Specialize Vec::from_elem<u8> to use calloc or memset

Fixes #38723.

src/liballoc_jemalloc/lib.rs

@@ -38,6 +38,10 @@ mod imp {
                        target_os = "dragonfly", target_os = "windows", target_env = "musl"),
                    link_name = "je_mallocx")]
         fn mallocx(size: size_t, flags: c_int) -> *mut c_void;
+        #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
+                       target_os = "dragonfly", target_os = "windows", target_env = "musl"),
+                   link_name = "je_calloc")]
+        fn calloc(size: size_t, flags: c_int) -> *mut c_void;
         #[cfg_attr(any(target_os = "macos", target_os = "android", target_os = "ios",
                        target_os = "dragonfly", target_os = "windows", target_env = "musl"),
                    link_name = "je_rallocx")]
@@ -56,6 +60,8 @@ mod imp {
         fn nallocx(size: size_t, flags: c_int) -> size_t;
     }
 
+    const MALLOCX_ZERO: c_int = 0x40;
+
     // The minimum alignment guaranteed by the architecture. This value is used to
     // add fast paths for low alignment values. In practice, the alignment is a
     // constant at the call site and the branch will be optimized out.
@@ -91,6 +97,16 @@ mod imp {
         unsafe { mallocx(size as size_t, flags) as *mut u8 }
     }
 
+    #[no_mangle]
+    pub extern "C" fn __rust_allocate_zeroed(size: usize, align: usize) -> *mut u8 {
+        if align <= MIN_ALIGN {
+            unsafe { calloc(size as size_t, 1) as *mut u8 }
+        } else {
+            let flags = align_to_flags(align) | MALLOCX_ZERO;
+            unsafe { mallocx(size as size_t, flags) as *mut u8 }
+        }
+    }
+
     #[no_mangle]
     pub extern "C" fn __rust_reallocate(ptr: *mut u8,
                                         _old_size: usize,
@@ -135,6 +151,11 @@ mod imp {
         bogus()
     }
 
+    #[no_mangle]
+    pub extern "C" fn __rust_allocate_zeroed(_size: usize, _align: usize) -> *mut u8 {
+        bogus()
+    }
+
     #[no_mangle]
     pub extern "C" fn __rust_reallocate(_ptr: *mut u8,
                                         _old_size: usize,