introduce StaticDebugAndDisplay blanket trait, use it for all errors

We will want to start boxing our errors to avoid generic error types
(and to reduce the size of error types, by collapsing multiple variants
into one). In order to do this, we need a trait that we can box which
expresses fmt::Debug and fmt::Display.

(Ideally we would also bound on std::error::Error but we can't express
this bound without std until Rust 1.81, and we can't have a std-only
bound since it would make the std feature non-additive.)

This IS A BREAKING CHANGE because it now requires all key parsing errors
to have a 'static bound. But the alternatives are pretty bad:

* We could continue dropping parsing errors and replacing them with
  Strings (or not replacing them at all..); or
* We could add a <Pk> bound to all of our error types

Author: apoelstra

src/blanket_traits.rs

@@ -17,6 +17,11 @@ use core::{fmt, hash};
 
 use crate::MiniscriptKey;
 
+/// Auxiliary trait indicating that a type implements both `Debug` and `Display`.
+pub trait StaticDebugAndDisplay: fmt::Debug + fmt::Display + 'static {}
+
+impl<T: fmt::Debug + fmt::Display + 'static> StaticDebugAndDisplay for T {}
+
 /// Blanket trait describing a key where all associated types implement `FromStr`,
 /// and all `FromStr` errors can be displayed.
 pub trait FromStrKey:
@@ -36,7 +41,7 @@ pub trait FromStrKey:
         + fmt::Debug
         + hash::Hash;
     /// Dummy type. Do not use.
-    type _Sha256FromStrErr: fmt::Debug + fmt::Display;
+    type _Sha256FromStrErr: StaticDebugAndDisplay;
     /// Dummy type. Do not use.
     type _Hash256: FromStr<Err = Self::_Hash256FromStrErr>
         + Clone
@@ -46,7 +51,7 @@ pub trait FromStrKey:
         + fmt::Debug
         + hash::Hash;
     /// Dummy type. Do not use.
-    type _Hash256FromStrErr: fmt::Debug + fmt::Display;
+    type _Hash256FromStrErr: StaticDebugAndDisplay;
     /// Dummy type. Do not use.
     type _Ripemd160: FromStr<Err = Self::_Ripemd160FromStrErr>
         + Clone
@@ -56,7 +61,7 @@ pub trait FromStrKey:
         + fmt::Debug
         + hash::Hash;
     /// Dummy type. Do not use.
-    type _Ripemd160FromStrErr: fmt::Debug + fmt::Display;
+    type _Ripemd160FromStrErr: StaticDebugAndDisplay;
     /// Dummy type. Do not use.
     type _Hash160: FromStr<Err = Self::_Hash160FromStrErr>
         + Clone
@@ -66,9 +71,9 @@ pub trait FromStrKey:
         + fmt::Debug
         + hash::Hash;
     /// Dummy type. Do not use.
-    type _Hash160FromStrErr: fmt::Debug + fmt::Display;
+    type _Hash160FromStrErr: StaticDebugAndDisplay;
     /// Dummy type. Do not use.
-    type _FromStrErr: fmt::Debug + fmt::Display;
+    type _FromStrErr: StaticDebugAndDisplay;
 }
 
 impl<T> FromStrKey for T
@@ -78,11 +83,11 @@ where
     Self::Hash256: FromStr,
     Self::Ripemd160: FromStr,
     Self::Hash160: FromStr,
-    <Self as FromStr>::Err: fmt::Debug + fmt::Display,
-    <<Self as MiniscriptKey>::Sha256 as FromStr>::Err: fmt::Debug + fmt::Display,
-    <Self::Hash256 as FromStr>::Err: fmt::Debug + fmt::Display,
-    <Self::Ripemd160 as FromStr>::Err: fmt::Debug + fmt::Display,
-    <Self::Hash160 as FromStr>::Err: fmt::Debug + fmt::Display,
+    <Self as FromStr>::Err: StaticDebugAndDisplay,
+    <<Self as MiniscriptKey>::Sha256 as FromStr>::Err: StaticDebugAndDisplay,
+    <Self::Hash256 as FromStr>::Err: StaticDebugAndDisplay,
+    <Self::Ripemd160 as FromStr>::Err: StaticDebugAndDisplay,
+    <Self::Hash160 as FromStr>::Err: StaticDebugAndDisplay,
 {
     type _Sha256 = <T as MiniscriptKey>::Sha256;
     type _Sha256FromStrErr = <<T as MiniscriptKey>::Sha256 as FromStr>::Err;