analyze: implement Box<T> rewrites (#1106)

This branch adds rewrites to convert pointers with the `FREE` permission
into `Box<T>`. This follows the dynamic ownership tracking proposal in
#1097.

Specific changes:
* Adds a new `dyn_owned: bool` field to `TypeDesc`, which indicates that
the pointer type should be wrapped in a dynamic ownership wrapper.
* Extends `ZeroizeType` handling to support zero-initializing some
pointer types (necessary to allow `malloc`/`calloc` of structs that
contain pointers).
* Adds support for `Box` and `dyn_owned` related casts in `mir_op` and
`rewrite::expr::convert`.
* Adds custom rewrite rules for `malloc`, `calloc`, `free`, and
`realloc`.

c2rust-analyze/src/dataflow/mod.rs

@@ -11,6 +11,18 @@ mod type_check;
 #[derive(Clone, Debug)]
 enum Constraint {
     /// Pointer `.0` must have a subset of the permissions of pointer `.1`.
+    ///
+    /// `Subset` and `SubsetExcept` have a special case involving `FREE` and `OFFSET` permissions.
+    /// The rewriter can't produce a cast that converts `Box<[T]>` to `Box<T>`; to avoid needing
+    /// such casts, we forbid assignment operations from discarding the `OFFSET` permission while
+    /// keeping `FREE`.  We implement this restriction by adding an additional requirement to the
+    /// definition of `Subset(L, R)`: if `L` contains `FREE` and `R` contains `OFFSET`, then `L`
+    /// must also contain `OFFSET`.  This is sufficient because all assignments and
+    /// pseudo-assignments generate `Subset` constraints.
+    ///
+    /// If `L` does not contain `FREE`, then no additional requirement applies, even if `R` does
+    /// contain `OFFSET`.  We allow discarding both `FREE` and `OFFSET` simultaneously during an
+    /// assignment.
     Subset(PointerId, PointerId),
     /// Pointer `.0` must have a subset of permissions of pointer `.1`, except
     /// for the provided permission set.
@@ -102,10 +114,25 @@ impl DataflowConstraints {
                     | PermissionSet::OFFSET_SUB
                     | PermissionSet::FREE;
 
-                (
-                    old_a & !(!old_b & (PROPAGATE_DOWN & !except)),
-                    old_b | (old_a & (PROPAGATE_UP & !except)),
-                )
+                let remove_a = !old_b & PROPAGATE_DOWN & !except;
+                let add_b = old_a & PROPAGATE_UP & !except;
+
+                // Special case: as documented on `Constraint::Subset`, if the subset has `FREE`,
+                // we propagate `OFFSET` in the opposite direction.  Specifically, if the superset
+                // has `OFFSET`, we add it to the subset, propagating "down".  (Propagating "up"
+                // here could allow `OFFSET` and `!OFFSET` to propagated up into the same
+                // `PointerId` through two different constraints, creating a conflict.)
+                let add_a = if old_a.contains(PermissionSet::FREE) {
+                    #[allow(bad_style)]
+                    let PROPAGATE_DOWN_WHEN_FREE =
+                        PermissionSet::OFFSET_ADD | PermissionSet::OFFSET_SUB;
+                    old_b & PROPAGATE_DOWN_WHEN_FREE & !except
+                } else {
+                    PermissionSet::empty()
+                };
+                debug_assert_eq!(add_a & remove_a, PermissionSet::empty());
+
+                ((old_a | add_a) & !remove_a, old_b | add_b)
             }
 
             fn all_perms(

c2rust-analyze/src/pointee_type/type_check.rs

@@ -299,8 +299,15 @@ impl<'tcx> TypeChecker<'tcx, '_> {
                 self.assign(dest_lty.label, arg_lty.label);
             }
             Callee::Free => {
-                // No constraints on `free`, since it doesn't reveal anything about the concrete
-                // type.
+                // Here we create a fresh inference variable and associate it with the argument
+                // pointer.  This doesn't constraint the type, since `free` doesn't reveal anything
+                // about the concrete type of the data, but it does ensure that the pointee type of
+                // the argument operand matches the pointee type of other pointers to the same
+                // allocation, which lets us remove a `void*` cast during rewriting.
+                let var = self.constraints.fresh_var();
+                assert_eq!(args.len(), 1);
+                let arg_lty = self.acx.type_of(&args[0]);
+                self.use_pointer_at_type(arg_lty.label, var);
             }
 
             Callee::Memcpy => {

c2rust-analyze/src/rewrite/expr/convert.rs

@@ -286,6 +286,119 @@ impl<'tcx> ConvertVisitor<'tcx> {
                 )
             }
 
