Initial commit

.gitignore

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+/target
+Cargo.lock

Cargo.toml

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+[package]
+name = "typeid-cast"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

src/lib.rs

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+use std::{any::TypeId, mem::ManuallyDrop};
+
+/// Safe cast using [`TypeId`].
+#[inline]
+pub fn cast<T: 'static, R: 'static>(t: T) -> Result<R, T> {
+    if TypeId::of::<T>() == TypeId::of::<R>() {
+        union U<T, R> {
+            t: ManuallyDrop<T>,
+            r: ManuallyDrop<R>,
+        }
+
+        // SAFETY: we've confirmed the types are the same.
+        Ok(unsafe { ManuallyDrop::into_inner(U { t: ManuallyDrop::new(t) }.r) })
+    } else {
+        Err(t)
+    }
+}