[zircon][rust] Use zr::slice_from_raw_parts in kernel FFI

Replace manual zero-length checks around `core::slice::from_raw_parts`
in `io_buffer_dispatcher_ffi` and `ioapic` with
`zr::slice_from_raw_parts`.

Bug: 448152804
Change-Id: I25cf1322a4fb4eed798cb718b8caac0f431513bf
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1835924
Reviewed-by: Corey Tabaka <eieio@google.com>
Fuchsia-Auto-Submit: Adam Barth <abarth@google.com>
Commit-Queue: Adam Barth <abarth@google.com>

src/lib/zr/src/ptr.rs

diff --git a/src/lib/zr/src/ptr.rs b/src/lib/zr/src/ptr.rs
index 2c60bee..580655ef 100644
--- a/src/lib/zr/src/ptr.rs
+++ b/src/lib/zr/src/ptr.rs
@@ -158,7 +158,6 @@
     if len == 0 {
         &[]
     } else {
-        debug_assert!(!data.is_null());
         // SAFETY: `len > 0`, and the caller guarantees `data` is valid for `len` elements.
         unsafe { core::slice::from_raw_parts(data, len) }
     }
@@ -179,7 +178,6 @@
     if len == 0 {
         &mut []
     } else {
-        debug_assert!(!data.is_null());
         // SAFETY: `len > 0`, and the caller guarantees `data` is uniquely valid for `len` elements.
         unsafe { core::slice::from_raw_parts_mut(data, len) }
     }

zircon/kernel/arch/x86/src/ioapic.rs

diff --git a/zircon/kernel/arch/x86/src/ioapic.rs b/zircon/kernel/arch/x86/src/ioapic.rs
index 0a8e1a2..622255e 100644
--- a/zircon/kernel/arch/x86/src/ioapic.rs
+++ b/zircon/kernel/arch/x86/src/ioapic.rs
@@ -366,19 +366,12 @@
     overrides: *const IoApicIsaOverride,
     num_overrides: usize,
 ) {
-    let io_apic_descs = if num_io_apic_descs > 0 {
-        // SAFETY: `io_apic_descs` points to a valid array of size `num_io_apic_descs` provided by
-        // early boot.
-        unsafe { core::slice::from_raw_parts(io_apic_descs, num_io_apic_descs) }
-    } else {
-        &[]
-    };
-    let overrides = if num_overrides > 0 {
-        // SAFETY: `overrides` points to a valid array of size `num_overrides` provided by early boot.
-        unsafe { core::slice::from_raw_parts(overrides, num_overrides) }
-    } else {
-        &[]
-    };
+    // SAFETY: The caller guarantees `io_apic_descs` points to `num_io_apic_descs` initialized
+    // descriptors whenever `num_io_apic_descs > 0`.
+    let io_apic_descs = unsafe { zr::slice_from_raw_parts(io_apic_descs, num_io_apic_descs) };
+    // SAFETY: The caller guarantees `overrides` points to `num_overrides` initialized entries
+    // whenever `num_overrides > 0`.
+    let overrides = unsafe { zr::slice_from_raw_parts(overrides, num_overrides) };
     apic_io_init_safe(io_apic_descs, overrides);
 }
 

zircon/kernel/object/io_buffer_dispatcher_ffi.rs

diff --git a/zircon/kernel/object/io_buffer_dispatcher_ffi.rs b/zircon/kernel/object/io_buffer_dispatcher_ffi.rs
index 7268310..596dab6 100644
--- a/zircon/kernel/object/io_buffer_dispatcher_ffi.rs
+++ b/zircon/kernel/object/io_buffer_dispatcher_ffi.rs
@@ -53,8 +53,8 @@
     name: *const c_char,
     len: usize,
 ) -> zx_status_t {
-    let name_bytes =
-        if len == 0 { &[] } else { unsafe { core::slice::from_raw_parts(name.cast(), len) } };
+    // SAFETY: The caller guarantees `name` points to `len` initialized bytes whenever `len > 0`.
+    let name_bytes = unsafe { zr::slice_from_raw_parts(name.cast(), len) };
     Status::result_into_raw(disp.set_name(name_bytes))
 }