Auto merge of #44890 - nvzqz:str-box-transmute, r=alexcrichton

Remove mem::transmute used in Box<str> conversions

Given that #44877 is failing, I decided to make a separate PR. This is done with the same motivation: to avoid `mem::transmute`-ing non `#[repr(C)]` types.

Author: bors

src/liballoc/boxed.rs

@@ -528,9 +528,7 @@ impl<'a> From<&'a str> for Box<str> {
 #[stable(feature = "boxed_str_conv", since = "1.19.0")]
 impl From<Box<str>> for Box<[u8]> {
     fn from(s: Box<str>) -> Self {
-        unsafe {
-            mem::transmute(s)
-        }
+        unsafe { Box::from_raw(Box::into_raw(s) as *mut [u8]) }
     }
 }
 

src/liballoc/str.rs

@@ -2047,10 +2047,8 @@ impl str {
     /// ```
     #[stable(feature = "box_str", since = "1.4.0")]
     pub fn into_string(self: Box<str>) -> String {
-        unsafe {
-            let slice = mem::transmute::<Box<str>, Box<[u8]>>(self);
-            String::from_utf8_unchecked(slice.into_vec())
-        }
+        let slice = Box::<[u8]>::from(self);
+        unsafe { String::from_utf8_unchecked(slice.into_vec()) }
     }
 
     /// Create a [`String`] by repeating a string `n` times.
@@ -2087,5 +2085,5 @@ impl str {
 /// ```
 #[stable(feature = "str_box_extras", since = "1.20.0")]
 pub unsafe fn from_boxed_utf8_unchecked(v: Box<[u8]>) -> Box<str> {
-    mem::transmute(v)
+    Box::from_raw(Box::into_raw(v) as *mut str)
 }

src/libstd/ffi/c_str.rs

@@ -311,8 +311,8 @@ impl CString {
     #[stable(feature = "cstr_memory", since = "1.4.0")]
     pub unsafe fn from_raw(ptr: *mut c_char) -> CString {
         let len = libc::strlen(ptr) + 1; // Including the NUL byte
-        let slice = slice::from_raw_parts(ptr, len as usize);
-        CString { inner: mem::transmute(slice) }
+        let slice = slice::from_raw_parts_mut(ptr, len as usize);
+        CString { inner: Box::from_raw(slice as *mut [c_char] as *mut [u8]) }
     }
 
     /// Transfers ownership of the string to a C caller.
@@ -480,7 +480,7 @@ impl CString {
     /// ```
     #[stable(feature = "into_boxed_c_str", since = "1.20.0")]
     pub fn into_boxed_c_str(self) -> Box<CStr> {
-        unsafe { mem::transmute(self.into_inner()) }
+        unsafe { Box::from_raw(Box::into_raw(self.into_inner()) as *mut CStr) }
     }
 
     // Bypass "move out of struct which implements [`Drop`] trait" restriction.
@@ -569,7 +569,7 @@ impl Borrow<CStr> for CString {
 impl<'a> From<&'a CStr> for Box<CStr> {
     fn from(s: &'a CStr) -> Box<CStr> {
         let boxed: Box<[u8]> = Box::from(s.to_bytes_with_nul());
-        unsafe { mem::transmute(boxed) }
+        unsafe { Box::from_raw(Box::into_raw(boxed) as *mut CStr) }
     }
 }
 
@@ -593,7 +593,7 @@ impl From<CString> for Box<CStr> {
 impl Default for Box<CStr> {
     fn default() -> Box<CStr> {
         let boxed: Box<[u8]> = Box::from([0]);
-        unsafe { mem::transmute(boxed) }
+        unsafe { Box::from_raw(Box::into_raw(boxed) as *mut CStr) }
     }
 }
 
@@ -817,7 +817,7 @@ impl CStr {
     #[inline]
     #[stable(feature = "cstr_from_bytes", since = "1.10.0")]
     pub unsafe fn from_bytes_with_nul_unchecked(bytes: &[u8]) -> &CStr {
-        mem::transmute(bytes)
+        &*(bytes as *const [u8] as *const CStr)
     }
 
     /// Returns the inner pointer to this C string.
@@ -913,7 +913,7 @@ impl CStr {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn to_bytes_with_nul(&self) -> &[u8] {
-        unsafe { mem::transmute(&self.inner) }
+        unsafe { &*(&self.inner as *const [c_char] as *const [u8]) }
     }
 
     /// Yields a [`&str`] slice if the `CStr` contains valid UTF-8.
@@ -1005,7 +1005,8 @@ impl CStr {
     /// ```
     #[stable(feature = "into_boxed_c_str", since = "1.20.0")]
     pub fn into_c_string(self: Box<CStr>) -> CString {
-        unsafe { mem::transmute(self) }
+        let raw = Box::into_raw(self) as *mut [u8];
+        CString { inner: unsafe { Box::from_raw(raw) } }
     }
 }
 

src/libstd/ffi/os_str.rs

@@ -10,7 +10,6 @@
 
 use borrow::{Borrow, Cow};
 use fmt;
-use mem;
 use ops;
 use cmp;
 use hash::{Hash, Hasher};
@@ -260,7 +259,8 @@ impl OsString {
     /// ```
     #[stable(feature = "into_boxed_os_str", since = "1.20.0")]
     pub fn into_boxed_os_str(self) -> Box<OsStr> {
-        unsafe { mem::transmute(self.inner.into_box()) }
+        let rw = Box::into_raw(self.inner.into_box()) as *mut OsStr;
+        unsafe { Box::from_raw(rw) }
     }
 }
 
