Reduce some unsafe with APIs from Rust 1.79 (#9203)

Notably `*mut [T]` now sports a `len` method to avoid the need for an
`unsafe` block to read the length field.

crates/wasmtime/src/runtime/vm/sys/custom/mmap.rs

@@ -67,7 +67,7 @@ impl Mmap {
 
     #[inline]
     pub fn len(&self) -> usize {
-        unsafe { (*self.memory.as_ptr()).len() }
+        self.memory.as_ptr().len()
     }
 
     pub unsafe fn make_executable(
@@ -102,7 +102,7 @@ impl Drop for Mmap {
     fn drop(&mut self) {
         unsafe {
             let ptr = self.memory.as_ptr().cast();
-            let len = (*self.memory.as_ptr()).len();
+            let len = self.memory.as_ptr().len();
             if len == 0 {
                 return;
             }

crates/wasmtime/src/runtime/vm/sys/miri/mmap.rs

@@ -68,7 +68,7 @@ impl Mmap {
     }
 
     pub fn len(&self) -> usize {
-        unsafe { (*self.memory.as_ptr()).len() }
+        self.memory.as_ptr().len()
     }
 
     pub unsafe fn make_executable(

crates/wasmtime/src/runtime/vm/sys/unix/mmap.rs

@@ -104,7 +104,7 @@ impl Mmap {
 
     #[inline]
     pub fn len(&self) -> usize {
-        unsafe { (*self.memory.as_ptr()).len() }
+        self.memory.as_ptr().len()
     }
 
     pub unsafe fn make_executable(
@@ -149,7 +149,7 @@ impl Drop for Mmap {
     fn drop(&mut self) {
         unsafe {
             let ptr = self.memory.as_ptr().cast();
-            let len = (*self.memory.as_ptr()).len();
+            let len = self.memory.as_ptr().len();
             if len == 0 {
                 return;
             }

crates/wasmtime/src/runtime/vm/sys/windows/mmap.rs

@@ -164,7 +164,7 @@ impl Mmap {
 
     #[inline]
     pub fn len(&self) -> usize {
-        unsafe { (*self.memory.as_ptr()).len() }
+        self.memory.as_ptr().len()
     }
 
     pub unsafe fn make_executable(