ZJIT: Add IntoUsize, ported from YJIT

Merged: https://github.com/ruby/ruby/pull/13276

zjit/src/cast.rs

@@ -1,3 +1,50 @@
+/// Trait for casting to [usize] that allows you to say `.as_usize()`.
+/// Implementation conditional on the cast preserving the numeric value on
+/// all inputs and being inexpensive.
+///
+/// [usize] is only guaranteed to be more than 16-bit wide, so we can't use
+/// `.into()` to cast an `u32` or an `u64` to a `usize` even though in all
+/// the platforms we support these casts are pretty much no-ops.
+/// We could say `as usize` or `.try_convert().unwrap()` everywhere for these
+/// casts but both options have undesirable consequences if and when we decide to
+/// support 32-bit platforms. Unfortunately we can't implement [::core::convert::From]
+/// for [usize] since both the trait and the type are external. Also, naming
+/// the method `into()` also causes a name conflict.
+pub(crate) trait IntoUsize {
+    /// Convert to usize. Implementation conditional on width of [usize].
+    fn as_usize(self) -> usize;
+}
+
+#[cfg(target_pointer_width = "64")]
+impl IntoUsize for u64 {
+    fn as_usize(self) -> usize {
+        self as usize
+    }
+}
+
+#[cfg(target_pointer_width = "64")]
+impl IntoUsize for u32 {
+    fn as_usize(self) -> usize {
+        self as usize
+    }
+}
+
+impl IntoUsize for u16 {
+    /// Alias for `.into()`. For convenience so you could use the trait for
+    /// all unsgined types.
+    fn as_usize(self) -> usize {
+        self.into()
+    }
+}
+
+impl IntoUsize for u8 {
+    /// Alias for `.into()`. For convenience so you could use the trait for
+    /// all unsgined types.
+    fn as_usize(self) -> usize {
+        self.into()
+    }
+}
+
 /// The `Into<u64>` Rust does not provide.
 /// Convert to u64 with assurance that the value is preserved.
 /// Currently, `usize::BITS == 64` holds for all platforms we support.