queue: check queue size at compile time

Emit a compile-time error if a queue size is invalid, instead of a
runtime error. This allows catching mistakes earlier, since the queue
size is always defined at compile time.

Unfortunately this means we must remove the corresponding test, as the
only way to test a compile-time failure (without using external
crates) is via doctests, which do not have access to private items
(which is the case for VirtQueue).

Signed-off-by: Carlos López <[email protected]>

src/queue.rs

@@ -21,7 +21,7 @@ use zerocopy::{AsBytes, FromBytes, FromZeroes};
 /// Each device can have zero or more virtqueues.
 ///
 /// * `SIZE`: The size of the queue. This is both the number of descriptors, and the number of slots
-///   in the available and used rings.
+///   in the available and used rings. It must be a power of 2 and fit in a [`u16`].
 #[derive(Debug)]
 pub struct VirtQueue<H: Hal, const SIZE: usize> {
     /// DMA guard
@@ -61,6 +61,8 @@ pub struct VirtQueue<H: Hal, const SIZE: usize> {
 }
 
 impl<H: Hal, const SIZE: usize> VirtQueue<H, SIZE> {
+    const SIZE_OK: () = assert!(SIZE.is_power_of_two() && SIZE <= u16::MAX as usize);
+
     /// Creates a new VirtQueue.
     ///
     /// * `indirect`: Whether to use indirect descriptors. This should be set if the
@@ -74,13 +76,13 @@ impl<H: Hal, const SIZE: usize> VirtQueue<H, SIZE> {
         indirect: bool,
         event_idx: bool,
     ) -> Result<Self> {
+        #[allow(clippy::let_unit_value)]
+        let _ = Self::SIZE_OK;
+
         if transport.queue_used(idx) {
             return Err(Error::AlreadyUsed);
         }
-        if !SIZE.is_power_of_two()
-            || SIZE > u16::MAX.into()
-            || transport.max_queue_size(idx) < SIZE as u32
-        {
+        if transport.max_queue_size(idx) < SIZE as u32 {
             return Err(Error::InvalidParam);
         }
         let size = SIZE as u16;
@@ -976,17 +978,6 @@ mod tests {
     use core::ptr::NonNull;
     use std::sync::{Arc, Mutex};
 
-    #[test]
-    fn invalid_queue_size() {
-        let mut header = VirtIOHeader::make_fake_header(MODERN_VERSION, 1, 0, 0, 4);
-        let mut transport = unsafe { MmioTransport::new(NonNull::from(&mut header)) }.unwrap();
-        // Size not a power of 2.
-        assert_eq!(
-            VirtQueue::<FakeHal, 3>::new(&mut transport, 0, false, false).unwrap_err(),
-            Error::InvalidParam
-        );
-    }
-
     #[test]
     fn queue_too_big() {
         let mut header = VirtIOHeader::make_fake_header(MODERN_VERSION, 1, 0, 0, 4);