Add print_stdout and print_stderr lints (#17446) (#18233)

# Objective

- Prevent usage of `println!`, `eprintln!` and the like because they
require `std`
- Fixes #17446

## Solution

- Enable the `print_stdout` and `print_stderr` clippy lints
- Replace all `println!` and `eprintln!` occurrences with `log::*` where
applicable or alternatively ignore the warnings

## Testing

- Run `cargo clippy --workspace` to ensure that there are no warnings
relating to printing to `stdout` or `stderr`

Cargo.toml

@@ -51,6 +51,8 @@ unwrap_or_default = "warn"
 needless_lifetimes = "allow"
 too_many_arguments = "allow"
 nonstandard_macro_braces = "warn"
+print_stdout = "warn"
+print_stderr = "warn"
 
 ptr_as_ptr = "warn"
 ptr_cast_constness = "warn"

crates/bevy_asset/src/asset_changed.rs

@@ -281,6 +281,7 @@ unsafe impl<A: AsAssetId> QueryFilter for AssetChanged<A> {
 }
 
 #[cfg(test)]
+#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
 mod tests {
     use crate::{AssetEvents, AssetPlugin, Handle};
     use alloc::{vec, vec::Vec};

crates/bevy_ecs/examples/change_detection.rs

@@ -8,6 +8,7 @@
 
 #![expect(
     clippy::std_instead_of_core,
+    clippy::print_stdout,
     reason = "Examples should not follow this lint"
 )]
 

crates/bevy_ecs/examples/events.rs

@@ -1,6 +1,8 @@
 //! In this example a system sends a custom event with a 50/50 chance during any frame.
 //! If an event was send, it will be printed by the console in a receiving system.
 
+#![expect(clippy::print_stdout, reason = "Allowed in examples.")]
+
 use bevy_ecs::{event::EventRegistry, prelude::*};
 
 fn main() {

crates/bevy_ecs/examples/resources.rs

@@ -3,6 +3,7 @@
 
 #![expect(
     clippy::std_instead_of_core,
+    clippy::print_stdout,
     reason = "Examples should not follow this lint"
 )]
 

crates/bevy_ecs/src/error/bevy_error.rs

@@ -139,6 +139,7 @@ std::thread_local! {
 
 /// When called, this will skip the currently configured panic hook when a [`BevyError`] backtrace has already been printed.
 #[cfg(feature = "backtrace")]
+#[expect(clippy::print_stdout, reason = "Allowed behind `std` feature gate.")]
 pub fn bevy_error_panic_hook(
     current_hook: impl Fn(&std::panic::PanicHookInfo),
 ) -> impl Fn(&std::panic::PanicHookInfo) {

crates/bevy_ecs/src/query/iter.rs

@@ -2585,6 +2585,7 @@ impl<T> Ord for NeutralOrd<T> {
 }
 
 #[cfg(test)]
+#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
 mod tests {
     use alloc::vec::Vec;
     use std::println;

crates/bevy_ecs/src/query/mod.rs

@@ -102,6 +102,7 @@ impl<T> DebugCheckedUnwrap for Option<T> {
 }
 
 #[cfg(test)]
+#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
 mod tests {
     use crate::{
         archetype::Archetype,

crates/bevy_ecs/src/schedule/executor/multi_threaded.rs

@@ -5,10 +5,9 @@ use bevy_utils::{default, syncunsafecell::SyncUnsafeCell};
 use concurrent_queue::ConcurrentQueue;
 use core::{any::Any, panic::AssertUnwindSafe};
 use fixedbitset::FixedBitSet;
-use std::{
+#[cfg(feature = "std")]
-    eprintln,
+use std::eprintln;
-    sync::{Mutex, MutexGuard},
+use std::sync::{Mutex, MutexGuard};
-};
 
