age-plugin: Add impls of state machine traits for Infallible

This enables representing a plugin without a recipient or identity
handler in the type system.

age-plugin/CHANGELOG.md

@@ -9,6 +9,10 @@ and this project adheres to Rust's notion of
 to 1.0.0 are beta releases.
 
 ## [Unreleased]
+### Added
+- `impl age_plugin::identity::IdentityPluginV1 for std::convert::Infallible`
+- `impl age_plugin::recipient::RecipientPluginV1 for std::convert::Infallible`
+
 ### Fixed
 - `age_plugin::run_state_machine` reverts to non-optional arguments, undoing the
   change from the previous release. The type arguments were impossible to set

age-plugin/src/identity.rs

@@ -7,7 +7,9 @@ use age_core::{
 };
 use base64::{prelude::BASE64_STANDARD_NO_PAD, Engine};
 use bech32::FromBase32;
+
 use std::collections::HashMap;
+use std::convert::Infallible;
 use std::io;
 
 use crate::{Callbacks, PLUGIN_IDENTITY_PREFIX};
@@ -49,6 +51,22 @@ pub trait IdentityPluginV1 {
     ) -> io::Result<HashMap<usize, Result<FileKey, Vec<Error>>>>;
 }
 
+impl IdentityPluginV1 for Infallible {
+    fn add_identity(&mut self, _: usize, _: &str, _: &[u8]) -> Result<(), Error> {
+        // This is never executed.
+        Ok(())
+    }
+
+    fn unwrap_file_keys(
+        &mut self,
+        _: Vec<Vec<Stanza>>,
+        _: impl Callbacks<Error>,
+    ) -> io::Result<HashMap<usize, Result<FileKey, Vec<Error>>>> {
+        // This is never executed.
+        Ok(HashMap::new())
+    }
+}
+
 /// The interface that age plugins can use to interact with an age implementation.
 struct BidirCallbacks<'a, 'b, R: io::Read, W: io::Write>(&'b mut BidirSend<'a, R, W>);
 

age-plugin/src/recipient.rs

@@ -7,6 +7,8 @@ use age_core::{
 };
 use base64::{prelude::BASE64_STANDARD_NO_PAD, Engine};
 use bech32::FromBase32;
+
+use std::convert::Infallible;
 use std::io;
 
 use crate::{Callbacks, PLUGIN_IDENTITY_PREFIX, PLUGIN_RECIPIENT_PREFIX};
@@ -48,6 +50,27 @@ pub trait RecipientPluginV1 {
     ) -> io::Result<Result<Vec<Vec<Stanza>>, Vec<Error>>>;
 }
 
+impl RecipientPluginV1 for Infallible {
+    fn add_recipient(&mut self, _: usize, _: &str, _: &[u8]) -> Result<(), Error> {
+        // This is never executed.
+        Ok(())
+    }
+
+    fn add_identity(&mut self, _: usize, _: &str, _: &[u8]) -> Result<(), Error> {
+        // This is never executed.
+        Ok(())
+    }
+
+    fn wrap_file_keys(
+        &mut self,
+        _: Vec<FileKey>,
+        _: impl Callbacks<Error>,
+    ) -> io::Result<Result<Vec<Vec<Stanza>>, Vec<Error>>> {
+        // This is never executed.
+        Ok(Ok(vec![]))
+    }
+}
+
 /// The interface that age plugins can use to interact with an age implementation.
 struct BidirCallbacks<'a, 'b, R: io::Read, W: io::Write>(&'b mut BidirSend<'a, R, W>);