From ac661188c8679e767c4f2abebee22b4db3128e99 Mon Sep 17 00:00:00 2001
From: Jakob Hellermann <jakob.hellermann@protonmail.com>
Date: Sun, 14 Mar 2021 00:36:16 +0000
Subject: [PATCH] better error message: specify which resource is missing
 (#1648)

---
 crates/bevy_ecs/src/system/system_param.rs | 28 ++++++++++++++++++----
 crates/bevy_ecs/src/world/mod.rs           | 12 +++++-----
 2 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/crates/bevy_ecs/src/system/system_param.rs b/crates/bevy_ecs/src/system/system_param.rs
index 8131a32760..60d8bbe9e0 100644
--- a/crates/bevy_ecs/src/system/system_param.rs
+++ b/crates/bevy_ecs/src/system/system_param.rs
@@ -234,7 +234,12 @@ impl<'a, T: Component> SystemParamFetch<'a> for ResState<T> {
     ) -> Self::Item {
         let column = world
             .get_populated_resource_column(state.component_id)
-            .expect("Requested resource does not exist");
+            .unwrap_or_else(|| {
+                panic!(
+                    "Requested resource does not exist: {}",
+                    std::any::type_name::<T>()
+                )
+            });
         Res {
             value: &*column.get_ptr().as_ptr().cast::<T>(),
             flags: *column.get_flags_mut_ptr(),
@@ -371,7 +376,12 @@ impl<'a, T: Component> SystemParamFetch<'a> for ResMutState<T> {
     ) -> Self::Item {
         let value = world
             .get_resource_unchecked_mut_with_id(state.component_id)
-            .expect("Requested resource does not exist");
+            .unwrap_or_else(|| {
+                panic!(
+                    "Requested resource does not exist: {}",
+                    std::any::type_name::<T>()
+                )
+            });
         ResMut {
             value: value.value,
             flags: value.flags,
@@ -603,7 +613,12 @@ impl<'a, T: 'static> SystemParamFetch<'a> for NonSendState<T> {
         NonSend {
             value: world
                 .get_non_send_with_id::<T>(state.component_id)
-                .expect("Requested non-send resource does not exist"),
+                .unwrap_or_else(|| {
+                    panic!(
+                        "Requested non-send resource does not exist: {}",
+                        std::any::type_name::<T>()
+                    )
+                }),
         }
     }
 }
@@ -692,7 +707,12 @@ impl<'a, T: 'static> SystemParamFetch<'a> for NonSendMutState<T> {
     ) -> Self::Item {
         let value = world
             .get_non_send_unchecked_mut_with_id(state.component_id)
-            .expect("Requested non-send resource does not exist");
+            .unwrap_or_else(|| {
+                panic!(
+                    "Requested non-send resource does not exist: {}",
+                    std::any::type_name::<T>()
+                )
+            });
         NonSendMut {
             value: value.value,
             flags: value.flags,
diff --git a/crates/bevy_ecs/src/world/mod.rs b/crates/bevy_ecs/src/world/mod.rs
index 43f87c7e91..952bb3ff4e 100644
--- a/crates/bevy_ecs/src/world/mod.rs
+++ b/crates/bevy_ecs/src/world/mod.rs
@@ -620,15 +620,15 @@ impl World {
         let component_id = self
             .components
             .get_resource_id(TypeId::of::<T>())
-            .expect("resource does not exist");
+            .unwrap_or_else(|| panic!("resource does not exist: {}", std::any::type_name::<T>()));
         let (ptr, mut flags) = {
             let resource_archetype = self.archetypes.resource_mut();
             let unique_components = resource_archetype.unique_components_mut();
-            let column = unique_components
-                .get_mut(component_id)
-                .expect("resource does not exist");
+            let column = unique_components.get_mut(component_id).unwrap_or_else(|| {
+                panic!("resource does not exist: {}", std::any::type_name::<T>())
+            });
             if column.is_empty() {
-                panic!("resource does not exist");
+                panic!("resource does not exist: {}", std::any::type_name::<T>());
             }
             // SAFE: if a resource column exists, row 0 exists as well. caller takes ownership of
             // the ptr value / drop is called when T is dropped
@@ -644,7 +644,7 @@ impl World {
         let unique_components = resource_archetype.unique_components_mut();
         let column = unique_components
             .get_mut(component_id)
-            .expect("resource does not exist");
+            .unwrap_or_else(|| panic!("resource does not exist: {}", std::any::type_name::<T>()));
         // SAFE: new location is immediately written to below
         let row = unsafe { column.push_uninit() };
         // SAFE: row was just allocated above