Rollup merge of rust-lang#70226 - RalfJung:checked, r=oli-obk

use checked casts and arithmetic in Miri engine

This is unfortunately pretty annoying because we have to cast back and forth between `u64` and `usize` more often that should be necessary, and that cast is considered fallible.

For example, should [this](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/mir/interpret/value/enum.ConstValue.html) really be `usize`?

Also, `LayoutDetails` uses `usize` for field indices, but in Miri we use `u64` to be able to also handle array indexing. Maybe methods like `mplace_field` should be suitably generalized to accept both `u64` and `usize`?

r? @oli-obk Cc @eddyb

src/librustc/mir/interpret/allocation.rs

@@ -1,18 +1,20 @@
 //! The virtual memory representation of the MIR interpreter.
 
+use std::borrow::Cow;
+use std::convert::TryFrom;
+use std::iter;
+use std::ops::{Deref, DerefMut, Range};
+
+use rustc_ast::ast::Mutability;
+use rustc_data_structures::sorted_map::SortedMap;
+use rustc_target::abi::HasDataLayout;
+
 use super::{
     read_target_uint, write_target_uint, AllocId, InterpResult, Pointer, Scalar, ScalarMaybeUndef,
 };
 
 use crate::ty::layout::{Align, Size};
 
-use rustc_ast::ast::Mutability;
-use rustc_data_structures::sorted_map::SortedMap;
-use rustc_target::abi::HasDataLayout;
-use std::borrow::Cow;
-use std::iter;
-use std::ops::{Deref, DerefMut, Range};
-
 // NOTE: When adding new fields, make sure to adjust the `Snapshot` impl in
 // `src/librustc_mir/interpret/snapshot.rs`.
 #[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash, RustcEncodable, RustcDecodable)]
@@ -90,7 +92,7 @@ impl<Tag> Allocation<Tag> {
     /// Creates a read-only allocation initialized by the given bytes
     pub fn from_bytes<'a>(slice: impl Into<Cow<'a, [u8]>>, align: Align) -> Self {
         let bytes = slice.into().into_owned();
-        let size = Size::from_bytes(bytes.len() as u64);
+        let size = Size::from_bytes(bytes.len());
         Self {
             bytes,
             relocations: Relocations::new(),
@@ -107,9 +109,8 @@ impl<Tag> Allocation<Tag> {
     }
 
     pub fn undef(size: Size, align: Align) -> Self {
-        assert_eq!(size.bytes() as usize as u64, size.bytes());
         Allocation {
-            bytes: vec![0; size.bytes() as usize],
+            bytes: vec![0; size.bytes_usize()],
             relocations: Relocations::new(),
             undef_mask: UndefMask::new(size, false),
             size,
@@ -152,7 +153,7 @@ impl Allocation<(), ()> {
 /// Raw accessors. Provide access to otherwise private bytes.
 impl<Tag, Extra> Allocation<Tag, Extra> {
     pub fn len(&self) -> usize {
-        self.size.bytes() as usize
+        self.size.bytes_usize()
     }
 
     /// Looks at a slice which may describe undefined bytes or describe a relocation. This differs
@@ -183,20 +184,15 @@ impl<'tcx, Tag: Copy, Extra: AllocationExtra<Tag>> Allocation<Tag, Extra> {
     #[inline]
     fn check_bounds(&self, offset: Size, size: Size) -> Range<usize> {
         let end = offset + size; // This does overflow checking.
-        assert_eq!(
-            end.bytes() as usize as u64,
-            end.bytes(),
-            "cannot handle this access on this host architecture"
-        );
-        let end = end.bytes() as usize;
+        let end = usize::try_from(end.bytes()).expect("access too big for this host architecture");
         assert!(
             end <= self.len(),
             "Out-of-bounds access at offset {}, size {} in allocation of size {}",
             offset.bytes(),
             size.bytes(),
             self.len()
         );
-        (offset.bytes() as usize)..end
+        offset.bytes_usize()..end
     }
 
