error: avoid leaking pem::PemError into api

Having `From<pem::PemError>` defined on the public `Error`
type means that the `pem` type leaks into rcgen's public API,
complicating semver incompatible updates.

This commit updates the sites that previously used this trait to
use the crate internal `ExternalError` extension trait to map the
`PemError` err to the generic rcgen `Error::PemError` err.

Additionally, the `rcgen::Error::PemError` variant is changed to hold
a `String` with the `pem::PemError` error string instead of the type
itself. This allows the `From` impl on `Error` to be removed, fixing the
type leak.

rcgen/src/error.rs

@@ -34,7 +34,7 @@ pub enum Error {
 	Time,
 	#[cfg(feature = "pem")]
 	/// Error from the pem crate
-	PemError(pem::PemError),
+	PemError(String),
 	/// Error generated by a remote key operation
 	RemoteKeyError,
 	/// Unsupported field when generating a CSR
@@ -98,13 +98,6 @@ impl fmt::Display for Error {
 
 impl std::error::Error for Error {}
 
-#[cfg(feature = "pem")]
-impl From<pem::PemError> for Error {
-	fn from(e: pem::PemError) -> Self {
-		Error::PemError(e)
-	}
-}
-
 /// A trait describing an error that can be converted into an `rcgen::Error`.
 ///
 /// We use this trait to avoid leaking external error types into the public API

rcgen/src/key_pair.rs

@@ -68,7 +68,7 @@ impl KeyPair {
 	/// Parses the key pair from the ASCII PEM format
 	#[cfg(feature = "pem")]
 	pub fn from_pem(pem_str: &str) -> Result<Self, Error> {
-		let private_key = pem::parse(pem_str)?;
+		let private_key = pem::parse(pem_str)._err()?;
 		let private_key_der: &[_] = private_key.contents();
 		Ok(private_key_der.try_into()?)
 	}
@@ -91,7 +91,7 @@ impl KeyPair {
 		pem_str: &str,
 		alg: &'static SignatureAlgorithm,
 	) -> Result<Self, Error> {
-		let private_key = pem::parse(pem_str)?;
+		let private_key = pem::parse(pem_str)._err()?;
 		let private_key_der: &[_] = private_key.contents();
 		Ok(Self::from_der_and_sign_algo(private_key_der, alg)?)
 	}
@@ -383,6 +383,12 @@ impl<T> ExternalError<T> for Result<T, ring::error::Unspecified> {
 	}
 }
 
+impl<T> ExternalError<T> for Result<T, pem::PemError> {
+	fn _err(self) -> Result<T, Error> {
+		self.map_err(|e| Error::PemError(e.to_string()))
+	}
+}
+
 pub(crate) trait PublicKeyData {
 	fn alg(&self) -> &SignatureAlgorithm;