Auto merge of #27252 - tbu-:pr_less_transmutes, r=alexcrichton

The replacements are functions that usually use a single `mem::transmute` in their body and restrict input and output via more concrete types than `T` and `U`. Worth noting are the `transmute` functions for slices and the `from_utf8*` family for mutable slices. Additionally, `mem::transmute` was often used for casting raw pointers, when you can already cast raw pointers just fine with `as`.

This builds upon #27233.

Author: bors

src/liballoc/arc.rs

@@ -193,7 +193,7 @@ impl<T> Arc<T> {
             weak: atomic::AtomicUsize::new(1),
             data: data,
         };
-        Arc { _ptr: unsafe { NonZero::new(mem::transmute(x)) } }
+        Arc { _ptr: unsafe { NonZero::new(Box::into_raw(x)) } }
     }
 }
 

src/libarena/lib.rs

@@ -152,7 +152,7 @@ unsafe fn destroy_chunk(chunk: &Chunk) {
     let fill = chunk.fill.get();
 
     while idx < fill {
-        let tydesc_data: *const usize = mem::transmute(buf.offset(idx as isize));
+        let tydesc_data = buf.offset(idx as isize) as *const usize;
         let (tydesc, is_done) = un_bitpack_tydesc_ptr(*tydesc_data);
         let (size, align) = ((*tydesc).size, (*tydesc).align);
 
@@ -305,7 +305,7 @@ impl<'longer_than_self> Arena<'longer_than_self> {
             let ptr = ptr as *mut T;
             // Write in our tydesc along with a bit indicating that it
             // has *not* been initialized yet.
-            *ty_ptr = mem::transmute(tydesc);
+            *ty_ptr = bitpack_tydesc_ptr(tydesc, false);
             // Actually initialize it
             ptr::write(&mut(*ptr), op());
             // Now that we are done, update the tydesc to indicate that
@@ -443,8 +443,7 @@ impl<T> TypedArenaChunk<T> {
     fn start(&self) -> *const u8 {
         let this: *const TypedArenaChunk<T> = self;
         unsafe {
-            mem::transmute(round_up(this.offset(1) as usize,
-                                    mem::align_of::<T>()))
+            round_up(this.offset(1) as usize, mem::align_of::<T>()) as *const u8
         }
     }
 
@@ -488,7 +487,7 @@ impl<T> TypedArena<T> {
         }
 
         let ptr: &mut T = unsafe {
-            let ptr: &mut T = mem::transmute(self.ptr.clone());
+            let ptr: &mut T = &mut *(self.ptr.get() as *mut T);
             ptr::write(ptr, object);
             self.ptr.set(self.ptr.get().offset(1));
             ptr

src/libcollections/btree/node.rs

@@ -25,7 +25,7 @@ use core::iter::Zip;
 use core::marker::PhantomData;
 use core::ops::{Deref, DerefMut, Index, IndexMut};
 use core::ptr::Unique;
-use core::{slice, mem, ptr, cmp, raw};
+use core::{slice, mem, ptr, cmp};
 use alloc::heap::{self, EMPTY};
 
 use borrow::Borrow;
@@ -357,7 +357,10 @@ impl<K, V> Node<K, V> {
 
     #[inline]
     pub fn as_slices_mut<'a>(&'a mut self) -> (&'a mut [K], &'a mut [V]) {
-        unsafe { mem::transmute(self.as_slices()) }
+        unsafe {(
+            slice::from_raw_parts_mut(*self.keys, self.len()),
+            slice::from_raw_parts_mut(*self.vals, self.len()),
+        )}
     }
 
     #[inline]
@@ -372,10 +375,7 @@ impl<K, V> Node<K, V> {
                     None => heap::EMPTY as *const Node<K,V>,
                     Some(ref p) => **p as *const Node<K,V>,
                 };
-                mem::transmute(raw::Slice {
-                    data: data,
-                    len: self.len() + 1
-                })
+                slice::from_raw_parts(data, self.len() + 1)
             }
         };
         NodeSlice {
@@ -390,6 +390,7 @@ impl<K, V> Node<K, V> {
 
     #[inline]
     pub fn as_slices_internal_mut<'b>(&'b mut self) -> MutNodeSlice<'b, K, V> {
+        // FIXME(#27620): Bad: This relies on structure layout!
         unsafe { mem::transmute(self.as_slices_internal()) }
     }
 

src/libcollections/lib.rs

@@ -47,7 +47,6 @@
 #![feature(oom)]
 #![feature(pattern)]
 #![feature(ptr_as_ref)]
-#![feature(raw)]
 #![feature(slice_patterns)]
 #![feature(staged_api)]
 #![feature(step_by)]

src/libcollections/str.rs

@@ -504,8 +504,7 @@ impl str {
     /// # Unsafety
     ///
     /// Caller must check both UTF-8 sequence boundaries and the boundaries
-    /// of the entire slice as
-    /// well.
+    /// of the entire slice as well.
     ///
     /// # Examples
     ///

src/libcollections/string.rs

@@ -977,7 +977,7 @@ impl ops::Index<ops::RangeFull> for String {
 
     #[inline]
     fn index(&self, _index: ops::RangeFull) -> &str {
-        unsafe { mem::transmute(&*self.vec) }
+        unsafe { str::from_utf8_unchecked(&self.vec) }
     }
 }
 
