First code

Author: mzabaluev

.gitignore

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+/target
+Cargo.lock

Cargo.toml

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+[package]
+name = "unwrap-infallible"
+version = "0.1.0"
+authors = ["Mikhail Zabaluev <[email protected]>"]
+edition = "2018"
+
+[features]
+default = []
+unstable = ["never_type"]
+never_type = []

src/lib.rs

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+#![cfg_attr(feature = "never_type", feature(never_type))]
+
+use core::hint::unreachable_unchecked;
+
+#[cfg(not(feature = "never_type"))]
+use core::convert::Infallible;
+
+pub trait UnwrapInfallible {
+    type Ok;
+    fn unwrap_infallible(self) -> Self::Ok;
+}
+
+#[cfg(feature = "never_type")]
+impl<T, E: From<!>> UnwrapInfallible for Result<T, E> {
+    type Ok = T;
+    fn unwrap_infallible(self) -> T {
+        self.unwrap_or_else(|_| {
+            unsafe { unreachable_unchecked() }
+        })
+    }
+}
+
+#[cfg(not(feature = "never_type"))]
+impl<T> UnwrapInfallible for Result<T, Infallible> {
+    type Ok = T;
+    fn unwrap_infallible(self) -> T {
+        self.unwrap_or_else(|_: Infallible| {
+            unsafe { unreachable_unchecked() }
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::UnwrapInfallible;
+    use core::convert::TryFrom;
+    use core::hint::unreachable_unchecked;
+
+    #[test]
+    fn with_infallible() {
+        let a = 42u8;
+        let a = u64::try_from(a).unwrap_infallible();
+        assert_eq!(a, 42u64);
+    }
+
+    #[cfg(feature = "never_type")]
+    #[test]
+    fn with_never_type() {
+        let r: Result<bool, !> = Ok(true);
+        assert!(r.unwrap_infallible());
+    }
+
+    enum MyInfallibleToken {}
+
+    #[cfg(feature = "never_type")]
+    impl From<!> for MyInfallibleToken {
+        fn from(_: !) -> Self {
+            unsafe { unreachable_unchecked() }
+        }
+    }
+
+    #[cfg(not(feature = "never_type"))]
+    impl<T> UnwrapInfallible for Result<T, MyInfallibleToken> {
+        type Ok = T;
+        fn unwrap_infallible(self) -> T {
+            self.unwrap_or_else(|_| {
+                unsafe { unreachable_unchecked() }
+            })
+        }
+    }
+
+    #[test]
+    fn with_custom_type() {
+        let r: Result<bool, MyInfallibleToken> = Ok(true);
+        assert!(r.unwrap_infallible());
+    }
+}