@@ -394,7 +394,7 @@ impl OsStr {
     }
 
     fn from_inner(inner: &Slice) -> &OsStr {
-        unsafe { mem::transmute(inner) }
+        unsafe { &*(inner as *const Slice as *const OsStr) }
     }
 
     /// Yields a [`&str`] slice if the `OsStr` is valid Unicode.
@@ -511,23 +511,24 @@ impl OsStr {
     /// [`OsString`]: struct.OsString.html
     #[stable(feature = "into_boxed_os_str", since = "1.20.0")]
     pub fn into_os_string(self: Box<OsStr>) -> OsString {
-        let inner: Box<Slice> = unsafe { mem::transmute(self) };
-        OsString { inner: Buf::from_box(inner) }
+        let boxed = unsafe { Box::from_raw(Box::into_raw(self) as *mut Slice) };
+        OsString { inner: Buf::from_box(boxed) }
     }
 
     /// Gets the underlying byte representation.
     ///
     /// Note: it is *crucial* that this API is private, to avoid
     /// revealing the internal, platform-specific encodings.
     fn bytes(&self) -> &[u8] {
-        unsafe { mem::transmute(&self.inner) }
+        unsafe { &*(&self.inner as *const _ as *const [u8]) }
     }
 }
 
 #[stable(feature = "box_from_os_str", since = "1.17.0")]
 impl<'a> From<&'a OsStr> for Box<OsStr> {
     fn from(s: &'a OsStr) -> Box<OsStr> {
-        unsafe { mem::transmute(s.inner.into_box()) }
+        let rw = Box::into_raw(s.inner.into_box()) as *mut OsStr;
+        unsafe { Box::from_raw(rw) }
     }
 }
 
@@ -548,7 +549,8 @@ impl From<OsString> for Box<OsStr> {
 #[stable(feature = "box_default_extra", since = "1.17.0")]
 impl Default for Box<OsStr> {
     fn default() -> Box<OsStr> {
-        unsafe { mem::transmute(Slice::empty_box()) }
+        let rw = Box::into_raw(Slice::empty_box()) as *mut OsStr;
+        unsafe { Box::from_raw(rw) }
     }
 }
 

src/libstd/path.rs

@@ -86,7 +86,6 @@ use fs;
 use hash::{Hash, Hasher};
 use io;
 use iter::{self, FusedIterator};
-use mem;
 use ops::{self, Deref};
 
 use ffi::{OsStr, OsString};
@@ -317,10 +316,10 @@ fn iter_after<A, I, J>(mut iter: I, mut prefix: J) -> Option<I>
 
 // See note at the top of this module to understand why these are used:
 fn os_str_as_u8_slice(s: &OsStr) -> &[u8] {
-    unsafe { mem::transmute(s) }
+    unsafe { &*(s as *const OsStr as *const [u8]) }
 }
 unsafe fn u8_slice_as_os_str(s: &[u8]) -> &OsStr {
-    mem::transmute(s)
+    &*(s as *const [u8] as *const OsStr)
 }
 
 // Detect scheme on Redox
@@ -1334,15 +1333,17 @@ impl PathBuf {
     /// [`Path`]: struct.Path.html
     #[stable(feature = "into_boxed_path", since = "1.20.0")]
     pub fn into_boxed_path(self) -> Box<Path> {
-        unsafe { mem::transmute(self.inner.into_boxed_os_str()) }
+        let rw = Box::into_raw(self.inner.into_boxed_os_str()) as *mut Path;
+        unsafe { Box::from_raw(rw) }
     }
 }
 
 #[stable(feature = "box_from_path", since = "1.17.0")]
 impl<'a> From<&'a Path> for Box<Path> {
     fn from(path: &'a Path) -> Box<Path> {
         let boxed: Box<OsStr> = path.inner.into();
-        unsafe { mem::transmute(boxed) }
+        let rw = Box::into_raw(boxed) as *mut Path;
+        unsafe { Box::from_raw(rw) }
     }
 }
 
@@ -1589,7 +1590,7 @@ impl Path {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new<S: AsRef<OsStr> + ?Sized>(s: &S) -> &Path {
-        unsafe { mem::transmute(s.as_ref()) }
+        unsafe { &*(s.as_ref() as *const OsStr as *const Path) }
     }
 
     /// Yields the underlying [`OsStr`] slice.
@@ -2312,7 +2313,8 @@ impl Path {
     /// [`PathBuf`]: struct.PathBuf.html
     #[stable(feature = "into_boxed_path", since = "1.20.0")]
     pub fn into_path_buf(self: Box<Path>) -> PathBuf {
-        let inner: Box<OsStr> = unsafe { mem::transmute(self) };
+        let rw = Box::into_raw(self) as *mut OsStr;
+        let inner = unsafe { Box::from_raw(rw) };
         PathBuf { inner: OsString::from(inner) }
     }
 }