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diff --git a/compiler/Expressions.ml b/compiler/Expressions.ml
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+open Types
+open Values
+
+type field_proj_kind =
+  | ProjAdt of TypeDeclId.id * VariantId.id option
+  | ProjOption of VariantId.id
+      (** Option is an assumed type, coming from the standard library *)
+  | ProjTuple of int
+[@@deriving show]
+(* arity of the tuple *)
+
+type projection_elem =
+  | Deref
+  | DerefBox
+  | Field of field_proj_kind * FieldId.id
+[@@deriving show]
+
+type projection = projection_elem list [@@deriving show]
+type place = { var_id : VarId.id; projection : projection } [@@deriving show]
+type borrow_kind = Shared | Mut | TwoPhaseMut [@@deriving show]
+
+type unop =
+  | Not
+  | Neg
+  | Cast of integer_type * integer_type
+      (** Cast an integer from a source type to a target type *)
+[@@deriving show, ord]
+
+(** A binary operation
+
+    Note that we merge checked binops and unchecked binops: we perform a
+    micro-pass on the MIR AST to remove the assertions introduced by rustc,
+    and later extract the binops which can fail (addition, substraction, etc.)
+    or have preconditions (division, remainder...) to monadic functions.
+ *)
+type binop =
+  | BitXor
+  | BitAnd
+  | BitOr
+  | Eq
+  | Lt
+  | Le
+  | Ne
+  | Ge
+  | Gt
+  | Div
+  | Rem
+  | Add
+  | Sub
+  | Mul
+  | Shl
+  | Shr
+[@@deriving show, ord]
+
+let all_binops =
+  [
+    BitXor;
+    BitAnd;
+    BitOr;
+    Eq;
+    Lt;
+    Le;
+    Ne;
+    Ge;
+    Gt;
+    Div;
+    Rem;
+    Add;
+    Sub;
+    Mul;
+    Shl;
+    Shr;
+  ]
+
+type operand =
+  | Copy of place
+  | Move of place
+  | Constant of ety * constant_value
+[@@deriving show]
+
+(** An aggregated ADT.
+
+    Note that ADTs are desaggregated at some point in MIR. For instance, if
+    we have in Rust:
+    {[
+      let ls = Cons(hd, tl);
+    ]}
+    
+    In MIR we have (yes, the discriminant update happens *at the end* for some
+    reason):
+    {[
+      (ls as Cons).0 = move hd;
+      (ls as Cons).1 = move tl;
+      discriminant(ls) = 0; // assuming [Cons] is the variant of index 0
+    ]}
+    
+    Note that in our semantics, we handle both cases (in case of desaggregated
+    initialization, [ls] is initialized to [⊥], then this [⊥] is expanded to
+    [Cons (⊥, ⊥)] upon the first assignment, at which point we can initialize
+    the field 0, etc.).
+ *)
+type aggregate_kind =
+  | AggregatedTuple
+  | AggregatedOption of VariantId.id * ety
+  (* TODO: AggregatedOption should be merged with AggregatedAdt *)
+  | AggregatedAdt of
+      TypeDeclId.id * VariantId.id option * erased_region list * ety list
+[@@deriving show]
+
+(* TODO: move the aggregate kind to operands *)
+type rvalue =
+  | Use of operand
+  | Ref of place * borrow_kind
+  | UnaryOp of unop * operand
+  | BinaryOp of binop * operand * operand
+  | Discriminant of place
+  | Aggregate of aggregate_kind * operand list
+[@@deriving show]