+            mir_op::RewriteKind::MallocSafe {
+                ref zero_ty,
+                elem_size,
+                single,
+            }
+            | mir_op::RewriteKind::CallocSafe {
+                ref zero_ty,
+                elem_size,
+                single,
+            } => {
+                // `malloc(n)` -> `Box::new(z)` or similar
+                assert!(matches!(hir_rw, Rewrite::Identity));
+                let zeroize_expr = generate_zeroize_expr(zero_ty);
+                let mut stmts = match *rw {
+                    mir_op::RewriteKind::MallocSafe { .. } => vec![
+                        Rewrite::Let(vec![("byte_len".into(), self.get_subexpr(ex, 0))]),
+                        Rewrite::Let1(
+                            "n".into(),
+                            Box::new(format_rewrite!("byte_len as usize / {elem_size}")),
+                        ),
+                    ],
+                    mir_op::RewriteKind::CallocSafe { .. } => vec![
+                        Rewrite::Let(vec![
+                            ("count".into(), self.get_subexpr(ex, 0)),
+                            ("size".into(), self.get_subexpr(ex, 1)),
+                        ]),
+                        format_rewrite!("assert_eq!(size, {elem_size})"),
+                        Rewrite::Let1("n".into(), Box::new(format_rewrite!("count as usize"))),
+                    ],
+                    _ => unreachable!(),
+                };
+                let expr = if single {
+                    stmts.push(Rewrite::Text("assert_eq!(n, 1)".into()));
+                    format_rewrite!("Box::new({})", zeroize_expr)
+                } else {
+                    stmts.push(Rewrite::Let1(
+                        "mut v".into(),
+                        Box::new(Rewrite::Text("Vec::with_capacity(n)".into())),
+                    ));
+                    stmts.push(format_rewrite!(
+                        "for i in 0..n {{\n    v.push({});\n}}",
+                        zeroize_expr,
+                    ));
+                    Rewrite::Text("v.into_boxed_slice()".into())
+                };
+                Rewrite::Block(stmts, Some(Box::new(expr)))
+            }
+
+            mir_op::RewriteKind::FreeSafe { single: _ } => {
+                // `free(p)` -> `drop(p)`
+                assert!(matches!(hir_rw, Rewrite::Identity));
+                Rewrite::Call("std::mem::drop".to_string(), vec![self.get_subexpr(ex, 0)])
+            }
+
+            mir_op::RewriteKind::ReallocSafe {
+                ref zero_ty,
+                elem_size,
+                src_single,
+                dest_single,
+            } => {
+                // `realloc(p, n)` -> `Box::new(...)`
+                assert!(matches!(hir_rw, Rewrite::Identity));
+                let zeroize_expr = generate_zeroize_expr(zero_ty);
+                let mut stmts = vec![
+                    Rewrite::Let(vec![
+                        ("src_ptr".into(), self.get_subexpr(ex, 0)),
+                        ("dest_byte_len".into(), self.get_subexpr(ex, 1)),
+                    ]),
+                    Rewrite::Let1(
+                        "dest_n".into(),
+                        Box::new(format_rewrite!("dest_byte_len as usize / {elem_size}")),
+                    ),
+                ];
+                if dest_single {
+                    stmts.push(Rewrite::Text("assert_eq!(dest_n, 1)".into()));
+                }
+                let expr = match (src_single, dest_single) {
+                    (false, false) => {
+                        stmts.push(Rewrite::Let1(
+                            "mut dest_ptr".into(),
+                            Box::new(Rewrite::Text("Vec::from(src_ptr)".into())),
+                        ));
+                        stmts.push(format_rewrite!(
+                            "dest_ptr.resize_with(dest_n, || {})",
+                            zeroize_expr,
+                        ));
+                        Rewrite::Text("dest_ptr.into_boxed_slice()".into())
+                    }
+                    (false, true) => {
+                        format_rewrite!(
+                            "src_ptr.into_iter().next().unwrap_or_else(|| {})",
+                            zeroize_expr
+                        )
+                    }
+                    (true, false) => {
+                        stmts.push(Rewrite::Let1(
+                            "mut dest_ptr".into(),
+                            Box::new(Rewrite::Text("Vec::with_capacity(dest_n)".into())),
+                        ));
+                        stmts.push(Rewrite::Text(
+                            "if dest_n >= 1 { dest_ptr.push(*src_ptr); }".into(),
+                        ));
+                        stmts.push(format_rewrite!(
+                            "dest_ptr.resize_with(dest_n, || {})",
+                            zeroize_expr,
+                        ));
+                        Rewrite::Text("dest_ptr.into_boxed_slice()".into())
+                    }
+                    (true, true) => Rewrite::Text("src_ptr".into()),
+                };
+                Rewrite::Block(stmts, Some(Box::new(expr)))
+            }
+
             mir_op::RewriteKind::CellGet => {
                 // `*x` to `Cell::get(x)`
                 assert!(matches!(hir_rw, Rewrite::Identity));
@@ -566,7 +679,7 @@ fn generate_zeroize_code(zero_ty: &ZeroizeType, lv: &str) -> String {
     match *zero_ty {
         ZeroizeType::Int => format!("{lv} = 0"),
         ZeroizeType::Bool => format!("{lv} = false"),
-        ZeroizeType::Iterable(ref elem_zero_ty) => format!(
+        ZeroizeType::Array(ref elem_zero_ty) => format!(
             "
             {{
                 for elem in {lv}.iter_mut() {{
@@ -576,7 +689,7 @@ fn generate_zeroize_code(zero_ty: &ZeroizeType, lv: &str) -> String {
         ",
             generate_zeroize_code(elem_zero_ty, "(*elem)")
         ),
-        ZeroizeType::Struct(ref fields) => {
+        ZeroizeType::Struct(_, ref fields) => {
             eprintln!("zeroize: {} fields on {lv}: {fields:?}", fields.len());
             let mut s = String::new();
             writeln!(s, "{{").unwrap();
@@ -594,6 +707,27 @@ fn generate_zeroize_code(zero_ty: &ZeroizeType, lv: &str) -> String {
     }
 }
 
