Implement TryFrom for a variety of types

src/time.rs

@@ -23,10 +23,22 @@
 pub struct Time(u64);
 
 impl Time {
-    /// Create a `webpki::Time` from a `std::time::SystemTime`.
+    /// Create a `webpki::Time` from a unix timestamp.
     ///
-    /// This will be replaced with a real `TryFrom<std::time::SystemTime>`
-    /// implementation when `TryFrom` is added to Rust Stable.
+    /// It is usually better to use the less error-prone
+    /// `webpki::Time::try_from(time: std::time::SystemTime)` instead when
+    /// `std::time::SystemTime` is available (when `#![no_std]` isn't being
+    /// used).
+    pub fn from_seconds_since_unix_epoch(secs: u64) -> Time {
+        Time(secs)
+    }
+}
+
+#[cfg(feature = "std")]
+impl core::convert::TryFrom<std::time::SystemTime> for Time {
+    type Error = ring::error::Unspecified;
+
+    /// Create a `webpki::Time` from a `std::time::SystemTime`.
     ///
     /// # Example:
     ///
@@ -36,28 +48,18 @@ impl Time {
     /// # extern crate ring;
     /// # extern crate webpki;
     /// #
-    /// #[cfg(feature = "std")]
+    /// #![cfg(feature = "std")]
+    /// use std::{convert::TryFrom, time::SystemTime};
+    ///
     /// # fn foo() -> Result<(), ring::error::Unspecified> {
-    /// let time = webpki::Time::try_from(std::time::SystemTime::now())?;
+    /// let time = webpki::Time::try_from(SystemTime::now())?;
     /// # Ok(())
     /// # }
     /// ```
-    ///
-    /// Requires the `std` feature.
-    #[cfg(feature = "std")]
-    pub fn try_from(time: std::time::SystemTime) -> Result<Time, ring::error::Unspecified> {
-        time.duration_since(std::time::UNIX_EPOCH)
+    fn try_from(value: std::time::SystemTime) -> Result<Self, Self::Error> {
+        value
+            .duration_since(std::time::UNIX_EPOCH)
             .map(|d| Time::from_seconds_since_unix_epoch(d.as_secs()))
             .map_err(|_| ring::error::Unspecified)
     }
-
-    /// Create a `webpki::Time` from a unix timestamp.
-    ///
-    /// It is usually better to use the less error-prone
-    /// `webpki::Time::try_from(time: &std::time::SystemTime)` instead when
-    /// `std::time::SystemTime` is available (when `#![no_std]` isn't being
-    /// used).
-    pub fn from_seconds_since_unix_epoch(secs: u64) -> Time {
-        Time(secs)
-    }
 }

src/webpki.rs

@@ -108,18 +108,22 @@ pub struct EndEntityCert<'a> {
     inner: cert::Cert<'a>,
 }
 
-impl<'a> EndEntityCert<'a> {
+impl<'a> core::convert::TryFrom<&'a [u8]> for EndEntityCert<'a> {
+    type Error = Error;
+
     /// Parse the ASN.1 DER-encoded X.509 encoding of the certificate
     /// `cert_der`.
-    pub fn from(cert_der: &'a [u8]) -> Result<Self, Error> {
+    fn try_from(cert_der: &'a [u8]) -> Result<Self, Self::Error> {
         Ok(Self {
             inner: cert::parse_cert(
                 untrusted::Input::from(cert_der),
                 cert::EndEntityOrCA::EndEntity,
             )?,
         })
     }
+}
 
+impl<'a> EndEntityCert<'a> {
     /// Verifies that the end-entity certificate is valid for use by a TLS
     /// server.
     ///

tests/integration.rs

@@ -12,6 +12,7 @@
 // ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 // OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
+use core::convert::TryFrom;
 extern crate webpki;
 
 static ALL_SIGALGS: &[&webpki::SignatureAlgorithm] = &[
@@ -35,7 +36,7 @@ static ALL_SIGALGS: &[&webpki::SignatureAlgorithm] = &[
 #[cfg(feature = "alloc")]
 #[test]
 pub fn netflix() {
-    let ee = include_bytes!("netflix/ee.der");
+    let ee: &[u8] = include_bytes!("netflix/ee.der");
     let inter = include_bytes!("netflix/inter.der");
     let ca = include_bytes!("netflix/ca.der");
 
@@ -45,7 +46,7 @@ pub fn netflix() {
     #[allow(clippy::unreadable_literal)] // TODO: Make this clear.
     let time = webpki::Time::from_seconds_since_unix_epoch(1492441716);
 
-    let cert = webpki::EndEntityCert::from(ee).unwrap();
+    let cert = webpki::EndEntityCert::try_from(ee).unwrap();
     assert_eq!(
         Ok(()),
         cert.verify_is_valid_tls_server_cert(ALL_SIGALGS, &anchors, &[inter], time)
@@ -54,7 +55,7 @@ pub fn netflix() {
 
 #[test]
 pub fn ed25519() {
-    let ee = include_bytes!("ed25519/ee.der");
+    let ee: &[u8] = include_bytes!("ed25519/ee.der");
     let ca = include_bytes!("ed25519/ca.der");
 
     let anchors = vec![webpki::trust_anchor_util::cert_der_as_trust_anchor(ca).unwrap()];
@@ -63,7 +64,7 @@ pub fn ed25519() {
     #[allow(clippy::unreadable_literal)] // TODO: Make this clear.
     let time = webpki::Time::from_seconds_since_unix_epoch(1547363522);
 
-    let cert = webpki::EndEntityCert::from(ee).unwrap();
+    let cert = webpki::EndEntityCert::try_from(ee).unwrap();
     assert_eq!(
         Ok(()),
         cert.verify_is_valid_tls_server_cert(ALL_SIGALGS, &anchors, &[], time)