Fix clippy warnings for Rust 1.79 (#1151)

This PR fixes new warnings generated by Clippy after upgrading Rust to
1.79.

- Only define `ImmixSpaceArg::mixed_age` when the "vo_bit" feature is
enabled. This eliminates a dead code warning.
- Use the associated constant `<integer_type>::MAX` (and `MIN`, too)
instead of the deprecated `max_value()` method and the
`::std::<integer_type>::MAX` constant.
-   Annotate the destination type of some `transmute` calls.
- Use the safe `std::ptr::NonNull::from()` converter instead of
`transmute`.
-   Initialize array elements of type `Atomic<MapState>` from constant.
- Remove `MutatorContext::barrier_impl`. It is not used by any bindings
right now, and it depends on the layout of `&dyn Trait`. Since `Barrier`
has `Downcast` as a supertrait, if a binding needs a reference to a
concrete `Barrier` implementation, it should just use downcast.

The warning against the `transmute` call in `mock_vm.rs` is suppressed
because it is almost impossible to annotate.

Also fixes compilation problems:

- Made benchmarks related to the "mock_test" feature conditionally
compiled so that we don't get compilation error when running `cargo
clippy --benches` or `cargo clippy --all-targets`.

benches/main.rs

@@ -19,18 +19,25 @@ use criterion::Criterion;
 // However, I will just keep these benchmarks here. If we find it not useful, and we do not plan to improve MockVM, we can delete
 // them.
 
-mod alloc;
-mod sft;
+#[cfg(feature = "mock_test")]
+mod mock_bench;
 
-fn bench_main(c: &mut Criterion) {
+pub fn bench_main(_c: &mut Criterion) {
+    #[cfg(feature = "mock_test")]
     match std::env::var("MMTK_BENCH") {
         Ok(bench) => match bench.as_str() {
-            "alloc" => alloc::bench(c),
-            "sft" => sft::bench(c),
+            "alloc" => mock_bench::alloc::bench(_c),
+            "sft" => mock_bench::sft::bench(_c),
             _ => panic!("Unknown benchmark {:?}", bench),
         },
         Err(_) => panic!("Need to name a benchmark by the env var MMTK_BENCH"),
     }
+
+    #[cfg(not(feature = "mock_test"))]
+    {
+        eprintln!("ERROR: Currently there are no benchmarks when the \"mock_test\" feature is not enabled.");
+        std::process::exit(1);
+    }
 }
 
 criterion_group!(benches, bench_main);

benches/mock_bench/mod.rs

@@ -0,0 +1,2 @@
+pub mod alloc;
+pub mod sft;

src/plan/generational/immix/global.rs

@@ -251,6 +251,7 @@ impl<VM: VMBinding> GenImmix<VM> {
                 // Any object is moved into the mature space, or is copied inside the mature space. We will unlog it.
                 unlog_object_when_traced: false,
                 // In GenImmix, young objects are not allocated in ImmixSpace directly.
+                #[cfg(feature = "vo_bit")]
                 mixed_age: false,
             },
         );

src/plan/immix/global.rs

@@ -138,6 +138,7 @@ impl<VM: VMBinding> Immix<VM> {
             ImmixSpaceArgs {
                 reset_log_bit_in_major_gc: false,
                 unlog_object_when_traced: false,
+                #[cfg(feature = "vo_bit")]
                 mixed_age: false,
             },
         )

src/plan/mutator_context.rs

@@ -153,13 +153,6 @@ impl<VM: VMBinding> MutatorContext<VM> for Mutator<VM> {
         self.mutator_tls
     }
 
-    /// Used by specialized barrier slow-path calls to avoid dynamic dispatches.
-    unsafe fn barrier_impl<B: Barrier<VM>>(&mut self) -> &mut B {
-        debug_assert!(self.barrier().is::<B>());
-        let (payload, _vptr) = std::mem::transmute::<_, (*mut B, *mut ())>(self.barrier());
-        &mut *payload
-    }
-
     fn barrier(&mut self) -> &mut dyn Barrier<VM> {
         &mut *self.barrier
     }
@@ -316,11 +309,6 @@ pub trait MutatorContext<VM: VMBinding>: Send + 'static {
     fn get_tls(&self) -> VMMutatorThread;
     /// Get active barrier trait object
     fn barrier(&mut self) -> &mut dyn Barrier<VM>;
-    /// Force cast the barrier trait object to a concrete implementation.
-    ///
-    /// # Safety
-    /// The safety of this function is ensured by a down-cast check.
-    unsafe fn barrier_impl<B: Barrier<VM>>(&mut self) -> &mut B;
 }
 
 /// This is used for plans to indicate the number of allocators reserved for the plan.