 #[cfg(feature = "trace")]
 use tracing::{info_span, Span};
@@ -283,7 +282,11 @@ impl<'scope, 'env: 'scope, 'sys> Context<'scope, 'env, 'sys> {
             .push(SystemResult { system_index })
             .unwrap_or_else(|error| unreachable!("{}", error));
         if let Err(payload) = res {
-            eprintln!("Encountered a panic in system `{}`!", &*system.name());
+            #[cfg(feature = "std")]
+            #[expect(clippy::print_stderr, reason = "Allowed behind `std` feature gate.")]
+            {
+                eprintln!("Encountered a panic in system `{}`!", &*system.name());
+            }
             // set the payload to propagate the error
             {
                 let mut panic_payload = self.environment.executor.panic_payload.lock().unwrap();
@@ -741,10 +744,14 @@ fn apply_deferred(
             system.apply_deferred(world);
         }));
         if let Err(payload) = res {
-            eprintln!(
+            #[cfg(feature = "std")]
-                "Encountered a panic when applying buffers for system `{}`!",
+            #[expect(clippy::print_stderr, reason = "Allowed behind `std` feature gate.")]
-                &*system.name()
+            {
-            );
+                eprintln!(
+                    "Encountered a panic when applying buffers for system `{}`!",
+                    &*system.name()
+                );
+            }
             return Err(payload);
         }
     }

crates/bevy_ecs/src/schedule/executor/simple.rs

@@ -118,6 +118,7 @@ impl SystemExecutor for SimpleExecutor {
             });
 
             #[cfg(feature = "std")]
+            #[expect(clippy::print_stderr, reason = "Allowed behind `std` feature gate.")]
             {
                 if let Err(payload) = std::panic::catch_unwind(f) {
                     eprintln!("Encountered a panic in system `{}`!", &*system.name());

crates/bevy_ecs/src/schedule/executor/single_threaded.rs

@@ -144,6 +144,7 @@ impl SystemExecutor for SingleThreadedExecutor {
             });
 
             #[cfg(feature = "std")]
+            #[expect(clippy::print_stderr, reason = "Allowed behind `std` feature gate.")]
             {
                 if let Err(payload) = std::panic::catch_unwind(f) {
                     eprintln!("Encountered a panic in system `{}`!", &*system.name());

crates/bevy_ecs/src/schedule/stepping.rs

@@ -823,6 +823,7 @@ impl ScheduleState {
 }
 
 #[cfg(all(test, feature = "bevy_debug_stepping"))]
+#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
 mod tests {
     use super::*;
     use crate::{prelude::*, schedule::ScheduleLabel};

crates/bevy_ecs/src/system/mod.rs

@@ -323,6 +323,7 @@ pub fn assert_system_does_not_conflict<Out, Params, S: IntoSystem<(), Out, Param
 }
 
 #[cfg(test)]
+#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
 mod tests {
     use alloc::{vec, vec::Vec};
     use bevy_utils::default;

crates/bevy_ecs/src/world/mod.rs

@@ -3663,6 +3663,7 @@ impl<T: Default> FromWorld for T {
 }
 
 #[cfg(test)]
+#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
 mod tests {
     use super::{FromWorld, World};
     use crate::{

crates/bevy_log/src/lib.rs

@@ -259,6 +259,7 @@ impl Default for LogPlugin {
 }
 
 impl Plugin for LogPlugin {
+    #[expect(clippy::print_stderr, reason = "Allowed during logger setup")]
     fn build(&self, app: &mut App) {
         #[cfg(feature = "trace")]
         {

crates/bevy_math/src/bounding/bounded2d/primitive_impls.rs

@@ -438,6 +438,7 @@ impl Bounded2d for Capsule2d {
 }
 
 #[cfg(test)]
+#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
 mod tests {
     use core::f32::consts::{FRAC_PI_2, FRAC_PI_3, FRAC_PI_4, FRAC_PI_6, TAU};
     use std::println;