     /// The last argument controls whether we error out when there are undefined
@@ -294,11 +290,10 @@ impl<'tcx, Tag: Copy, Extra: AllocationExtra<Tag>> Allocation<Tag, Extra> {
         cx: &impl HasDataLayout,
         ptr: Pointer<Tag>,
     ) -> InterpResult<'tcx, &[u8]> {
-        assert_eq!(ptr.offset.bytes() as usize as u64, ptr.offset.bytes());
-        let offset = ptr.offset.bytes() as usize;
+        let offset = ptr.offset.bytes_usize();
         Ok(match self.bytes[offset..].iter().position(|&c| c == 0) {
             Some(size) => {
-                let size_with_null = Size::from_bytes((size + 1) as u64);
+                let size_with_null = Size::from_bytes(size) + Size::from_bytes(1);
                 // Go through `get_bytes` for checks and AllocationExtra hooks.
                 // We read the null, so we include it in the request, but we want it removed
                 // from the result, so we do subslicing.
@@ -343,7 +338,7 @@ impl<'tcx, Tag: Copy, Extra: AllocationExtra<Tag>> Allocation<Tag, Extra> {
         let (lower, upper) = src.size_hint();
         let len = upper.expect("can only write bounded iterators");
         assert_eq!(lower, len, "can only write iterators with a precise length");
-        let bytes = self.get_bytes_mut(cx, ptr, Size::from_bytes(len as u64))?;
+        let bytes = self.get_bytes_mut(cx, ptr, Size::from_bytes(len))?;
         // `zip` would stop when the first iterator ends; we want to definitely
         // cover all of `bytes`.
         for dest in bytes {
@@ -386,7 +381,7 @@ impl<'tcx, Tag: Copy, Extra: AllocationExtra<Tag>> Allocation<Tag, Extra> {
         } else {
             match self.relocations.get(&ptr.offset) {
                 Some(&(tag, alloc_id)) => {
-                    let ptr = Pointer::new_with_tag(alloc_id, Size::from_bytes(bits as u64), tag);
+                    let ptr = Pointer::new_with_tag(alloc_id, Size::from_bytes(bits), tag);
                     return Ok(ScalarMaybeUndef::Scalar(ptr.into()));
                 }
                 None => {}
@@ -433,7 +428,7 @@ impl<'tcx, Tag: Copy, Extra: AllocationExtra<Tag>> Allocation<Tag, Extra> {
         };
 
         let bytes = match val.to_bits_or_ptr(type_size, cx) {
-            Err(val) => val.offset.bytes() as u128,
+            Err(val) => u128::from(val.offset.bytes()),
             Ok(data) => data,
         };
 
@@ -524,7 +519,7 @@ impl<'tcx, Tag: Copy, Extra> Allocation<Tag, Extra> {
             )
         };
         let start = ptr.offset;
-        let end = start + size;
+        let end = start + size; // `Size` addition
 
         // Mark parts of the outermost relocations as undefined if they partially fall outside the
         // given range.
@@ -563,7 +558,7 @@ impl<'tcx, Tag, Extra> Allocation<Tag, Extra> {
     #[inline]
     fn check_defined(&self, ptr: Pointer<Tag>, size: Size) -> InterpResult<'tcx> {
         self.undef_mask
-            .is_range_defined(ptr.offset, ptr.offset + size)
+            .is_range_defined(ptr.offset, ptr.offset + size) // `Size` addition
             .or_else(|idx| throw_ub!(InvalidUndefBytes(Some(Pointer::new(ptr.alloc_id, idx)))))
     }
 
