diff --git a/.circleci/config.yml b/.circleci/config.yml
index 6b4b9417..0b1b9803 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -9,6 +9,7 @@ jobs:
       - image: rust:<< parameters.image >>
     environment:
       RUST_BACKTRACE: 1
+      RUSTFLAGS: -D warnings
     steps:
       - checkout
       - restore_cache:
@@ -36,6 +37,7 @@ jobs:
       xcode: "9.0"
     environment:
       RUST_BACKTRACE: 1
+      RUSTFLAGS: -D warnings
     steps:
       - checkout
       - run: curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain << parameters.version >>
@@ -61,6 +63,6 @@ workflows:
     jobs:
       - linux:
           name: openssl
-          image: 1.37.0-stretch
+          image: 1.37.0
       - macos:
           version: 1.37.0
diff --git a/appveyor.yml b/appveyor.yml
index e7cea7aa..473dd0e9 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,5 +1,5 @@
 environment:
-  RUST_VERSION: 1.32.0
+  RUST_VERSION: 1.37.0
   TARGET: x86_64-pc-windows-msvc
 install:
   - ps: Start-FileDownload "https://static.rust-lang.org/dist/rust-${env:RUST_VERSION}-${env:TARGET}.exe"
diff --git a/src/imp/openssl.rs b/src/imp/openssl.rs
index cc83f2e6..421eda29 100644
--- a/src/imp/openssl.rs
+++ b/src/imp/openssl.rs
@@ -14,7 +14,7 @@ use self::openssl::x509::{X509, X509VerifyResult};
 use std::error;
 use std::fmt;
 use std::io;
-use std::sync::{Once, ONCE_INIT};
+use std::sync::Once;
 
 use {Protocol, TlsAcceptorBuilder, TlsConnectorBuilder};
 use self::openssl::pkey::Private;
@@ -90,7 +90,7 @@ fn supported_protocols(
 }
 
 fn init_trust() {
-    static ONCE: Once = ONCE_INIT;
+    static ONCE: Once = Once::new();
     ONCE.call_once(|| openssl_probe::init_ssl_cert_env_vars());
 }
 
@@ -120,17 +120,10 @@ pub enum Error {
 }
 
 impl error::Error for Error {
-    fn description(&self) -> &str {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
         match *self {
-            Error::Normal(ref e) => error::Error::description(e),
-            Error::Ssl(ref e, _) => error::Error::description(e),
-        }
-    }
-
-    fn cause(&self) -> Option<&error::Error> {
-        match *self {
-            Error::Normal(ref e) => error::Error::cause(e),
-            Error::Ssl(ref e, _) => error::Error::cause(e),
+            Error::Normal(ref e) => error::Error::source(e),
+            Error::Ssl(ref e, _) => error::Error::source(e),
         }
     }
 }
diff --git a/src/imp/schannel.rs b/src/imp/schannel.rs
index d38e6f37..5b0a3961 100644
--- a/src/imp/schannel.rs
+++ b/src/imp/schannel.rs
@@ -34,12 +34,8 @@ fn convert_protocols(min: Option<::Protocol>, max: Option<::Protocol>) -> &'stat
 pub struct Error(io::Error);
 
 impl error::Error for Error {
-    fn description(&self) -> &str {
-        error::Error::description(&self.0)
-    }
-
-    fn cause(&self) -> Option<&error::Error> {
-        error::Error::cause(&self.0)
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        error::Error::source(&self.0)
     }
 }
 
diff --git a/src/imp/security_framework.rs b/src/imp/security_framework.rs
index 4008b080..5060f410 100644
--- a/src/imp/security_framework.rs
+++ b/src/imp/security_framework.rs
@@ -16,7 +16,7 @@ use std::error;
 use std::fmt;
 use std::io;
 use std::sync::Mutex;
-use std::sync::{Once, ONCE_INIT};
+use std::sync::Once;
 
 #[cfg(not(target_os = "ios"))]
 use self::security_framework::os::macos::certificate::{PropertyType, SecCertificateExt};
@@ -31,7 +31,7 @@ use self::security_framework_sys::base::errSecParam;
 
 use {Protocol, TlsAcceptorBuilder, TlsConnectorBuilder};
 
-static SET_AT_EXIT: Once = ONCE_INIT;
+static SET_AT_EXIT: Once = Once::new();
 
 #[cfg(not(target_os = "ios"))]
 lazy_static! {
@@ -51,12 +51,8 @@ fn convert_protocol(protocol: Protocol) -> SslProtocol {
 pub struct Error(base::Error);
 
 impl error::Error for Error {
-    fn description(&self) -> &str {
-        error::Error::description(&self.0)
-    }
-
-    fn cause(&self) -> Option<&error::Error> {
-        error::Error::cause(&self.0)
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        error::Error::source(&self.0)
     }
 }
 
@@ -97,7 +93,7 @@ impl Identity {
         let identity_cert = identity.certificate()?.to_der();
 
         Ok(Identity {
-            identity: identity,
+            identity,
             chain: import
                 .cert_chain
                 .unwrap_or(vec![])
diff --git a/src/lib.rs b/src/lib.rs
index 34f3fa0a..3edcb86f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -132,12 +132,8 @@ pub type Result<T> = result::Result<T, Error>;
 pub struct Error(imp::Error);
 
 impl error::Error for Error {
-    fn description(&self) -> &str {
-        error::Error::description(&self.0)
-    }
-
-    fn cause(&self) -> Option<&error::Error> {
-        error::Error::cause(&self.0)
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        error::Error::source(&self.0)
     }
 }
 
@@ -271,11 +267,7 @@ impl<S> error::Error for HandshakeError<S>
 where
     S: Any + fmt::Debug,
 {
-    fn description(&self) -> &str {
-        "handshake error"
-    }
-
-    fn cause(&self) -> Option<&error::Error> {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
         match *self {
             HandshakeError::Failure(ref e) => Some(e),
             HandshakeError::WouldBlock(_) => None,