(c2rust-analyze) Support ptr-to-ptr casts between safely transmutab…

…le types, for now limited to same-sized integers (#839)

- Fixes #840

This introduces the concept of equivalent/compatible/safely transmutable
types:
https://github.com/immunant/c2rust/blob/2915b8d0c71add21dee1f9d540958ea863478212/c2rust-analyze/src/util.rs#L356-L380

Thus, we can now allow ptr-to-ptr casts between safely transmutable
pointee types, whereas previously they were only allowed for equal
types. In particular, this enables support for string casts, which are
produced by `c2rust transpile` as `b"" as *const u8 as *const
core::ffi::c_char`, where `c_char = i8`. Thus, this fixes #840.

New tests are added in `string_casts.rs` to cover various ptr casts,
though some of them crash in the rewriter due to having implicitly
inserted MIR statements like implicit `&raw`s, which are inserted with
`addr_of!`s. Thus, for some of these (where it works), there are
versions with explicit `addr_of!`s that succeed end-to-end.

c2rust-analyze/src/dataflow/type_check.rs

@@ -2,7 +2,9 @@ use super::DataflowConstraints;
 use crate::c_void_casts::CVoidCastDirection;
 use crate::context::{AnalysisCtxt, LTy, PermissionSet, PointerId};
 use crate::panic_detail;
-use crate::util::{describe_rvalue, is_null_const, ty_callee, Callee, RvalueDesc};
+use crate::util::{
+    describe_rvalue, is_null_const, is_transmutable_ptr_cast, ty_callee, Callee, RvalueDesc,
+};
 use assert_matches::assert_matches;
 use rustc_hir::def_id::DefId;
 use rustc_middle::mir::{
@@ -43,11 +45,6 @@ struct TypeChecker<'tcx, 'a> {
     equiv_constraints: Vec<(PointerId, PointerId)>,
 }
 
-fn is_castable_to<'tcx>(_from_lty: LTy<'tcx>, _to_lty: LTy<'tcx>) -> bool {
-    // TODO: implement
-    true
-}
-
 impl<'tcx> TypeChecker<'tcx, '_> {
     fn add_edge(&mut self, src: PointerId, dest: PointerId) {
         // Copying `src` to `dest` can discard permissions, but can't add new ones.
@@ -105,6 +102,7 @@ impl<'tcx> TypeChecker<'tcx, '_> {
     fn visit_cast(&mut self, cast_kind: CastKind, op: &Operand<'tcx>, to_lty: LTy<'tcx>) {
         let to_ty = to_lty.ty;
         let from_lty = self.acx.type_of(op);
+        let from_ty = from_lty.ty;
 
         match cast_kind {
             CastKind::PointerFromExposedAddress => {
@@ -114,31 +112,40 @@ impl<'tcx> TypeChecker<'tcx, '_> {
                     panic!("Creating non-null pointers from exposed addresses not supported");
                 }
             }
-            CastKind::Pointer(PointerCast::Unsize) => {
-                let pointee_to_ty = to_ty
-                    .builtin_deref(true)
-                    .unwrap_or_else(|| panic!("unsize cast has non-pointer output {:?}?", to_ty))
-                    .ty;
-
-                assert_matches!(from_lty.kind(), TyKind::Ref(..) | TyKind::RawPtr(..));
-                let pointee_from_lty = assert_matches!(from_lty.args, [lty] => lty);
-
-                let elem_to_ty = assert_matches!(pointee_to_ty.kind(), &TyKind::Slice(ty) => ty);
-                assert!(matches!(pointee_from_lty.kind(), TyKind::Array(..)));
-                let elem_from_lty = assert_matches!(pointee_from_lty.args, [lty] => lty);
-                assert_eq!(elem_from_lty.ty, elem_to_ty);
-                assert_eq!(pointee_from_lty.label, PointerId::NONE);
-                self.do_assign_pointer_ids(to_lty.label, from_lty.label);
+            CastKind::PointerExposeAddress => {
+                // Allow, as [`CastKind::PointerFromExposedAddress`] is the dangerous one,
+                // and we'll catch (not allow) that above.
+                // This becomes no longer a pointer, so we don't need to add any dataflow constraints
+                // (until we try to handle [`CastKind::PointerFromExposedAddress`], if we do).
             }
-            CastKind::Pointer(..) => {
-                // The source and target types are both pointers, and they have identical pointee types.
-                // TODO: remove or move check to `is_castable_to`
-                assert!(from_lty.args[0].ty == to_lty.args[0].ty);
-                assert!(is_castable_to(from_lty, to_lty));
+            CastKind::Pointer(ptr_cast) => {
+                // All of these [`PointerCast`]s are type checked by rustc already.
+                // They don't involve arbitrary raw ptr to raw ptr casts
+                // ([PointerCast::MutToConstPointer`] doesn't allow changing types),
+                // which we need to check for safe transmutability,
+                // and which are (currently) covered in [`CastKind::Misc`].
+                // That's why there's a `match` here that does nothing;
+                // it ensures if [`PointerCast`] is changed in a future `rustc` version,
+                // this won't compile until we've checked that this reasoning is still accurate.
+                match ptr_cast {
+                    PointerCast::ReifyFnPointer => {}
+                    PointerCast::UnsafeFnPointer => {}
+                    PointerCast::ClosureFnPointer(_) => {}
+                    PointerCast::MutToConstPointer => {}
+                    PointerCast::ArrayToPointer => {}
+                    PointerCast::Unsize => {}
+                }
+                self.do_assign_pointer_ids(to_lty.label, from_lty.label)
+                // TODO add other dataflow constraints
             }
-            _ => {
-                // A cast such as `T as U`
-                assert!(is_castable_to(from_lty, to_lty));
+            CastKind::Misc => {
+                match is_transmutable_ptr_cast(from_ty, to_ty) {
+                    Some(true) => {
+                        // TODO add other dataflow constraints
+                    },
+                    Some(false) => ::log::error!("TODO: unsupported ptr-to-ptr cast between pointee types not yet supported as safely transmutable: `{from_ty:?} as {to_ty:?}`"),
+                    None => {}, // not a ptr cast (no dataflow constraints needed); let rustc typeck this
+                };
             }
         }
 
