro_support::MacroCallsite = $crate::callsite2! {
name: concat!(
name: $crate::__macro_support::concat!(
"event ",
file!(),
":",
Expand Down Expand Up @@ -639,7 +639,7 @@ macro_rules! event {
if $crate::level_enabled!($lvl) {
use $crate::__macro_support::*;
static CALLSITE: $crate::__macro_support::MacroCallsite = $crate::callsite2! {
name: concat!(
name: $crate::__macro_support::concat!(
"event ",
file!(),
":",
Expand Down Expand Up @@ -2171,7 +2171,7 @@ macro_rules! __tracing_log {
macro_rules! __mk_format_string {
// === base case ===
(@ { $(,)* $($out:expr),* $(,)* } $(,)*) => {
concat!( $($out),*)
$crate::__macro_support::concat!( $($out),*)
};

// === recursive case (more tts), ===
Expand Down

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