Auto merge of #15827 - moonlightdrive:use-iflet-not-match, r=mbrubeck

replace match with if let

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Use `if let` instead of a `match` with a single pattern.

I'm not clear if/how this needs to be tested (beyond what's been checked-off below), so please let me know if there's more to do. Thanks!

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15810

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/style/properties/properties.mako.rs

@@ -1802,13 +1802,10 @@ pub fn apply_declarations<'a, F, I>(viewport_size: Size2D<Au>,
     let mut custom_properties = None;
     let mut seen_custom = HashSet::new();
     for declaration in iter_declarations() {
-        match *declaration {
-            PropertyDeclaration::Custom(ref name, ref value) => {
-                ::custom_properties::cascade(
-                    &mut custom_properties, &inherited_custom_properties,
-                    &mut seen_custom, name, value)
-            }
-            _ => {}
+        if let PropertyDeclaration::Custom(ref name, ref value) = *declaration {
+            ::custom_properties::cascade(
+                &mut custom_properties, &inherited_custom_properties,
+                &mut seen_custom, name, value)
         }
     }