@@ -295,12 +302,21 @@ impl<'tcx> TypeChecker<'tcx, '_> {
         }
     }
 
-    /// Unify corresponding `PointerId`s in `lty1` and `lty2`.
+    /// Unify corresponding [`PointerId`]s in `lty1` and `lty2`.
+    ///
+    /// The two inputs must have identical underlying types.
+    /// For any position where the underlying type has a pointer,
+    /// this function unifies the [`PointerId`]s that `lty1` and `lty2` have at that position.
+    /// For example, given
+    ///
+    /// ```
+    /// # fn(
+    /// lty1: *mut /*l1*/ *const /*l2*/ u8,
+    /// lty2: *mut /*l3*/ *const /*l4*/ u8,
+    /// # ) {}
+    /// ```
     ///
-    /// The two inputs must have identical underlying types.  For any position where the underlying
-    /// type has a pointer, this function unifies the `PointerId`s that `lty1` and `lty2` have at
-    /// that position.  For example, given `lty1 = *mut /*l1*/ *const /*l2*/ u8` and `lty2 = *mut
-    /// /*l3*/ *const /*l4*/ u8`, this function will unify `l1` with `l3` and `l2` with `l4`.
+    /// this function will unify `l1` with `l3` and `l2` with `l4`.
     fn do_unify(&mut self, lty1: LTy<'tcx>, lty2: LTy<'tcx>) {
         assert_eq!(
             self.acx.tcx().erase_regions(lty1.ty),

c2rust-analyze/src/util.rs

@@ -12,6 +12,7 @@ use rustc_middle::ty::{
     self, AdtDef, DefIdTree, EarlyBinder, Subst, SubstsRef, Ty, TyCtxt, TyKind, UintTy,
 };
 use rustc_span::symbol::Symbol;
+use rustc_type_ir::IntTy;
 use std::fmt::Debug;
 
 #[derive(Debug)]
@@ -356,6 +357,90 @@ pub fn is_null_const(constant: Constant) -> bool {
 pub trait PhantomLifetime<'a> {}
 impl<'a, T: ?Sized> PhantomLifetime<'a> for T {}
 