@@ -1016,15 +1016,15 @@ impl ops::Deref for String {
 
     #[inline]
     fn deref(&self) -> &str {
-        unsafe { mem::transmute(&self.vec[..]) }
+        unsafe { str::from_utf8_unchecked(&self.vec) }
     }
 }
 
 #[stable(feature = "derefmut_for_string", since = "1.2.0")]
 impl ops::DerefMut for String {
     #[inline]
     fn deref_mut(&mut self) -> &mut str {
-        unsafe { mem::transmute(&mut self.vec[..]) }
+        unsafe { mem::transmute(&mut *self.vec) }
     }
 }
 

src/libcollections/vec.rs

@@ -1714,7 +1714,7 @@ impl<T> DoubleEndedIterator for IntoIter<T> {
                 } else {
                     self.end = self.end.offset(-1);
 
-                    Some(ptr::read(mem::transmute(self.end)))
+                    Some(ptr::read(self.end))
                 }
             }
         }

src/libcore/any.rs

@@ -146,7 +146,7 @@ impl Any {
                 let to: TraitObject = transmute(self);
 
                 // Extract the data pointer
-                Some(transmute(to.data))
+                Some(&*(to.data as *const T))
             }
         } else {
             None
@@ -164,7 +164,7 @@ impl Any {
                 let to: TraitObject = transmute(self);
 
                 // Extract the data pointer
-                Some(transmute(to.data))
+                Some(&mut *(to.data as *const T as *mut T))
             }
         } else {
             None

src/libcore/cmp.rs

@@ -19,6 +19,7 @@
 
 use self::Ordering::*;
 
+use mem;
 use marker::Sized;
 use option::Option::{self, Some, None};
 
@@ -114,6 +115,10 @@ pub enum Ordering {
 }
 
 impl Ordering {
+    unsafe fn from_i8_unchecked(v: i8) -> Ordering {
+        mem::transmute(v)
+    }
+
     /// Reverse the `Ordering`.
     ///
     /// * `Less` becomes `Greater`.
@@ -155,7 +160,7 @@ impl Ordering {
             //
             // NB. it is safe because of the explicit discriminants
             // given above.
-            ::mem::transmute::<_, Ordering>(-(self as i8))
+            Ordering::from_i8_unchecked(-(self as i8))
         }
     }
 }

src/libcore/fmt/mod.rs

@@ -90,7 +90,7 @@ pub trait Write {
     fn write_char(&mut self, c: char) -> Result {
         let mut utf_8 = [0u8; 4];
         let bytes_written = c.encode_utf8(&mut utf_8).unwrap_or(0);
-        self.write_str(unsafe { mem::transmute(&utf_8[..bytes_written]) })
+        self.write_str(unsafe { str::from_utf8_unchecked(&utf_8[..bytes_written]) })
     }
 
     /// Glue for usage of the `write!` macro with implementers of this trait.
@@ -1320,7 +1320,7 @@ impl Display for char {
         } else {
             let mut utf8 = [0; 4];
             let amt = self.encode_utf8(&mut utf8).unwrap_or(0);
-            let s: &str = unsafe { mem::transmute(&utf8[..amt]) };
+            let s: &str = unsafe { str::from_utf8_unchecked(&utf8[..amt]) };
             f.pad(s)
         }
     }

src/libcore/ptr.rs

@@ -16,13 +16,13 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use mem;
 use clone::Clone;
 use intrinsics;
 use ops::Deref;
 use fmt;
 use option::Option::{self, Some, None};
 use marker::{PhantomData, Send, Sized, Sync};
+use mem;
 use nonzero::NonZero;
 
 use cmp::{PartialEq, Eq, Ord, PartialOrd};
@@ -100,7 +100,7 @@ pub unsafe fn swap<T>(x: *mut T, y: *mut T) {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn replace<T>(dest: *mut T, mut src: T) -> T {
-    mem::swap(mem::transmute(dest), &mut src); // cannot overlap
+    mem::swap(&mut *dest, &mut src); // cannot overlap
     src
 }
 
@@ -327,15 +327,14 @@ impl<T: ?Sized> Clone for *mut T {
 
 // Equality for extern "C" fn pointers
 mod externfnpointers {
-    use mem;
     use cmp::PartialEq;
 
     #[stable(feature = "rust1", since = "1.0.0")]
     impl<_R> PartialEq for extern "C" fn() -> _R {
         #[inline]
         fn eq(&self, other: &extern "C" fn() -> _R) -> bool {
-            let self_: *const () = unsafe { mem::transmute(*self) };
-            let other_: *const () = unsafe { mem::transmute(*other) };
+            let self_ = *self as usize;
+            let other_ = *other as usize;
             self_ == other_
         }
     }
@@ -345,9 +344,9 @@ mod externfnpointers {
             impl<_R,$($p),*> PartialEq for extern "C" fn($($p),*) -> _R {
                 #[inline]
                 fn eq(&self, other: &extern "C" fn($($p),*) -> _R) -> bool {
-                    let self_: *const () = unsafe { mem::transmute(*self) };
+                    let self_ = *self as usize;
 
-                    let other_: *const () = unsafe { mem::transmute(*other) };
+                    let other_ = *other as usize;
                     self_ == other_
                 }
             }

src/libcore/slice.rs

@@ -33,7 +33,6 @@
 // * The `raw` and `bytes` submodules.
 // * Boilerplate trait implementations.
 
