transpile: const macros: allow non-variable DeclRefs

for example, mentioning functions should be accepted even in const contexts

Author: fw-immunant

c2rust-transpile/src/translator/mod.rs

@@ -3338,13 +3338,15 @@ impl<'c> Translation<'c> {
                     .ok_or_else(|| format_err!("Missing declref {:?}", decl_id))?
                     .kind;
                 if ctx.is_const {
-                    // TODO Determining which declarations have been declared within the scope of the const macro expr
-                    // vs. which are out-of-scope of the const macro is non-trivial,
-                    // so for now, we don't allow const macros referencing any declarations.
-                    return Err(format_translation_err!(
-                        self.ast_context.display_loc(src_loc),
-                        "Cannot yet refer to declarations in a const expr",
-                    ));
+                    if let CDeclKind::Variable { .. } = decl {
+                        // TODO Determining which variables have been declared within the scope of the const macro expr
+                        // vs. which are out-of-scope of the const macro is non-trivial,
+                        // so for now, we don't allow const macros referencing any variables.
+                        return Err(format_translation_err!(
+                            self.ast_context.display_loc(src_loc),
+                            "Cannot yet refer to variables in a const expr",
+                        ));
+                    }
 
                     #[allow(unreachable_code)] // TODO temporary (see above).
                     if let CDeclKind::Variable {

c2rust-transpile/tests/snapshots/macros.c

@@ -405,3 +405,7 @@ unsigned int ntlm_v2_blob_len(struct ntlmdata *ntlm) { return NTLMv2_BLOB_LEN; }
 int late_init_var() {
   return LATE_INIT_VAR;
 }
+
+void to_be_compared(void) {}
+#define FUNCTION_COMPARISON to_be_compared == to_be_compared
+bool function_comparison = FUNCTION_COMPARISON;

c2rust-transpile/tests/snapshots/[email protected]

@@ -429,6 +429,13 @@ pub unsafe extern "C" fn late_init_var() -> core::ffi::c_int {
         i
     });
 }
+#[no_mangle]
+pub unsafe extern "C" fn to_be_compared() {}
+pub const FUNCTION_COMPARISON: core::ffi::c_int =
+    (Some(to_be_compared as unsafe extern "C" fn() -> ())
+        == Some(to_be_compared as unsafe extern "C" fn() -> ())) as core::ffi::c_int;
+#[no_mangle]
+pub static mut function_comparison: bool = unsafe { FUNCTION_COMPARISON != 0 };
 unsafe extern "C" fn run_static_initializers() {
     global_static_const_ptr_arithmetic = PTR_ARITHMETIC;
     global_static_const_indexing = NESTED_STR[LITERAL_FLOAT as core::ffi::c_int as usize];