io::Error::downcast: avoid reallocation in case of failure

Author: a1phyr

library/std/src/io/error.rs

@@ -950,19 +950,19 @@ impl Error {
     where
         E: error::Error + Send + Sync + 'static,
     {
-        match self.repr.into_data() {
-            ErrorData::Custom(b) if b.error.is::<E>() => {
-                let res = (*b).error.downcast::<E>();
-
-                // downcast is a really trivial and is marked as inline, so
-                // it's likely be inlined here.
-                //
-                // And the compiler should be able to eliminate the branch
-                // that produces `Err` here since b.error.is::<E>()
-                // returns true.
-                Ok(*res.unwrap())
+        if let ErrorData::Custom(c) = self.repr.data()
+            && c.error.is::<E>()
+        {
+            if let ErrorData::Custom(b) = self.repr.into_data()
+                && let Ok(err) = b.error.downcast::<E>()
+            {
+                Ok(*err)
+            } else {
+                // Safety: We have just checked that the condition is true
+                unsafe { crate::hint::unreachable_unchecked() }
             }
-            repr_data => Err(Self { repr: Repr::new(repr_data) }),
+        } else {
+            Err(self)
         }
     }
 

library/std/src/io/error/repr_bitpacked.rs

@@ -133,15 +133,6 @@ unsafe impl Send for Repr {}
 unsafe impl Sync for Repr {}
 
 impl Repr {
-    pub(super) fn new(dat: ErrorData<Box<Custom>>) -> Self {
-        match dat {
-            ErrorData::Os(code) => Self::new_os(code),
-            ErrorData::Simple(kind) => Self::new_simple(kind),
-            ErrorData::SimpleMessage(simple_message) => Self::new_simple_message(simple_message),
-            ErrorData::Custom(b) => Self::new_custom(b),
-        }
-    }
-
     pub(super) fn new_custom(b: Box<Custom>) -> Self {
         let p = Box::into_raw(b).cast::<u8>();
         // Should only be possible if an allocator handed out a pointer with

library/std/src/io/error/repr_unpacked.rs

@@ -10,9 +10,6 @@ pub(super) struct Repr(Inner);
 
 impl Repr {
     #[inline]
-    pub(super) fn new(dat: ErrorData<Box<Custom>>) -> Self {
-        Self(dat)
-    }
     pub(super) fn new_custom(b: Box<Custom>) -> Self {
         Self(Inner::Custom(b))
     }