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)
         }
     }