src/plan/sticky/immix/global.rs

@@ -329,6 +329,7 @@ impl<VM: VMBinding> StickyImmix<VM> {
                 // Along with the option above, we unlog them again during tracing.
                 reset_log_bit_in_major_gc: true,
                 // In StickyImmix, both young and old objects are allocated in the ImmixSpace.
+                #[cfg(feature = "vo_bit")]
                 mixed_age: true,
             },
         );

src/policy/immix/immixspace.rs

@@ -77,6 +77,8 @@ pub struct ImmixSpaceArgs {
     /// instance contain young objects, their VO bits need to be updated during this GC.  Currently
     /// only StickyImmix is affected.  GenImmix allocates young objects in a separete CopySpace
     /// nursery and its VO bits can be cleared in bulk.
+    // Currently only used when "vo_bit" is enabled.  Using #[cfg(...)] to eliminate dead code warning.
+    #[cfg(feature = "vo_bit")]
     pub mixed_age: bool,
 }
 

src/policy/sft_map.rs

@@ -103,8 +103,12 @@ pub(crate) type SFTRawPointer = *const (dyn SFT + Sync + 'static);
 /// see if 128 bits atomic instructions are available.
 #[cfg(target_pointer_width = "64")]
 type AtomicDoubleWord = portable_atomic::AtomicU128;
+#[cfg(target_pointer_width = "64")]
+type DoubleWord = u128;
 #[cfg(target_pointer_width = "32")]
 type AtomicDoubleWord = portable_atomic::AtomicU64;
+#[cfg(target_pointer_width = "32")]
+type DoubleWord = u64;
 
 /// The type we store SFT raw pointer as. It basically just double word sized atomic integer.
 /// This type provides an abstraction so we can access SFT easily.
@@ -128,7 +132,7 @@ impl SFTRefStorage {
     }
 
     pub fn new(sft: SFTRawPointer) -> Self {
-        let val = unsafe { std::mem::transmute(sft) };
+        let val: DoubleWord = unsafe { std::mem::transmute(sft) };
         Self(AtomicDoubleWord::new(val))
     }
 
@@ -140,7 +144,7 @@ impl SFTRefStorage {
 
     // Store a raw SFT pointer with the release ordering.
     pub fn store(&self, sft: SFTRawPointer) {
-        let val = unsafe { std::mem::transmute(sft) };
+        let val: DoubleWord = unsafe { std::mem::transmute(sft) };
         self.0.store(val, Ordering::Release)
     }
 }

src/util/address.rs

@@ -134,7 +134,7 @@ impl Address {
     /// The lowest possible address.
     pub const ZERO: Self = Address(0);
     /// The highest possible address.
-    pub const MAX: Self = Address(usize::max_value());
+    pub const MAX: Self = Address(usize::MAX);
 
     /// creates Address from a pointer
     pub fn from_ptr<T>(ptr: *const T) -> Address {
@@ -164,7 +164,6 @@ impl Address {
     /// It is unsafe and the user needs to be aware that they are creating an invalid address.
     /// The max address should only be used as unininitialized or sentinel values in performance critical code (where you dont want to use `Option<Address>`).
     pub unsafe fn max() -> Address {
-        use std::usize;
         Address(usize::MAX)
     }
 

src/util/constants.rs

@@ -78,8 +78,8 @@ mod java_specific_constants {
     pub const LOG_BITS_IN_LONG: u8 = LOG_BITS_IN_BYTE + LOG_BYTES_IN_LONG;
     pub const BITS_IN_LONG: usize = 1 << LOG_BITS_IN_LONG;
 
-    pub const MAX_INT: usize = i32::max_value() as usize; // 0x7fff_ffff
-    pub const MIN_INT: usize = i32::min_value() as u32 as usize; // 0x8000_0000
+    pub const MAX_INT: usize = i32::MAX as usize; // 0x7fff_ffff
+    pub const MIN_INT: usize = i32::MIN as u32 as usize; // 0x8000_0000
 }
 pub(crate) use java_specific_constants::*;
 

src/util/erase_vm.rs

@@ -17,7 +17,8 @@ macro_rules! define_erased_vm_mut_ref {
         pub struct $new_type<'a>(usize, PhantomData<&'a ()>);
         impl<'a> $new_type<'a> {
             pub fn new<VM: VMBinding>(r: &'a mut $orig_type) -> Self {
-                Self(unsafe { std::mem::transmute(r) }, PhantomData)
+                let worker_as_usize: usize = unsafe { std::mem::transmute(r) };
+                Self(worker_as_usize, PhantomData)
             }
             pub fn into_mut<VM: VMBinding>(self) -> &'a mut $orig_type {
                 unsafe { std::mem::transmute(self.0) }

src/util/heap/layout/byte_map_mmapper.rs

@@ -12,7 +12,6 @@ use std::sync::Mutex;
 
 use atomic::Atomic;
 use std::io::Result;
-use std::mem::transmute;
 
 const MMAP_NUM_CHUNKS: usize = if LOG_BYTES_IN_ADDRESS_SPACE == 32 {
     1 << (LOG_BYTES_IN_ADDRESS_SPACE as usize - LOG_MMAP_CHUNK_BYTES)
@@ -134,11 +133,22 @@ impl Mmapper for ByteMapMmapper {
 
