From 7adcdd2fd33fa935d53881f248fd1b7c2fbe39b8 Mon Sep 17 00:00:00 2001
From: Georg Semmler <github@weiznich.de>
Date: Wed, 24 Aug 2022 17:23:15 +0200
Subject: [PATCH] Improve some error messages generated by nightly rustc

This commit introduces a new feature flags that let's user opt into the
nightly only `#[rustc_on_unimplemented]` attribute. This attribute can
be used to improve error messages generated by missing trait
implementations. This commit adds annotations to two locations in bevy
to improve error messages submitted as part of
https://github.com/weiznich/rust-foundation-community-grant/issues/3

Addresses https://github.com/bevyengine/bevy/issues/1519
---
 Cargo.toml                                 | 1 +
 crates/bevy_ecs/Cargo.toml                 | 1 +
 crates/bevy_ecs/src/lib.rs                 | 1 +
 crates/bevy_ecs/src/query/fetch.rs         | 8 ++++++++
 crates/bevy_ecs/src/system/system_param.rs | 4 ++++
 crates/bevy_internal/Cargo.toml            | 1 +
 6 files changed, 16 insertions(+)

diff --git a/Cargo.toml b/Cargo.toml
index b0920d9ecd0fa..3a44ac572e53e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -114,6 +114,7 @@ debug_asset_server = ["bevy_internal/debug_asset_server"]
 
 # Enable animation support, and glTF animation loading
 animation = ["bevy_internal/animation"]
+nightly-error-messages = ["bevy_internal/nightly-error-messages"]
 
 [dependencies]
 bevy_dylib = { path = "crates/bevy_dylib", version = "0.9.0-dev", default-features = false, optional = true }
diff --git a/crates/bevy_ecs/Cargo.toml b/crates/bevy_ecs/Cargo.toml
index 0f23192b313d3..0d9e8c95737f9 100644
--- a/crates/bevy_ecs/Cargo.toml
+++ b/crates/bevy_ecs/Cargo.toml
@@ -12,6 +12,7 @@ categories = ["game-engines", "data-structures"]
 [features]
 trace = []
 default = ["bevy_reflect"]
+nightly-error-messages = []
 
 [dependencies]
 bevy_ptr = { path = "../bevy_ptr", version = "0.9.0-dev" }
diff --git a/crates/bevy_ecs/src/lib.rs b/crates/bevy_ecs/src/lib.rs
index 3b3e3e2b18d37..56520f9deafc6 100644
--- a/crates/bevy_ecs/src/lib.rs
+++ b/crates/bevy_ecs/src/lib.rs
@@ -1,5 +1,6 @@
 #![warn(clippy::undocumented_unsafe_blocks)]
 #![doc = include_str!("../README.md")]
+#![cfg_attr(feature = "nightly-error-messages", feature(rustc_attrs))]
 
 #[cfg(target_pointer_width = "16")]
 compile_error!("bevy_ecs cannot safely compile for a 16-bit platform.");
diff --git a/crates/bevy_ecs/src/query/fetch.rs b/crates/bevy_ecs/src/query/fetch.rs
index 21fab0db52367..beaa3d2996982 100644
--- a/crates/bevy_ecs/src/query/fetch.rs
+++ b/crates/bevy_ecs/src/query/fetch.rs
@@ -325,6 +325,14 @@ use std::{cell::UnsafeCell, marker::PhantomData};
 /// [`WorldQuery::update_archetype_component_access`] exactly reflects the results of
 /// [`WorldQuery::matches_component_set`], [`WorldQuery::archetype_fetch`], and
 /// [`WorldQuery::table_fetch`].
+#[cfg_attr(
+    feature = "nightly-error-messages",
+    rustc_on_unimplemented(on(
+        not(any(_Self = "& _", _Self = "&mut _", _Self = "()", _Self = "(_, _)")),
+        message = "Using a `WorldQuery` object as parameter to `Query` requires the usage of a reference",
+        label = "consider using `& {Self}` here"
+    ),)
+)]
 pub unsafe trait WorldQuery: for<'w> WorldQueryGats<'w> {
     /// The read-only variant of this [`WorldQuery`], which satisfies the [`ReadOnlyWorldQuery`] trait.
     type ReadOnly: ReadOnlyWorldQuery<State = Self::State>;
diff --git a/crates/bevy_ecs/src/system/system_param.rs b/crates/bevy_ecs/src/system/system_param.rs
index 8762bd64c4255..1b24d3fca67c5 100644
--- a/crates/bevy_ecs/src/system/system_param.rs
+++ b/crates/bevy_ecs/src/system/system_param.rs
@@ -252,6 +252,10 @@ impl_param_set!();
 /// # schedule.add_system_to_stage("update", write_resource_system.after("first"));
 /// # schedule.run_once(&mut world);
 /// ```
+#[cfg_attr(
+    feature = "nightly-error-messages",
+    rustc_on_unimplemented(note = "consider adding `#[derive(bevy::Resource)]` to `{Self}`")
+)]
 pub trait Resource: Send + Sync + 'static {}
 
 /// Shared borrow of a [`Resource`].
diff --git a/crates/bevy_internal/Cargo.toml b/crates/bevy_internal/Cargo.toml
index d4be55fc72878..fa856022c5470 100644
--- a/crates/bevy_internal/Cargo.toml
+++ b/crates/bevy_internal/Cargo.toml
@@ -22,6 +22,7 @@ trace_chrome = [ "bevy_log/tracing-chrome" ]
 trace_tracy = ["bevy_render?/tracing-tracy", "bevy_log/tracing-tracy" ]
 wgpu_trace = ["bevy_render/wgpu_trace"]
 debug_asset_server = ["bevy_asset/debug_asset_server"]
+nightly-error-messages = ["bevy_ecs/nightly-error-messages"]
 
 # Image format support for texture loading (PNG and HDR are enabled by default)
 hdr = ["bevy_render/hdr"]