-use mem::transmute;
 use clone::Clone;
 use cmp::{Ordering, PartialEq, PartialOrd, Eq, Ord};
 use cmp::Ordering::{Less, Equal, Greater};
@@ -148,7 +147,7 @@ macro_rules! slice_ref {
             // Use a non-null pointer value
             &mut *(1 as *mut _)
         } else {
-            transmute(ptr)
+            mem::transmute(ptr)
         }
     }};
 }
@@ -261,7 +260,7 @@ impl<T> SliceExt for [T] {
 
     #[inline]
     unsafe fn get_unchecked(&self, index: usize) -> &T {
-        transmute(self.repr().data.offset(index as isize))
+        &*(self.repr().data.offset(index as isize))
     }
 
     #[inline]
@@ -430,7 +429,7 @@ impl<T> SliceExt for [T] {
 
     #[inline]
     unsafe fn get_unchecked_mut(&mut self, index: usize) -> &mut T {
-        transmute((self.repr().data as *mut T).offset(index as isize))
+        &mut *(self.repr().data as *mut T).offset(index as isize)
     }
 
     #[inline]
@@ -547,8 +546,7 @@ impl<T> ops::Index<usize> for [T] {
 
     fn index(&self, index: usize) -> &T {
         assert!(index < self.len());
-
-        unsafe { mem::transmute(self.repr().data.offset(index as isize)) }
+        unsafe { self.get_unchecked(index) }
     }
 }
 
@@ -557,8 +555,7 @@ impl<T> ops::IndexMut<usize> for [T] {
     #[inline]
     fn index_mut(&mut self, index: usize) -> &mut T {
         assert!(index < self.len());
-
-        unsafe { mem::transmute(self.repr().data.offset(index as isize)) }
+        unsafe { self.get_unchecked_mut(index) }
     }
 }
 
@@ -1427,7 +1424,7 @@ pub fn mut_ref_slice<'a, A>(s: &'a mut A) -> &'a mut [A] {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn from_raw_parts<'a, T>(p: *const T, len: usize) -> &'a [T] {
-    transmute(RawSlice { data: p, len: len })
+    mem::transmute(RawSlice { data: p, len: len })
 }
 
 /// Performs the same functionality as `from_raw_parts`, except that a mutable
@@ -1439,7 +1436,38 @@ pub unsafe fn from_raw_parts<'a, T>(p: *const T, len: usize) -> &'a [T] {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn from_raw_parts_mut<'a, T>(p: *mut T, len: usize) -> &'a mut [T] {
-    transmute(RawSlice { data: p, len: len })
+    mem::transmute(RawSlice { data: p, len: len })
+}
+
+#[inline]
+fn check_types<T,U>() {
+    assert!(mem::size_of::<T>() == mem::size_of::<U>());
+    assert!(mem::align_of::<T>() % mem::align_of::<U>() == 0)
+}
+
+/// Reinterprets a slice of one type as a slice of another type.
+///
+/// Both types have to have the same size and the type that is converted to
+/// must have equal or less restrictive alignment.
+///
+/// # Panics
+///
+/// This functions panics if the above preconditions about the types are not
+/// met.
+#[inline]
+unsafe fn transmute<T,U>(slice: &[T]) -> &[U] {
+    check_types::<T,U>();
+    from_raw_parts(slice.as_ptr() as *const U, slice.len())
+}
+
+/// Reinterprets a mutable slice of one type as a mutable slice of another
+/// type.
+///
+/// Equivalent of `slice::transmute` for mutable slices.
+#[inline]
+unsafe fn transmute_mut<T,U>(slice: &mut [T]) -> &mut [U] {
+    check_types::<T,U>();
+    from_raw_parts_mut(slice.as_mut_ptr() as *mut U, slice.len())
 }
 
 //
@@ -1580,9 +1608,9 @@ macro_rules! impl_int_slice {
             #[inline]
             fn as_signed(&self) -> &[$s] { unsafe { transmute(self) } }
             #[inline]
-            fn as_unsigned_mut(&mut self) -> &mut [$u] { unsafe { transmute(self) } }
+            fn as_unsigned_mut(&mut self) -> &mut [$u] { unsafe { transmute_mut(self) } }
             #[inline]
-            fn as_signed_mut(&mut self) -> &mut [$s] { unsafe { transmute(self) } }
+            fn as_signed_mut(&mut self) -> &mut [$s] { unsafe { transmute_mut(self) } }
         }
     }
 }