Auto merge of #18265 - hiikezoe:dont-fallback-to-discrete-animation, r=birtles

Don't fallback to discrete animations within animate().

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https://bugzilla.mozilla.org/show_bug.cgi?id=1394287
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<!-- 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

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components/style/properties/helpers/animated_properties.mako.rs

@@ -2623,13 +2623,7 @@ where
             (&SVGLength::Length(ref this), &SVGLength::Length(ref other)) => {
                 Ok(SVGLength::Length(this.animate(other, procedure)?))
             },
-            _ => {
-                // FIXME(nox): Is this correct for addition and accumulation?
-                // I think an error should be returned if it's not
-                // an interpolation.
-                let (this_weight, other_weight) = procedure.weights();
-                Ok(if this_weight > other_weight { self.clone() } else { other.clone() })
-            },
+            _ => Err(()),
         }
     }
 }
@@ -2649,10 +2643,7 @@ where
             (&SVGStrokeDashArray::Values(ref this), &SVGStrokeDashArray::Values(ref other)) => {
                 Ok(SVGStrokeDashArray::Values(this.animate(other, procedure)?))
             },
-            _ => {
-                let (this_weight, other_weight) = procedure.weights();
-                Ok(if this_weight > other_weight { self.clone() } else { other.clone() })
-            },
+            _ => Err(()),
         }
     }
 }
@@ -2684,13 +2675,7 @@ where
             (&SVGOpacity::Opacity(ref this), &SVGOpacity::Opacity(ref other)) => {
                 Ok(SVGOpacity::Opacity(this.animate(other, procedure)?))
             },
-            _ => {
-                // FIXME(nox): Is this correct for addition and accumulation?
-                // I think an error should be returned if it's not
-                // an interpolation.
-                let (this_weight, other_weight) = procedure.weights();
-                Ok(if this_weight > other_weight { self.clone() } else { other.clone() })
-            },
+            _ => Err(()),
         }
     }
 }