Implement TryFrom<Key> for ProprietaryKey

TryFrom` became available in Rust 1.34 so we can use it now we have
bumped our MSRV.

Implement `TryFrom<Key>` for `ProprietaryKey` and deprecate the
`from_key` method.

src/util/psbt/map/global.rs

@@ -12,6 +12,8 @@
 // If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
 //
 
+use core::convert::TryFrom;
+
 use crate::prelude::*;
 
 use crate::io::{self, Cursor, Read};
@@ -191,7 +193,7 @@ impl PartiallySignedTransaction {
                                 return Err(Error::InvalidKey(pair.key).into())
                             }
                         }
-                        PSBT_GLOBAL_PROPRIETARY => match proprietary.entry(raw::ProprietaryKey::from_key(pair.key.clone())?) {
+                        PSBT_GLOBAL_PROPRIETARY => match proprietary.entry(raw::ProprietaryKey::try_from(pair.key.clone())?) {
                             btree_map::Entry::Vacant(empty_key) => {
                                 empty_key.insert(pair.value);
                             },

src/util/psbt/map/input.rs

@@ -16,6 +16,7 @@ use crate::prelude::*;
 use crate::io;
 use core::fmt;
 use core::str::FromStr;
+use core::convert::TryFrom;
 
 use secp256k1;
 use crate::blockdata::script::Script;
@@ -356,7 +357,7 @@ impl Input {
                 }
             }
             PSBT_IN_PROPRIETARY => {
-                let key = raw::ProprietaryKey::from_key(raw_key.clone())?;
+                let key = raw::ProprietaryKey::try_from(raw_key.clone())?;
                 match self.proprietary.entry(key) {
                     btree_map::Entry::Vacant(empty_key) => {
                         empty_key.insert(raw_value);

src/util/psbt/map/output.rs

@@ -249,7 +249,7 @@ impl Output {
                 }
             }
             PSBT_OUT_PROPRIETARY => {
-                let key = raw::ProprietaryKey::from_key(raw_key.clone())?;
+                let key = raw::ProprietaryKey::try_from(raw_key.clone())?;
                 match self.proprietary.entry(key) {
                     btree_map::Entry::Vacant(empty_key) => {
                         empty_key.insert(raw_value);

src/util/psbt/raw.rs

@@ -20,6 +20,7 @@
 
 use crate::prelude::*;
 use core::fmt;
+use core::convert::TryFrom;
 
 use crate::io;
 use crate::consensus::encode::{self, ReadExt, WriteExt, Decodable, Encodable, VarInt, serialize, deserialize, MAX_VEC_SIZE};
@@ -160,12 +161,9 @@ impl<Subtype> Decodable for ProprietaryKey<Subtype> where Subtype: Copy + From<u
 impl<Subtype> ProprietaryKey<Subtype> where Subtype: Copy + From<u8> + Into<u8> {
     /// Constructs [ProprietaryKey] from [Key]; returns
     /// [Error::InvalidProprietaryKey] if `key` do not starts with 0xFC byte
+    #[deprecated(since = "0.29.0", note = "use try_from instead")]
     pub fn from_key(key: Key) -> Result<Self, Error> {
-        if key.type_value != 0xFC {
+        Self::try_from(key)
-            return Err(Error::InvalidProprietaryKey)
-        }
-
-        Ok(deserialize(&key.key)?)
     }
 
     /// Constructs full [Key] corresponding to this proprietary key type
@@ -176,3 +174,21 @@ impl<Subtype> ProprietaryKey<Subtype> where Subtype: Copy + From<u8> + Into<u8>
         }
     }
 }
+
+impl<Subtype> TryFrom<Key> for ProprietaryKey<Subtype>
+where
+    Subtype:Copy + From<u8> + Into<u8> {
+    type Error = Error;
+
+    /// Constructs a [`ProprietaryKey`] from a [`Key`].
+    ///
+    /// # Errors
+    /// Returns [`Error::InvalidProprietaryKey`] if `key` does not start with `0xFC` byte.
+    fn try_from(key: Key) -> Result<Self, Self::Error> {
+        if key.type_value != 0xFC {
+            return Err(Error::InvalidProprietaryKey)
+        }
+
+        Ok(deserialize(&key.key)?)
+    }
+}