Make Query::single (and friends) return a Result (#18082)

# Objective

As discussed in #14275, Bevy is currently too prone to panic, and makes
the easy / beginner-friendly way to do a large number of operations just
to panic on failure.

This is seriously frustrating in library code, but also slows down
development, as many of the `Query::single` panics can actually safely
be an early return (these panics are often due to a small ordering issue
or a change in game state.

More critically, in most "finished" products, panics are unacceptable:
any unexpected failures should be handled elsewhere. That's where the
new

With the advent of good system error handling, we can now remove this.

Note: I was instrumental in a) introducing this idea in the first place
and b) pushing to make the panicking variant the default. The
introduction of both `let else` statements in Rust and the fancy system
error handling work in 0.16 have changed my mind on the right balance
here.

## Solution

1. Make `Query::single` and `Query::single_mut` (and other random
related methods) return a `Result`.
2. Handle all of Bevy's internal usage of these APIs.
3. Deprecate `Query::get_single` and friends, since we've moved their
functionality to the nice names.
4. Add detailed advice on how to best handle these errors.

Generally I like the diff here, although `get_single().unwrap()` in
tests is a bit of a downgrade.

## Testing

I've done a global search for `.single` to track down any missed
deprecated usages.

As to whether or not all the migrations were successful, that's what CI
is for :)

## Future work

~~Rename `Query::get_single` and friends to `Query::single`!~~

~~I've opted not to do this in this PR, and smear it across two releases
in order to ease the migration. Successive deprecations are much easier
to manage than the semantics and types shifting under your feet.~~

Cart has convinced me to change my mind on this; see
#18082 (comment).

## Migration guide

`Query::single`, `Query::single_mut` and their `QueryState` equivalents
now return a `Result`. Generally, you'll want to:

1. Use Bevy 0.16's system error handling to return a `Result` using the
`?` operator.
2. Use a `let else Ok(data)` block to early return if it's an expected
failure.
3. Use `unwrap()` or `Ok` destructuring inside of tests.

The old `Query::get_single` (etc) methods which did this have been
deprecated.

Author: alice-i-cecile

crates/bevy_core_pipeline/src/oit/mod.rs

@@ -162,7 +162,7 @@ fn configure_depth_texture_usages(
     }
 
     // Find all the render target that potentially uses OIT
-    let primary_window = p.get_single().ok();
+    let primary_window = p.single().ok();
     let mut render_target_has_oit = <HashSet<_>>::default();
     for (camera, has_oit) in &cameras {
         if has_oit {

crates/bevy_dev_tools/src/picking_debug.rs

@@ -260,7 +260,7 @@ pub fn debug_draw(
             .map(|(entity, camera)| {
                 (
                     entity,
-                    camera.target.normalize(primary_window.get_single().ok()),
+                    camera.target.normalize(primary_window.single().ok()),
                 )
             })
             .filter_map(|(entity, target)| Some(entity).zip(target))

crates/bevy_ecs/compile_fail/tests/ui/query_lifetime_safety.rs

@@ -27,29 +27,29 @@ fn main() {
         }
 
         {
-            let data: &Foo = query.single();
-            let mut data2: Mut<Foo> = query.single_mut();
+            let data: &Foo = query.single().unwrap();
+            let mut data2: Mut<Foo> = query.single_mut().unwrap();
             //~^ E0502
             assert_eq!(data, &mut *data2); // oops UB
         }
 
         {
-            let mut data2: Mut<Foo> = query.single_mut();
-            let data: &Foo = query.single();
+            let mut data2: Mut<Foo> = query.single_mut().unwrap();
+            let data: &Foo = query.single().unwrap();
             //~^ E0502
             assert_eq!(data, &mut *data2); // oops UB
         }
 
         {
-            let data: &Foo = query.get_single().unwrap();
-            let mut data2: Mut<Foo> = query.get_single_mut().unwrap();
+            let data: &Foo = query.single().unwrap();
+            let mut data2: Mut<Foo> = query.single_mut().unwrap();
             //~^ E0502
             assert_eq!(data, &mut *data2); // oops UB
         }
 
