transpile: Override pointer difference expression type to PtrDiff

Author: Rua

c2rust-transpile/src/c_ast/mod.rs

@@ -1316,19 +1316,29 @@ impl TypedAstContext {
                         let lhs_resolved_ty = self.ast_context.resolve_type(lhs_type_id.ctype);
                         let rhs_resolved_ty = self.ast_context.resolve_type(rhs_type_id.ctype);
 
-                        let neither_ptr = !lhs_resolved_ty.kind.is_pointer()
-                            && !rhs_resolved_ty.kind.is_pointer();
-
-                        if op.all_types_same() && neither_ptr {
-                            if CTypeKind::PULLBACK_KINDS.contains(&lhs_resolved_ty.kind) {
+                        if op == CBinOp::Subtract
+                            && lhs_resolved_ty.kind.is_pointer()
+                            && rhs_resolved_ty.kind.is_pointer()
+                        {
+                            // Pointer difference operator should return `ptrdiff_t`.
+                            let new_type_id =
+                                self.ast_context.type_for_kind(&CTypeKind::PtrDiff).unwrap();
+                            Some(CQualTypeId::new(new_type_id))
+                        } else {
+                            let neither_ptr = !lhs_resolved_ty.kind.is_pointer()
+                                && !rhs_resolved_ty.kind.is_pointer();
+
+                            if op.all_types_same() && neither_ptr {
+                                if CTypeKind::PULLBACK_KINDS.contains(&lhs_resolved_ty.kind) {
+                                    Some(lhs_type_id)
+                                } else {
+                                    Some(rhs_type_id)
+                                }
+                            } else if op.is_bitshift() {
                                 Some(lhs_type_id)
                             } else {
-                                Some(rhs_type_id)
+                                return;
                             }
-                        } else if op.is_bitshift() {
-                            Some(lhs_type_id)
-                        } else {
-                            return;
                         }
                     }
                     CExprKind::Unary(_ty, op, e, _idk) => op.expected_result_type(

c2rust-transpile/tests/snapshots/snapshots__transpile@exprs.c.2021.clang15.snap

@@ -90,6 +90,5 @@ pub unsafe extern "C" fn pointer_arithmetic() {
     let mut p1: *mut ::core::ffi::c_int = &raw mut i1 as *mut ::core::ffi::c_int;
     let mut p2: *mut ::core::ffi::c_int = &raw mut i2 as *mut ::core::ffi::c_int;
     let mut diff: ptrdiff_t = p1.offset_from(p2);
-    let mut diff_int: ::core::ffi::c_int =
-        p1.offset_from(p2) as ::core::ffi::c_long as ::core::ffi::c_int;
+    let mut diff_int: ::core::ffi::c_int = p1.offset_from(p2) as ::core::ffi::c_int;
 }

c2rust-transpile/tests/snapshots/snapshots__transpile@exprs.c.2024.clang15.snap

@@ -90,6 +90,5 @@ pub unsafe extern "C" fn pointer_arithmetic() {
     let mut p1: *mut ::core::ffi::c_int = &raw mut i1 as *mut ::core::ffi::c_int;
     let mut p2: *mut ::core::ffi::c_int = &raw mut i2 as *mut ::core::ffi::c_int;
     let mut diff: ptrdiff_t = p1.offset_from(p2);
-    let mut diff_int: ::core::ffi::c_int =
-        p1.offset_from(p2) as ::core::ffi::c_long as ::core::ffi::c_int;
+    let mut diff_int: ::core::ffi::c_int = p1.offset_from(p2) as ::core::ffi::c_int;
 }