+/// Determine if `from` can be safely transmuted to `to`,
+/// which is defined as `*(from as *const To)` being a safe operation,
+/// where `from: *const From` and assuming `*from` already was safe.
+///
+/// Note that this is one-way, and is slightly different from [`core::mem::transmute`],
+/// and more similar to [`core::mem::transmute_copy`].
+///
+/// This forms a reflexive, transitive, and non-symmetric (one-way) relation, named `~` below.
+/// Formally, `A ~ B` iff whenever `*a` is well-defined (i.e., not UB),
+/// `*(a as *const B)` is also well-defined, where `a: *const A`.
+///
+/// However, safe transmutability is difficult to check completely,
+/// so this function only checks a subset of it,
+/// with these formal rules for all types `A`, `B`:
+///
+/// 1. `A = B => A ~ B`
+/// 2. `A ~ B => *A ~ *B`
+/// 3. `uN ~ iN`, `iN ~ uN`, where `N` is an integer width
+/// 4. `A ~ B, N > 0 => [A; N] ~ B`, where `const N: usize`
+///
+/// Note: 5. `A ~ B => [A] ~ B` is not a rule because it would be unsound for zero-length slices,
+/// which we cannot check unlike for arrays, which we need for translated string literals.
+///
+/// Thus, [`true`] means it is definitely transmutable,
+/// while [`false`] means it may not be transmutable.
+///
+/// Also note that for `A ~ B`, we need at least
+/// * `size_of::<A>() >= size_of::<B>()`
+/// * `align_of::<A>() >= align_of::<B>()`
+///
+/// For rules 1 and 3, this obviously holds.
+/// For rule 2, this holds as long as
+/// `A ~ B` implies that (`*B` is a fat ptr implies `*A` is a fat ptr).
+///
+/// For rule 1, this holds trivially.  
+/// For rule 2, this holds because `**A` and `**B` are always thin ptrs.  
+/// For rule 3, this holds trivially.  
+/// For rule 4, this holds because  if `*A` is a fat ptr,
+/// `A` is unsized, and thus `[A; N]` is ill-formed to begin with.  
+/// For (almost) rule 5, this holds because `*[A]` is always a fat ptr.
+pub fn is_transmutable_to<'tcx>(from: Ty<'tcx>, to: Ty<'tcx>) -> bool {
+    let transmutable_ints = || {
+        use IntTy::*;
+        use UintTy::*;
+        match (from.kind(), to.kind()) {
+            (ty::Uint(u), ty::Int(i)) | (ty::Int(i), ty::Uint(u)) => {
+                matches!(
+                    (u, i),
+                    (Usize, Isize) | (U8, I8) | (U16, I16) | (U32, I32) | (U64, I64)
+                )
+            }
+            _ => false,
+        }
+    };
+
+    let one_way_transmutable = || match *from.kind() {
+        ty::Array(from, n) => {
+            is_transmutable_to(from, to) && {
+                let is_zero = n.kind().try_to_scalar_int().unwrap().is_null();
+                !is_zero
+            }
+        }
+        _ => false,
+    };
+
+    from == to
+        || is_transmutable_ptr_cast(from, to).unwrap_or(false)
+        || transmutable_ints()
+        || one_way_transmutable()
+}
+
+/// Determine if `from as to` is a ptr-to-ptr cast.
+/// and if it is, if the pointee types are [safely transmutable](is_transmutable_to).
+///
+/// This returns [`Some`]`(is_transmutable)` if they're both pointers,
+/// and [`None`] if its some other types.
+///
+/// See [`is_transmutable_to`] for the definition of safe transmutability.
+pub fn is_transmutable_ptr_cast<'tcx>(from: Ty<'tcx>, to: Ty<'tcx>) -> Option<bool> {
+    let from = from.builtin_deref(true)?.ty;
+    let to = to.builtin_deref(true)?.ty;
+    Some(is_transmutable_to(from, to))
+}
+
 #[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
 pub enum TestAttr {
     /// `#[c2rust_analyze_test::fixed_signature]`: Mark all pointers in the function signature as

c2rust-analyze/tests/analyze/string_casts.rs

@@ -1,9 +1,35 @@
-#[cfg(any())]
-pub fn cast_only(s: *const u8) {
+pub fn cast_ptr_to_ptr(s: *const u8) {
     s as *const core::ffi::c_char;
 }
 
+pub fn deep_cast_ptr_to_ptr(x: *const *const u8) {
+    x as *const *const i8;
+}
+
+/// For the below disabled (`#[cfg(any())]`ed) tests, they currently crash in the rewriter
+/// due to it not being able to handle implicitly inserted `&raw` MIR statements yet.
+/// Thus, they also have `*_explicit` versions where
+/// a `std::ptr::addr_of!` is used to make the `&raw` explicit.
+///
+/// Also note that `addr_of!` (with a `use std::ptr::addr_of`)
+/// and `::core::ptr::addr_of!` don't work either,
+/// though `std::ptr::addr_of`, `::std::ptr::addr_of!`,
+/// and `core::ptr::addr_of!` do work.
+
+#[cfg(any())]
+pub fn cast_array_to_ptr(s: &[u8; 1]) {
+    s as *const u8;
+}
+
+pub fn cast_array_to_ptr_explicit(s: &[u8; 1]) {
+    std::ptr::addr_of!(*s) as *const u8;
+}
+
 #[cfg(any())]
 pub fn cast_from_literal() {
-    b"" as *const u8 as *const core::ffi::c_char;
+    b"\0" as *const u8 as *const core::ffi::c_char;
+}
+
+pub fn cast_from_literal_explicit() {
+    std::ptr::addr_of!(*b"\0") as *const u8 as *const core::ffi::c_char;
 }

c2rust-analyze/tests/filecheck/addr_of.rs

@@ -25,8 +25,8 @@ fn shared_ref_with_struct() {
     let y = std::ptr::addr_of!(x.a);
 }
 
-// CHECK-LABEL: fn cast_array_to_ptr_explicit(s: &[u8; 0]) {
-pub fn cast_array_to_ptr_explicit(s: &[u8; 0]) {
+// CHECK-LABEL: fn cast_array_to_ptr_explicit(s: &[u8; 1]) {
+pub fn cast_array_to_ptr_explicit(s: &[u8; 1]) {
     // CHECK-DAG: &*(std::ptr::addr_of!(*s)) as *const u8
     std::ptr::addr_of!(*s) as *const u8;
 }