         {
-            let mut data2: Mut<Foo> = query.get_single_mut().unwrap();
-            let data: &Foo = query.get_single().unwrap();
+            let mut data2: Mut<Foo> = query.single_mut().unwrap();
+            let data: &Foo = query.single().unwrap();
             //~^ E0502
             assert_eq!(data, &mut *data2); // oops UB
         }

crates/bevy_ecs/compile_fail/tests/ui/query_lifetime_safety.stderr

@@ -45,9 +45,9 @@ error[E0502]: cannot borrow `query` as immutable because it is also borrowed as
 error[E0502]: cannot borrow `query` as mutable because it is also borrowed as immutable
   --> tests/ui/query_lifetime_safety.rs:45:39
    |
-44 |             let data: &Foo = query.get_single().unwrap();
+44 |             let data: &Foo = query.single().unwrap();
    |                              ----- immutable borrow occurs here
-45 |             let mut data2: Mut<Foo> = query.get_single_mut().unwrap();
+45 |             let mut data2: Mut<Foo> = query.single_mut().unwrap();
    |                                       ^^^^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
 46 |
 47 |             assert_eq!(data, &mut *data2); // oops UB
@@ -56,9 +56,9 @@ error[E0502]: cannot borrow `query` as mutable because it is also borrowed as im
 error[E0502]: cannot borrow `query` as immutable because it is also borrowed as mutable
   --> tests/ui/query_lifetime_safety.rs:52:30
    |
-51 |             let mut data2: Mut<Foo> = query.get_single_mut().unwrap();
+51 |             let mut data2: Mut<Foo> = query.single_mut().unwrap();
    |                                       ----- mutable borrow occurs here
-52 |             let data: &Foo = query.get_single().unwrap();
+52 |             let data: &Foo = query.single().unwrap();
    |                              ^^^^^ immutable borrow occurs here
 53 |
 54 |             assert_eq!(data, &mut *data2); // oops UB

crates/bevy_ecs/compile_fail/tests/ui/query_to_readonly.rs

@@ -35,13 +35,13 @@ fn for_loops(mut query: Query<&mut Foo>) {
 fn single_mut_query(mut query: Query<&mut Foo>) {
     // this should fail to compile
     {
-        let mut mut_foo = query.single_mut();
+        let mut mut_foo = query.single_mut().unwrap();
 
         // This solves "temporary value dropped while borrowed"
         let readonly_query = query.as_readonly();
         //~^ E0502
 
-        let ref_foo = readonly_query.single();
+        let ref_foo = readonly_query.single().unwrap();
 
         *mut_foo = Foo;
 
@@ -55,7 +55,7 @@ fn single_mut_query(mut query: Query<&mut Foo>) {
 
         let ref_foo = readonly_query.single();
 
-        let mut mut_foo = query.single_mut();
+        let mut mut_foo = query.single_mut().unwrap();
         //~^ E0502
 
         println!("{ref_foo:?}");

crates/bevy_ecs/compile_fail/tests/ui/query_transmute_safety.rs

@@ -22,8 +22,8 @@ fn main() {
         let mut query_a = lens_a.query();
         let mut query_b = lens_b.query();
 
-        let a = query_a.single_mut();
-        let b = query_b.single_mut(); // oops 2 mutable references to same Foo
+        let a = query_a.single_mut().unwrap();
+        let b = query_b.single_mut().unwrap(); // oops 2 mutable references to same Foo
         assert_eq!(*a, *b);
     }
 
@@ -34,8 +34,8 @@ fn main() {
         let mut query_b = lens.query();
         //~^ E0499
 
-        let a = query_a.single_mut();
-        let b = query_b.single_mut(); // oops 2 mutable references to same Foo
+        let a = query_a.single_mut().unwrap();
+        let b = query_b.single_mut().unwrap(); // oops 2 mutable references to same Foo
         assert_eq!(*a, *b);
     }
 }

crates/bevy_ecs/src/change_detection.rs

@@ -1557,7 +1557,7 @@ mod tests {
         // Since the world is always ahead, as long as changes can't get older than `u32::MAX` (which we ensure),
         // the wrapping difference will always be positive, so wraparound doesn't matter.
         let mut query = world.query::<Ref<C>>();
-        assert!(query.single(&world).is_changed());
+        assert!(query.single(&world).unwrap().is_changed());
     }
 