 impl ByteMapMmapper {
     pub fn new() -> Self {
-        // Hacky because AtomicU8 does not implement Copy
-        // Should be fiiine because AtomicXXX has the same bit representation as XXX
+        // Because AtomicU8 does not implement Copy, it is a compilation error to usen the
+        // expression `[Atomic::new(MapState::Unmapped); MMAP_NUM_CHUNKS]` because that involves
+        // copying.  We must define a constant for it.
+        //
+        // TODO: Use the inline const expression `const { Atomic::new(MapState::Unmapped) }` after
+        // we bump MSRV to 1.79.
+
+        // If we declare a const Atomic, Clippy will warn about const items being interior mutable.
+        // Using inline const expression will eliminate this warning, but that is experimental until
+        // 1.79.  Fix it after we bump MSRV.
+        #[allow(clippy::declare_interior_mutable_const)]
+        const INITIAL_ENTRY: Atomic<MapState> = Atomic::new(MapState::Unmapped);
+
         ByteMapMmapper {
             lock: Mutex::new(()),
-            mapped: unsafe { transmute([MapState::Unmapped; MMAP_NUM_CHUNKS]) },
+            mapped: [INITIAL_ENTRY; MMAP_NUM_CHUNKS],
             strategy: Atomic::new(MmapStrategy::Normal),
         }
     }

src/util/heap/layout/fragmented_mapper.rs

@@ -9,7 +9,6 @@ use atomic::{Atomic, Ordering};
 use std::cell::UnsafeCell;
 use std::fmt;
 use std::io::Result;
-use std::mem::transmute;
 use std::sync::Mutex;
 
 const MMAP_NUM_CHUNKS: usize = 1 << (33 - LOG_MMAP_CHUNK_BYTES);
@@ -242,8 +241,20 @@ impl FragmentedMapper {
     }
 
     fn new_slab() -> Box<Slab> {
-        let mapped: Box<Slab> =
-            Box::new(unsafe { transmute([MapState::Unmapped; MMAP_NUM_CHUNKS]) });
+        // Because AtomicU8 does not implement Copy, it is a compilation error to usen the
+        // expression `[Atomic::new(MapState::Unmapped); MMAP_NUM_CHUNKS]` because that involves
+        // copying.  We must define a constant for it.
+        //
+        // TODO: Use the inline const expression `const { Atomic::new(MapState::Unmapped) }` after
+        // we bump MSRV to 1.79.
+
+        // If we declare a const Atomic, Clippy will warn about const items being interior mutable.
+        // Using inline const expression will eliminate this warning, but that is experimental until
+        // 1.79.  Fix it after we bump MSRV.
+        #[allow(clippy::declare_interior_mutable_const)]
+        const INITIAL_ENTRY: Atomic<MapState> = Atomic::new(MapState::Unmapped);
+
+        let mapped: Box<Slab> = Box::new([INITIAL_ENTRY; MMAP_NUM_CHUNKS]);
         mapped
     }
 

src/util/heap/space_descriptor.rs

@@ -35,7 +35,7 @@ impl SpaceDescriptor {
         let top = end == vm_layout().heap_end;
         if vm_layout().force_use_contiguous_spaces {
             let space_index = if start > vm_layout().heap_end {
-                ::std::usize::MAX
+                usize::MAX
             } else {
                 start >> vm_layout().space_shift_64()
             };

src/util/int_array_freelist.rs

@@ -1,5 +1,5 @@
 use super::freelist::*;
-use std::{mem, ptr::NonNull};
+use std::ptr::NonNull;
 
 #[derive(Debug)]
 pub struct IntArrayFreeList {
@@ -42,11 +42,12 @@ impl IntArrayFreeList {
         iafl
     }
     pub fn from_parent(parent: &IntArrayFreeList, ordinal: i32) -> Self {
+        let parent_ptr = std::ptr::NonNull::from(parent);
         let iafl = IntArrayFreeList {
             head: -(1 + ordinal),
             heads: parent.heads,
             table: None,
-            parent: Some(unsafe { mem::transmute(parent) }),
+            parent: Some(parent_ptr),
         };
         debug_assert!(-iafl.head <= iafl.heads);
         iafl

src/util/options.rs

@@ -8,7 +8,7 @@ use strum_macros::EnumString;
 
 /// The default stress factor. This is set to the max usize,
 /// which means we will never trigger a stress GC for the default value.
-pub const DEFAULT_STRESS_FACTOR: usize = usize::max_value();
+pub const DEFAULT_STRESS_FACTOR: usize = usize::MAX;
 
 /// The zeroing approach to use for new object allocations.
 /// Affects each plan differently.

src/util/test_util/mock_vm.rs

@@ -44,7 +44,12 @@ lazy_static! {
 // we do not store them for access after the mock method returns.
 macro_rules! lifetime {
     ($e: expr) => {
-        unsafe { std::mem::transmute($e) }
+        unsafe {
+            // The dynamic nature of this lifetime-removal macro makes it impossible to reason
+            // about the source and destination types of the `transmute`.
+            #[allow(clippy::missing_transmute_annotations)]
+            std::mem::transmute($e)
+        }
     };
 }