Seal the Params trait

This avoids situations like #41 where people try to implement the trait
only to find out that you simply can't.

src/params.rs

@@ -53,12 +53,27 @@ bitflags::bitflags! {
     }
 }
 
+// See https://rust-lang.github.io/api-guidelines/future-proofing.html for more information
+mod sealed {
+    /// Dummy trait to prevent [`Param`] from being implemented outside of NIH-plug. This is not
+    /// possible because of the way `ParamPtr` works, so it's best to just make it flat out impossible.
+    pub trait Sealed {}
+}
+pub(crate) use sealed::Sealed;
+
 /// Describes a single parameter of any type. Most parameter implementations also have a field
 /// called `value` that and a field called `smoothed`. The former stores the latest unsmoothed
 /// value, and the latter can be used to access the smoother. These two fields should be used in DSP
 /// code to either get the parameter's current (smoothed) value. In UI code the getters from this
 /// trait should be used instead.
-pub trait Param: Display {
+///
+/// # Sealed
+///
+/// This trait cannot be implemented outside of NIH-plug itself. If you want to create new
+/// abstractions around parameters, consider wrapping them in a struct instead. Then use the
+/// `#[nested(id_prefix = "foo")]` syntax from the [`Params`] trait to reuse that wrapper in
+/// multiple places.
+pub trait Param: Display + sealed::Sealed {
     /// The plain parameter type.
     type Plain: PartialEq;
 

src/params/boolean.rs

@@ -61,6 +61,9 @@ impl Display for BoolParam {
     }
 }
 
+// `Params` can not be implemented outside of NIH-plug itself because `ParamPtr` is also closed
+impl super::Sealed for BoolParam {}
+
 impl Param for BoolParam {
     type Plain = bool;
 

src/params/enums.rs

@@ -110,6 +110,9 @@ impl Display for EnumParamInner {
     }
 }
 
+// `Params` can not be implemented outside of NIH-plug itself because `ParamPtr` is also closed
+impl<T: Enum + PartialEq> super::Sealed for EnumParam<T> {}
+
 impl<T: Enum + PartialEq> Param for EnumParam<T> {
     type Plain = T;
 
@@ -188,6 +191,9 @@ impl<T: Enum + PartialEq> Param for EnumParam<T> {
     }
 }
 
+// `Params` can not be implemented outside of NIH-plug itself because `ParamPtr` is also closed
+impl super::Sealed for EnumParamInner {}
+
 impl Param for EnumParamInner {
     type Plain = i32;
 

src/params/float.rs

@@ -86,6 +86,9 @@ impl Display for FloatParam {
     }
 }
 
+// `Params` can not be implemented outside of NIH-plug itself because `ParamPtr` is also closed
+impl super::Sealed for FloatParam {}
+
 impl Param for FloatParam {
     type Plain = f32;
 

src/params/integer.rs

@@ -78,6 +78,9 @@ impl Display for IntParam {
     }
 }
 
+// `Params` can not be implemented outside of NIH-plug itself because `ParamPtr` is also closed
+impl super::Sealed for IntParam {}
+
 impl Param for IntParam {
     type Plain = i32;