docs(CHANGELOG): add entry for diagnostic(…) directives

CHANGELOG.md

@@ -40,11 +40,62 @@ Bottom level categories:
 
 ## Unreleased
 
+## Major changes
+
+### The `diagnostic(…);` directive is now supported in WGSL
+
+Naga now parses `diagnostic(…);` directives according to the WGSL spec. This allows users to control certain lints, similar to Rust's `allow`, `warn`, and `deny` attributes. For example, in standard WGSL (but, notably, not Naga yet—see <https://github.com/gfx-rs/wgpu/issues/4369>) this snippet would emit a uniformity error:
+
+```wgsl
+@group(0) @binding(0) var s : sampler;
+@group(0) @binding(2) var tex : texture_2d<f32>;
+@group(1) @binding(0) var<storage, read> ro_buffer : array<f32, 4>;
+
+@fragment
+fn main(@builtin(position) p : vec4f) -> @location(0) vec4f {
+  if ro_buffer[0] == 0 {
+    // Emits a derivative uniformity error during validation.
+    return textureSample(tex, s, vec2(0.,0.));
+  }
+
+  return vec4f(0.);
+}
+```
+
+…but we can now silence it with the `off` severity level, like so:
+
+```wgsl
+// Disable the diagnosic with this…
+diagnostic(off, derivative_uniformity);
+
+@group(0) @binding(0) var s : sampler;
+@group(0) @binding(2) var tex : texture_2d<f32>;
+@group(1) @binding(0) var<storage, read> ro_buffer : array<f32, 4>;
+
+@fragment
+fn main(@builtin(position) p : vec4f) -> @location(0) vec4f {
+  if ro_buffer[0] == 0 {
+    // Look ma, no error!
+    return textureSample(tex, s, vec2(0.,0.));
+  }
+
+  return vec4f(0.);
+}
+```
+
+There are some limitations to keep in mind with this new functionality:
+
+- We do not yet support `diagnostic(…)` rules in attribute position (i.e., `@diagnostic(…) fn my_func { … }`). This is being tracked in <https://github.com/gfx-rs/wgpu/issues/5320>. We expect that rules in `fn` attribute position will be relaxed shortly (see <https://github.com/gfx-rs/wgpu/pull/6353>), but the prioritization for statement positions is unclear. If you are blocked by not being able to parse `diagnostic(…)` rules in statement positions, please let us know in that issue, so we can determine how to prioritize it!
+- Standard WGSL specifies `error`, `warning`, `info`, and `off` severity levels. These are all technically usable now! A caveat, though: warning- and info-level are only emitted to `stderr` via the `log` façade, rather than being reported through a `Result::Err` in Naga or the `CompilationInfo` interface in `wgpu{,-core}`. This will require breaking changes in Naga to fix, and is being tracked by <https://github.com/gfx-rs/wgpu/issues/6458>.
+- Not all lints can be controlled with `diagnostic(…)` rules. In fact, only the `derivative_uniformity` triggering rule exists in the WGSL standard. That said, Naga contributors are excited to see how this level of control unlocks a new ecosystem of configurable diagnostics.
+- Finally, `diagnostic(…)` rules are not yet emitted in WGSL output. This means that `wgsl-in` → `wgsl-out` is currently a lossy process. We felt that it was important to unblock users who needed `diagnostic(…)` rules (i.e., <https://github.com/gfx-rs/wgpu/issues/3135>) before we took significant effort to fix this (tracked in <https://github.com/gfx-rs/wgpu/issues/6496>).
+
+By @ErichDonGubler in [#6456](https://github.com/gfx-rs/wgpu/pull/6456), [#6148](https://github.com/gfx-rs/wgpu/pull/6148).
+
 ### New Features
 
 #### Naga
 
-- Parse `diagnostic(…)` directives, but don't implement any triggering rules yet. By @ErichDonGubler in [#6456](https://github.com/gfx-rs/wgpu/pull/6456).
 - Fix an issue where `naga` CLI would incorrectly skip the first positional argument when `--stdin-file-path` was specified. By @ErichDonGubler in [#6480](https://github.com/gfx-rs/wgpu/pull/6480).
 - Fix textureNumLevels in the GLSL backend. By @magcius in [#6483](https://github.com/gfx-rs/wgpu/pull/6483).