crates/bevy_math/src/direction.rs

@@ -69,10 +69,13 @@ fn assert_is_normalized(message: &str, length_squared: f32) {
     } else if length_error_squared > 2e-4 {
         // Length error is approximately 1e-4 or more.
         #[cfg(feature = "std")]
-        eprintln!(
+        #[expect(clippy::print_stderr, reason = "Allowed behind `std` feature gate.")]
-            "Warning: {message} The length is {}.",
+        {
-            ops::sqrt(length_squared)
+            eprintln!(
-        );
+                "Warning: {message} The length is {}.",
+                ops::sqrt(length_squared)
+            );
+        }
     }
 }
 

crates/bevy_mikktspace/examples/generate.rs

@@ -5,6 +5,7 @@
     clippy::useless_conversion,
     reason = "Crate auto-generated with many non-idiomatic decisions. See #7372 for details."
 )]
+#![expect(clippy::print_stdout, reason = "Allowed in examples.")]
 
 use glam::{Vec2, Vec3};
 

crates/bevy_reflect/examples/reflect_docs.rs

@@ -6,6 +6,8 @@
 //!
 //! These scenarios can readily be achieved by using `bevy_reflect` with the `documentation` feature.
 
+#![expect(clippy::print_stdout, reason = "Allowed in examples.")]
+
 use bevy_reflect::{Reflect, TypeInfo, Typed};
 
 fn main() {

crates/bevy_remote/src/builtin_methods.rs

@@ -1758,6 +1758,7 @@ fn get_resource_type_registration<'r>(
 }
 
 #[cfg(test)]
+#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
 mod tests {
     /// A generic function that tests serialization and deserialization of any type
     /// implementing Serialize and Deserialize traits.

crates/bevy_tasks/examples/busy_behavior.rs

@@ -2,6 +2,8 @@
 //! for 100ms. It's expected to take about a second to run (assuming the machine has >= 4 logical
 //! cores)
 
+#![expect(clippy::print_stdout, reason = "Allowed in examples.")]
+
 use bevy_platform_support::time::Instant;
 use bevy_tasks::TaskPoolBuilder;
 use core::time::Duration;

crates/bevy_tasks/examples/idle_behavior.rs

@@ -2,6 +2,8 @@
 //! spinning. Other than the one thread, the system should remain idle, demonstrating good behavior
 //! for small workloads.
 
+#![expect(clippy::print_stdout, reason = "Allowed in examples.")]
+
 use bevy_platform_support::time::Instant;
 use bevy_tasks::TaskPoolBuilder;
 use core::time::Duration;

crates/bevy_time/src/lib.rs

@@ -175,6 +175,7 @@ pub fn time_system(
 }
 
 #[cfg(test)]
+#[expect(clippy::print_stdout, reason = "Allowed in tests.")]
 mod tests {
     use crate::{Fixed, Time, TimePlugin, TimeUpdateStrategy, Virtual};
     use bevy_app::{App, FixedUpdate, Startup, Update};

crates/bevy_transform/src/components/transform.rs

@@ -27,7 +27,10 @@ fn assert_is_normalized(message: &str, length_squared: f32) {
     } else if length_error_squared > 2e-4 {
         // Length error is approximately 1e-4 or more.
         #[cfg(feature = "std")]
-        eprintln!("Warning: {message}",);
+        #[expect(clippy::print_stderr, reason = "Allowed behind `std` feature gate.")]
+        {
+            eprintln!("Warning: {message}",);
+        }
     }
 }
 

tools/build-easefunction-graphs/src/main.rs

@@ -1,4 +1,7 @@
 //! Generates graphs for the `EaseFunction` docs.
+
+#![expect(clippy::print_stdout, reason = "Allowed in tools.")]
+
 use std::path::PathBuf;
 
 use bevy_math::curve::{CurveExt, EaseFunction, EasingCurve, JumpAt};

tools/example-showcase/src/main.rs

@@ -1,5 +1,7 @@
 //! Tool to run all examples or generate a showcase page for the Bevy website.
 
+#![expect(clippy::print_stdout, reason = "Allowed in tools.")]
+
 use core::{
     fmt::Display,
     hash::{Hash, Hasher},