# Objective
There's a lot of code duplication in `access.rs`. The same logic is
duplicated between components and resources. This also takes up
unnecessary memory in `Access`, as it relies on bitsets spanning the
entire `ComponentId` range.
## Solution
Since resources are now a special kind of component, this can be
removed.
## Limitations
Since `!Send` data queries used `Access` resources, `!Send` data queries
now conflict with broad queries.
```rust
// 0.18
fn system(q1_: Query<EntityMut>, q2_: NonSend<R>) {} // valid, does not conflict
// 0.19
fn system(q1_: Query<EntityMut>, q2_: NonSend<R>) {} // invalid, does conflict
```
Given how rarely non-send data is used, I recommend using
```
// 0.19
fn system(q1_: Query<EntityMut, Without<R>>, q2_: NonSend<R>) {} // works again
```
If this is also unacceptable, this PR is blocked on the `!Send` data
removal from the ECS (or some hacky workaround).
## Extra Attention
@chescock brought `AssetChanged` to my attention. It has a weird access
pattern. See the following example:
```rust
fn system(c: Query<&mut AssetChanges<Mesh>>, r: Query<(), AssetChanged<Mesh>>) {}
```
System `c` registers access with `add_write` for `AssetChanges<Mesh>`,
while `r` registers access with `add_read` for both `Mesh` and
`AssetChanges<Mesh>`. This system is invalid, and I've added a test to
reflect that. However, since this stuff is tricky, I would like some
extra eyes on it. Currently, it looks *fine*.
This makes the [New Bevy Renderer](#2535) the default (and only) renderer. The new renderer isn't _quite_ ready for the final release yet, but I want as many people as possible to start testing it so we can identify bugs and address feedback prior to release.
The examples are all ported over and operational with a few exceptions:
* I removed a good portion of the examples in the `shader` folder. We still have some work to do in order to make these examples possible / ergonomic / worthwhile: #3120 and "high level shader material plugins" are the big ones. This is a temporary measure.
* Temporarily removed the multiple_windows example: doing this properly in the new renderer will require the upcoming "render targets" changes. Same goes for the render_to_texture example.
* Removed z_sort_debug: entity visibility sort info is no longer available in app logic. we could do this on the "render app" side, but i dont consider it a priority.
# Objective
- Improve error descriptions and help understand how to fix them
- I noticed one today that could be expanded, it seemed like a good starting point
## Solution
- Start something like https://github.com/rust-lang/rust/tree/master/compiler/rustc_error_codes/src/error_codes
- Remove sentence about Rust mutability rules which is not very helpful in the error message
I decided to start the error code with B for Bevy so that they're not confused with error code from rust (which starts with E)
Longer term, there are a few more evolutions that can continue this:
- the code samples should be compiled check, and even executed for some of them to check they have the correct error code in a panic
- the error could be build on a page in the website like https://doc.rust-lang.org/error-index.html
- most panic should have their own error code
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