+/// Generate an expression to produce a zeroized version of a value.
+fn generate_zeroize_expr(zero_ty: &ZeroizeType) -> String {
+    match *zero_ty {
+        ZeroizeType::Int => format!("0"),
+        ZeroizeType::Bool => format!("false"),
+        ZeroizeType::Array(ref elem_zero_ty) => format!(
+            "std::array::from_fn(|| {})",
+            generate_zeroize_expr(elem_zero_ty)
+        ),
+        ZeroizeType::Struct(ref name, ref fields) => {
+            let mut s = String::new();
+            write!(s, "{} {{\n", name).unwrap();
+            for (name, field_zero_ty) in fields {
+                write!(s, "{}: {},\n", name, generate_zeroize_expr(field_zero_ty),).unwrap();
+            }
+            write!(s, "}}\n").unwrap();
+            s
+        }
+    }
+}
+
 fn take_prefix_while<'a, T>(slice: &mut &'a [T], mut pred: impl FnMut(&'a T) -> bool) -> &'a [T] {
     let i = slice.iter().position(|x| !pred(x)).unwrap_or(slice.len());
     let (a, b) = slice.split_at(i);
@@ -614,14 +748,14 @@ pub fn convert_cast_rewrite(kind: &mir_op::RewriteKind, hir_rw: Rewrite) -> Rewr
             Rewrite::Ref(Box::new(elem), mutbl_from_bool(mutbl))
         }
 
-        mir_op::RewriteKind::MutToImm => {
-            // `p` -> `&*p`
+        mir_op::RewriteKind::Reborrow { mutbl } => {
+            // `p` -> `&*p` / `&mut *p`
             let hir_rw = match fold_mut_to_imm(hir_rw) {
                 Ok(folded_rw) => return folded_rw,
                 Err(rw) => rw,
             };
             let place = Rewrite::Deref(Box::new(hir_rw));
-            Rewrite::Ref(Box::new(place), hir::Mutability::Not)
+            Rewrite::Ref(Box::new(place), mutbl_from_bool(mutbl))
         }
 
         mir_op::RewriteKind::OptionUnwrap => {
@@ -661,6 +795,33 @@ pub fn convert_cast_rewrite(kind: &mir_op::RewriteKind, hir_rw: Rewrite) -> Rewr
             Rewrite::MethodCall(ref_method, Box::new(hir_rw), vec![])
         }
 
+        mir_op::RewriteKind::DynOwnedUnwrap => {
+            Rewrite::MethodCall("unwrap".to_string(), Box::new(hir_rw), vec![])
+        }
+        mir_op::RewriteKind::DynOwnedTake => {
+            // `p` -> `mem::replace(&mut p, Err(()))`
+            Rewrite::Call(
+                "std::mem::replace".to_string(),
+                vec![
+                    Rewrite::Ref(Box::new(hir_rw), hir::Mutability::Mut),
+                    Rewrite::Text("Err(())".into()),
+                ],
+            )
+        }
+        mir_op::RewriteKind::DynOwnedWrap => {
+            Rewrite::Call("std::result::Result::<_, ()>::Ok".to_string(), vec![hir_rw])
+        }
+
+        mir_op::RewriteKind::DynOwnedDowngrade { mutbl } => {
+            let ref_method = if mutbl {
+                "as_deref_mut".into()
+            } else {
+                "as_deref".into()
+            };
+            let hir_rw = Rewrite::MethodCall(ref_method, Box::new(hir_rw), vec![]);
+            Rewrite::MethodCall("unwrap".into(), Box::new(hir_rw), vec![])
+        }
+
         mir_op::RewriteKind::CastRefToRaw { mutbl } => {
             // `addr_of!(*p)` is cleaner than `p as *const _`; we don't know the pointee
             // type here, so we can't emit `p as *const T`.