From 26535a099dbe32a3781d80ca987bb28054d17de5 Mon Sep 17 00:00:00 2001
From: Robbert van der Helm <mail@robbertvanderhelm.nl>
Date: Tue, 22 Nov 2022 17:43:26 +0100
Subject: [PATCH] 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         | 17 ++++++++++++++++-
 src/params/boolean.rs |  3 +++
 src/params/enums.rs   |  6 ++++++
 src/params/float.rs   |  3 +++
 src/params/integer.rs |  3 +++
 5 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/src/params.rs b/src/params.rs
index 44a3838c..537f332d 100644
--- a/src/params.rs
+++ b/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;
 
diff --git a/src/params/boolean.rs b/src/params/boolean.rs
index 9cbe4231..b3b91b34 100644
--- a/src/params/boolean.rs
+++ b/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;
 
diff --git a/src/params/enums.rs b/src/params/enums.rs
index adc2702d..817bdc3e 100644
--- a/src/params/enums.rs
+++ b/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;
 
diff --git a/src/params/float.rs b/src/params/float.rs
index da2c4db6..572de863 100644
--- a/src/params/float.rs
+++ b/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;
 
diff --git a/src/params/integer.rs b/src/params/integer.rs
index 79fb5d86..5f0e3b9f 100644
--- a/src/params/integer.rs
+++ b/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;