     #[test]

crates/bevy_ecs/src/query/builder.rs

@@ -33,7 +33,7 @@ use super::{FilteredAccess, QueryData, QueryFilter};
 ///     .build();
 ///
 /// // Consume the QueryState
-/// let (entity, b) = query.single(&world);
+/// let (entity, b) = query.single(&world).unwrap();
 /// ```
 pub struct QueryBuilder<'w, D: QueryData = (), F: QueryFilter = ()> {
     access: FilteredAccess<ComponentId>,
@@ -297,13 +297,13 @@ mod tests {
             .with::<A>()
             .without::<C>()
             .build();
-        assert_eq!(entity_a, query_a.single(&world));
+        assert_eq!(entity_a, query_a.single(&world).unwrap());
 
         let mut query_b = QueryBuilder::<Entity>::new(&mut world)
             .with::<A>()
             .without::<B>()
             .build();
-        assert_eq!(entity_b, query_b.single(&world));
+        assert_eq!(entity_b, query_b.single(&world).unwrap());
     }
 
     #[test]
@@ -319,13 +319,13 @@ mod tests {
             .with_id(component_id_a)
             .without_id(component_id_c)
             .build();
-        assert_eq!(entity_a, query_a.single(&world));
+        assert_eq!(entity_a, query_a.single(&world).unwrap());
 
         let mut query_b = QueryBuilder::<Entity>::new(&mut world)
             .with_id(component_id_a)
             .without_id(component_id_b)
             .build();
-        assert_eq!(entity_b, query_b.single(&world));
+        assert_eq!(entity_b, query_b.single(&world).unwrap());
     }
 
     #[test]
@@ -385,7 +385,7 @@ mod tests {
             .data::<&B>()
             .build();
 
-        let entity_ref = query.single(&world);
+        let entity_ref = query.single(&world).unwrap();
 
         assert_eq!(entity, entity_ref.id());
 
@@ -408,7 +408,7 @@ mod tests {
             .ref_id(component_id_b)
             .build();
 
-        let entity_ref = query.single(&world);
+        let entity_ref = query.single(&world).unwrap();
 
         assert_eq!(entity, entity_ref.id());
 

crates/bevy_ecs/src/query/error.rs

@@ -104,7 +104,7 @@ impl<'w> PartialEq for QueryEntityError<'w> {
 impl<'w> Eq for QueryEntityError<'w> {}
 
 /// An error that occurs when evaluating a [`Query`](crate::system::Query) or [`QueryState`](crate::query::QueryState) as a single expected result via
-/// [`get_single`](crate::system::Query::get_single) or [`get_single_mut`](crate::system::Query::get_single_mut).
+/// [`single`](crate::system::Query::single) or [`single_mut`](crate::system::Query::single_mut).
 #[derive(Debug, Error)]
 pub enum QuerySingleError {
     /// No entity fits the query.

crates/bevy_ecs/src/query/mod.rs

@@ -763,8 +763,8 @@ mod tests {
         let _: Option<&Foo> = q.get(&world, e).ok();
         let _: Option<&Foo> = q.get_manual(&world, e).ok();
         let _: Option<[&Foo; 1]> = q.get_many(&world, [e]).ok();
-        let _: Option<&Foo> = q.get_single(&world).ok();
-        let _: &Foo = q.single(&world);
+        let _: Option<&Foo> = q.single(&world).ok();
+        let _: &Foo = q.single(&world).unwrap();
 
         // system param
         let mut q = SystemState::<Query<&mut Foo>>::new(&mut world);
@@ -776,9 +776,9 @@ mod tests {
 
         let _: Option<&Foo> = q.get(e).ok();
         let _: Option<[&Foo; 1]> = q.get_many([e]).ok();
-        let _: Option<&Foo> = q.get_single().ok();
+        let _: Option<&Foo> = q.single().ok();
         let _: [&Foo; 1] = q.many([e]);
-        let _: &Foo = q.single();
+        let _: &Foo = q.single().unwrap();
     }
 
     // regression test for https://github.com/bevyengine/bevy/pull/8029