@@ -643,7 +638,7 @@ impl<Tag, Extra> Allocation<Tag, Extra> {
         if defined.ranges.len() <= 1 {
             self.undef_mask.set_range_inbounds(
                 dest.offset,
-                dest.offset + size * repeat,
+                dest.offset + size * repeat, // `Size` operations
                 defined.initial,
             );
             return;
@@ -721,10 +716,10 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
         for i in 0..length {
             new_relocations.extend(relocations.iter().map(|&(offset, reloc)| {
                 // compute offset for current repetition
-                let dest_offset = dest.offset + (i * size);
+                let dest_offset = dest.offset + size * i; // `Size` operations
                 (
                     // shift offsets from source allocation to destination allocation
-                    offset + dest_offset - src.offset,
+                    (offset + dest_offset) - src.offset, // `Size` operations
                     reloc,
                 )
             }));
@@ -861,18 +856,18 @@ impl UndefMask {
         if amount.bytes() == 0 {
             return;
         }
-        let unused_trailing_bits = self.blocks.len() as u64 * Self::BLOCK_SIZE - self.len.bytes();
+        let unused_trailing_bits =
+            u64::try_from(self.blocks.len()).unwrap() * Self::BLOCK_SIZE - self.len.bytes();
         if amount.bytes() > unused_trailing_bits {
             let additional_blocks = amount.bytes() / Self::BLOCK_SIZE + 1;
-            assert_eq!(additional_blocks as usize as u64, additional_blocks);
             self.blocks.extend(
                 // FIXME(oli-obk): optimize this by repeating `new_state as Block`.
-                iter::repeat(0).take(additional_blocks as usize),
+                iter::repeat(0).take(usize::try_from(additional_blocks).unwrap()),
             );
         }
         let start = self.len;
         self.len += amount;
-        self.set_range_inbounds(start, start + amount, new_state);
+        self.set_range_inbounds(start, start + amount, new_state); // `Size` operation
     }
 }
 
@@ -881,7 +876,5 @@ fn bit_index(bits: Size) -> (usize, usize) {
     let bits = bits.bytes();
     let a = bits / UndefMask::BLOCK_SIZE;
     let b = bits % UndefMask::BLOCK_SIZE;
-    assert_eq!(a as usize as u64, a);
-    assert_eq!(b as usize as u64, b);
-    (a as usize, b as usize)
+    (usize::try_from(a).unwrap(), usize::try_from(b).unwrap())
 }

src/librustc/mir/interpret/mod.rs

@@ -95,6 +95,27 @@ mod pointer;
 mod queries;
 mod value;
 
+use std::convert::TryFrom;
+use std::fmt;
+use std::io;
+use std::num::NonZeroU32;
+use std::sync::atomic::{AtomicU32, Ordering};
+
+use byteorder::{BigEndian, LittleEndian, ReadBytesExt, WriteBytesExt};
+use rustc_ast::ast::LitKind;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::sync::{HashMapExt, Lock};
+use rustc_data_structures::tiny_list::TinyList;
+use rustc_hir::def_id::DefId;
+use rustc_macros::HashStable;
+use rustc_serialize::{Decodable, Encodable, Encoder};
+
+use crate::mir;
+use crate::ty::codec::TyDecoder;
+use crate::ty::layout::{self, Size};
+use crate::ty::subst::GenericArgKind;
+use crate::ty::{self, Instance, Ty, TyCtxt};
+
 pub use self::error::{
     struct_error, ConstEvalErr, ConstEvalRawResult, ConstEvalResult, ErrorHandled, FrameInfo,
     InterpError, InterpErrorInfo, InterpResult, InvalidProgramInfo, MachineStopType,
@@ -107,24 +128,6 @@ pub use self::allocation::{Allocation, AllocationExtra, Relocations, UndefMask};
 
 pub use self::pointer::{CheckInAllocMsg, Pointer, PointerArithmetic};
 
