diff --git a/openssl-sys/Cargo.toml b/openssl-sys/Cargo.toml
index e09bac56265c3575cb6175767ef6f35726e3cae3..a1c57e6e57cf4376cf9f1e909c7c7f84e5ae6506 100644
--- a/openssl-sys/Cargo.toml
+++ b/openssl-sys/Cargo.toml
@@ -18,9 +18,10 @@ vendored = ['openssl-src']
 libc = "0.2"
 
 [build-dependencies]
-pkg-config = "0.3.9"
 cc = "1.0"
 openssl-src = { version = "111.0.1", optional = true }
+pkg-config = "0.3.9"
+rustc_version = "0.2"
 
 [target.'cfg(target_env = "msvc")'.build-dependencies]
 vcpkg = "0.2"
diff --git a/openssl-sys/build/main.rs b/openssl-sys/build/main.rs
index 4c8b33e76161fc3127abb4e86486a3d7f9d2b2b1..a1f163d71cf36354963d0c2957052e3e86396d0c 100644
--- a/openssl-sys/build/main.rs
+++ b/openssl-sys/build/main.rs
@@ -2,6 +2,7 @@ extern crate cc;
 #[cfg(feature = "vendored")]
 extern crate openssl_src;
 extern crate pkg_config;
+extern crate rustc_version;
 #[cfg(target_env = "msvc")]
 extern crate vcpkg;
 
@@ -41,6 +42,8 @@ fn env(name: &str) -> Option<OsString> {
 }
 
 fn main() {
+    check_rustc_versions();
+
     let target = env::var("TARGET").unwrap();
 
     let (lib_dir, include_dir) = find::get_openssl(&target);
@@ -90,6 +93,14 @@ fn main() {
     }
 }
 
+fn check_rustc_versions() {
+    let version = rustc_version::version().unwrap();
+
+    if version >= rustc_version::Version::new(1, 31, 0) {
+        println!("cargo:rustc-cfg=const_fn");
+    }
+}
+
 /// Validates the header files found in `include_dir` and then returns the
 /// version string of OpenSSL.
 fn validate_headers(include_dirs: &[PathBuf]) -> Version {
diff --git a/openssl-sys/src/err.rs b/openssl-sys/src/err.rs
index 0804dbe736b36771eb8209dc70f81f177dc8c387..ecaec3ed0645007b1374acb824d6aafc61f21afe 100644
--- a/openssl-sys/src/err.rs
+++ b/openssl-sys/src/err.rs
@@ -5,16 +5,24 @@ pub const ERR_TXT_STRING: c_int = 0x02;
 
 pub const ERR_LIB_PEM: c_int = 9;
 
-pub fn ERR_GET_LIB(l: c_ulong) -> c_int {
-    ((l >> 24) & 0x0FF) as c_int
-}
-
-pub fn ERR_GET_FUNC(l: c_ulong) -> c_int {
-    ((l >> 12) & 0xFFF) as c_int
-}
-
-pub fn ERR_GET_REASON(l: c_ulong) -> c_int {
-    (l & 0xFFF) as c_int
+const_fn! {
+    pub const fn ERR_PACK(l: c_int, f: c_int, r: c_int) -> c_ulong {
+        ((l as c_ulong & 0x0FF) << 24) |
+        ((f as c_ulong & 0xFFF) << 12) |
+        ((r as c_ulong & 0xFFF))
+    }
+
+    pub const fn ERR_GET_LIB(l: c_ulong) -> c_int {
+        ((l >> 24) & 0x0FF) as c_int
+    }
+
+    pub const fn ERR_GET_FUNC(l: c_ulong) -> c_int {
+        ((l >> 12) & 0xFFF) as c_int
+    }
+
+    pub const fn ERR_GET_REASON(l: c_ulong) -> c_int {
+        (l & 0xFFF) as c_int
+    }
 }
 
 #[repr(C)]
diff --git a/openssl-sys/src/macros.rs b/openssl-sys/src/macros.rs
index 77bf005d5df01ad6bdfe5db14f924782198257e5..84320b7265e22eb7ea32e7a65dfcf9c561734dd4 100644
--- a/openssl-sys/src/macros.rs
+++ b/openssl-sys/src/macros.rs
@@ -67,3 +67,21 @@ macro_rules! stack {
         }
     };
 }
+
+#[cfg(const_fn)]
+macro_rules! const_fn {
+    ($(pub const fn $name:ident($($arg:ident: $t:ty),*) -> $ret:ty $b:block)*) => {
+        $(
+            pub const fn $name($($arg: $t),*) -> $ret $b
+        )*
+    }
+}
+
+#[cfg(not(const_fn))]
+macro_rules! const_fn {
+    ($(pub const fn $name:ident($($arg:ident: $t:ty),*) -> $ret:ty $b:block)*) => {
+        $(
+            pub fn $name($($arg: $t),*) -> $ret $b
+        )*
+    }
+}