Avoid mismatch when generating structs that represent scalar data but also include ZSTs (rust-lang#2794)

This change considers another case for the code generation of scalar data.
In particular, when the expected type is an ADT, we previously
considered two cases: either there is no field or there is one. But in
cases where the ADT includes a ZST, the ADT will contain multiple
fields: one associated to the scalar data, and other fields associated
to the ZSTs. In those cases, we ended up crashing as in rust-lang#2680:

```rust
error: internal compiler error: Kani unexpectedly panicked at panicked at cprover_bindings/src/goto_program/expr.rs:804:9:
                                assertion failed: `(left == right)`
                                  left: `2`,
                                 right: `1`: Error in struct_expr; mismatch in number of fields and values.
                                    StructTag("tag-_161424092592971241517604180463813633314")
                                    [Expr { value: IntConstant(0), typ: Unsignedbv { width: 8 }, location: None, size_of_annotation: None }].

thread 'rustc' panicked at cprover_bindings/src/goto_program/expr.rs:804:9:
assertion failed: `(left == right)`
  left: `2`,
 right: `1`: Error in struct_expr; mismatch in number of fields and values.
	StructTag("tag-_161424092592971241517604180463813633314")
	[Expr { value: IntConstant(0), typ: Unsignedbv { width: 8 }, location: None, size_of_annotation: None }]
```

With the changes in this PR, that's no longer the case.

Resolves rust-lang#2364 
Resolves rust-lang#2680

Author: adpaco-aws

kani-compiler/src/codegen_cprover_gotoc/codegen/operand.rs

@@ -303,38 +303,39 @@ impl<'tcx> GotocCtx<'tcx> {
                 }
             }
             (Scalar::Int(_), ty::Adt(adt, subst)) => {
-                if adt.is_struct() || adt.is_union() {
-                    // in this case, we must have a one variant ADT. there are two cases
-                    let variant = &adt.variants().raw[0];
-                    // if there is no field, then it's just a ZST
-                    if variant.fields.is_empty() {
-                        if adt.is_struct() {
-                            let overall_t = self.codegen_ty(ty);
-                            Expr::struct_expr_from_values(overall_t, vec![], &self.symbol_table)
-                        } else {
-                            unimplemented!()
-                        }
-                    } else {
-                        // otherwise, there is just one field, which is stored as the scalar data
-                        let field = &variant.fields[0usize.into()];
-                        let fty = field.ty(self.tcx, subst);
-
-                        let overall_t = self.codegen_ty(ty);
-                        if adt.is_struct() {
-                            self.codegen_single_variant_single_field(s, span, overall_t, fty)
-                        } else {
-                            unimplemented!()
-                        }
-                    }
-                } else {
-                    // if it's an enum
+                if adt.is_struct() {
+                    // In this case, we must have a one variant ADT.
+                    let variant = adt.non_enum_variant();
+                    let overall_type = self.codegen_ty(ty);
+                    // There must be at least one field associated with the scalar data.
+                    // Any additional fields correspond to ZSTs.
+                    let field_types: Vec<Ty<'_>> =
+                        variant.fields.iter().map(|f| f.ty(self.tcx, subst)).collect();
+                    // Check that there is a single non-ZST field.
+                    let non_zst_types: Vec<_> =
+                        field_types.iter().filter(|t| !self.is_zst(**t)).collect();
+                    assert!(
+                        non_zst_types.len() == 1,
+                        "error: expected exactly one field whose type is not a ZST"
+                    );
+                    let field_values: Vec<Expr> = field_types
+                        .iter()
+                        .map(|t| {
+                            if self.is_zst(*t) {
+                                Expr::init_unit(self.codegen_ty(*t), &self.symbol_table)
+                            } else {
+                                self.codegen_scalar(s, *t, span)
+                            }
+                        })
+                        .collect();
+                    Expr::struct_expr_from_values(overall_type, field_values, &self.symbol_table)
+                } else if adt.is_enum() {
                     let layout = self.layout_of(ty);
                     let overall_t = self.codegen_ty(ty);
                     match &layout.variants {
                         Variants::Single { index } => {
                             // here we must have one variant
                             let variant = &adt.variants()[*index];
-
                             match variant.fields.len() {
                                 0 => Expr::struct_expr_from_values(
                                     overall_t,
@@ -398,6 +399,9 @@ impl<'tcx> GotocCtx<'tcx> {
                             }
                         },
                     }
+                } else {
+                    // if it's a union
+                    unimplemented!()
                 }
             }
             (Scalar::Int(int), ty::Tuple(_)) => {

tests/cargo-kani/codegen-scalar-with-phantom/Cargo.toml

@@ -0,0 +1,11 @@
+# Copyright Kani Contributors
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+
+[package]
+name = "codegen-scalar-with-phantom"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

tests/cargo-kani/codegen-scalar-with-phantom/check_phantom_data.expected

@@ -0,0 +1,4 @@
+Status: SUCCESS\
+Description: "assertion failed: C.x == 0"
+
+VERIFICATION:- SUCCESSFUL

tests/cargo-kani/codegen-scalar-with-phantom/src/lib.rs

@@ -0,0 +1,21 @@
+// Copyright Kani Contributors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! Check that we can codegen structs with scalar and phantom data.
+//!
+//! Note: Phantom data is represented with ZSTs, which are already covered by
+//! the test `codegen-scalar-with-zsts`, but we include this one as well for
+//! completeness.
+
+use std::marker::PhantomData;
+
+pub struct Foo<R> {
+    x: u8,
+    _t: PhantomData<R>,
+}
+
+#[kani::proof]
+fn check_phantom_data() {
+    const C: Foo<usize> = Foo { x: 0, _t: PhantomData };
+    assert_eq!(C.x, 0);
+}

tests/cargo-kani/codegen-scalar-with-zsts/Cargo.toml

@@ -0,0 +1,11 @@
+# Copyright Kani Contributors
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+
+[package]
+name = "codegen-scalar-with-zsts"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

tests/cargo-kani/codegen-scalar-with-zsts/check_zst.expected

@@ -0,0 +1,4 @@
+Status: SUCCESS\
+Description: "assertion failed: C.x == 0"
+
+VERIFICATION:- SUCCESSFUL

tests/cargo-kani/codegen-scalar-with-zsts/src/lib.rs

@@ -0,0 +1,17 @@
+// Copyright Kani Contributors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! Check that we can codegen structs with scalar and ZSTs.
+
+struct Empty {}
+
+pub struct Foo {
+    x: u8,
+    _t: Empty,
+}
+
+#[kani::proof]
+fn check_zst() {
+    const C: Foo = Foo { x: 0, _t: Empty {} };
+    assert_eq!(C.x, 0);
+}