-use crate::mir;
-use crate::ty::codec::TyDecoder;
-use crate::ty::layout::{self, Size};
-use crate::ty::subst::GenericArgKind;
-use crate::ty::{self, Instance, Ty, TyCtxt};
-use byteorder::{BigEndian, LittleEndian, ReadBytesExt, WriteBytesExt};
-use rustc_ast::ast::LitKind;
-use rustc_data_structures::fx::FxHashMap;
-use rustc_data_structures::sync::{HashMapExt, Lock};
-use rustc_data_structures::tiny_list::TinyList;
-use rustc_hir::def_id::DefId;
-use rustc_macros::HashStable;
-use rustc_serialize::{Decodable, Encodable, Encoder};
-use std::fmt;
-use std::io;
-use std::num::NonZeroU32;
-use std::sync::atomic::{AtomicU32, Ordering};
-
 /// Uniquely identifies one of the following:
 /// - A constant
 /// - A static
@@ -264,8 +267,8 @@ impl<'s> AllocDecodingSession<'s> {
         D: TyDecoder<'tcx>,
     {
         // Read the index of the allocation.
-        let idx = decoder.read_u32()? as usize;
-        let pos = self.state.data_offsets[idx] as usize;
+        let idx = usize::try_from(decoder.read_u32()?).unwrap();
+        let pos = usize::try_from(self.state.data_offsets[idx]).unwrap();
 
         // Decode the `AllocDiscriminant` now so that we know if we have to reserve an
         // `AllocId`.

src/librustc/mir/interpret/pointer.rs

@@ -62,9 +62,9 @@ pub trait PointerArithmetic: layout::HasDataLayout {
     /// This should be called by all the other methods before returning!
     #[inline]
     fn truncate_to_ptr(&self, (val, over): (u64, bool)) -> (u64, bool) {
-        let val = val as u128;
+        let val = u128::from(val);
         let max_ptr_plus_1 = 1u128 << self.pointer_size().bits();
-        ((val % max_ptr_plus_1) as u64, over || val >= max_ptr_plus_1)
+        (u64::try_from(val % max_ptr_plus_1).unwrap(), over || val >= max_ptr_plus_1)
     }
 
     #[inline]
@@ -73,17 +73,16 @@ pub trait PointerArithmetic: layout::HasDataLayout {
         self.truncate_to_ptr(res)
     }
 
-    // Overflow checking only works properly on the range from -u64 to +u64.
     #[inline]
-    fn overflowing_signed_offset(&self, val: u64, i: i128) -> (u64, bool) {
-        // FIXME: is it possible to over/underflow here?
+    fn overflowing_signed_offset(&self, val: u64, i: i64) -> (u64, bool) {
         if i < 0 {
             // Trickery to ensure that `i64::MIN` works fine: compute `n = -i`.
             // This formula only works for true negative values; it overflows for zero!
             let n = u64::MAX - (i as u64) + 1;
             let res = val.overflowing_sub(n);
             self.truncate_to_ptr(res)
         } else {
+            // `i >= 0`, so the cast is safe.
             self.overflowing_offset(val, i as u64)
         }
     }
@@ -96,7 +95,7 @@ pub trait PointerArithmetic: layout::HasDataLayout {
 
     #[inline]
     fn signed_offset<'tcx>(&self, val: u64, i: i64) -> InterpResult<'tcx, u64> {
-        let (res, over) = self.overflowing_signed_offset(val, i128::from(i));
+        let (res, over) = self.overflowing_signed_offset(val, i);
         if over { throw_ub!(PointerArithOverflow) } else { Ok(res) }
     }
 }
@@ -189,14 +188,14 @@ impl<'tcx, Tag> Pointer<Tag> {
     }
 
     #[inline]
-    pub fn overflowing_signed_offset(self, i: i128, cx: &impl HasDataLayout) -> (Self, bool) {
+    pub fn overflowing_signed_offset(self, i: i64, cx: &impl HasDataLayout) -> (Self, bool) {
         let (res, over) = cx.data_layout().overflowing_signed_offset(self.offset.bytes(), i);
         (Pointer::new_with_tag(self.alloc_id, Size::from_bytes(res), self.tag), over)
     }
 
     #[inline(always)]
     pub fn wrapping_signed_offset(self, i: i64, cx: &impl HasDataLayout) -> Self {
-        self.overflowing_signed_offset(i128::from(i), cx).0
+        self.overflowing_signed_offset(i, cx).0
     }
 
     #[inline(always)]