fix: Allow macros to change types on each iteration of a comptime loop

compiler/noirc_frontend/src/elaborator/types.rs

@@ -633,7 +633,7 @@ impl<'context> Elaborator<'context> {
         0
     }
 
-    pub fn unify(
+    pub(super) fn unify(
         &mut self,
         actual: &Type,
         expected: &Type,
@@ -644,6 +644,22 @@ impl<'context> Elaborator<'context> {
         }
     }
 
+    /// Do not apply type bindings even after a successful unification.
+    /// This function is used by the interpreter for some comptime code
+    /// which can change types e.g. on each iteration of a for loop.
+    pub fn unify_without_applying_bindings(
+        &mut self,
+        actual: &Type,
+        expected: &Type,
+        file: fm::FileId,
+        make_error: impl FnOnce() -> TypeCheckError,
+    ) {
+        let mut bindings = TypeBindings::new();
+        if actual.try_unify(expected, &mut bindings).is_err() {
+            self.errors.push((make_error().into(), file));
+        }
+    }
+
     /// Wrapper of Type::unify_with_coercions using self.errors
     pub(super) fn unify_with_coercions(
         &mut self,

compiler/noirc_frontend/src/hir/comptime/interpreter.rs

@@ -243,7 +243,7 @@
             }
         } else {
             let name = self.elaborator.interner.function_name(&function);
             unreachable!("Non-builtin, lowlevel or oracle builtin fn '{name}'")
         }
     }
 
@@ -1303,9 +1303,11 @@
                     // Macro calls are typed as type variables during type checking.
                     // Now that we know the type we need to further unify it in case there
                     // are inconsistencies or the type needs to be known.
+                    // We don't commit any type bindings made this way in case the type of
+                    // the macro result changes across loop iterations.
                     let expected_type = self.elaborator.interner.id_type(id);
                     let actual_type = result.get_type();
-                    self.unify(&actual_type, &expected_type, location);
+                    self.unify_without_binding(&actual_type, &expected_type, location);
                 }
                 Ok(result)
             }
@@ -1319,16 +1321,14 @@
         }
     }
 
-    fn unify(&mut self, actual: &Type, expected: &Type, location: Location) {
-        // We need to swap out the elaborator's file since we may be
-        // in a different one currently, and it uses that for the error location.
-        let old_file = std::mem::replace(&mut self.elaborator.file, location.file);
-        self.elaborator.unify(actual, expected, || TypeCheckError::TypeMismatch {
-            expected_typ: expected.to_string(),
-            expr_typ: actual.to_string(),
-            expr_span: location.span,
+    fn unify_without_binding(&mut self, actual: &Type, expected: &Type, location: Location) {
+        self.elaborator.unify_without_applying_bindings(actual, expected, location.file, || {
+            TypeCheckError::TypeMismatch {
+                expected_typ: expected.to_string(),
+                expr_typ: actual.to_string(),
+                expr_span: location.span,
+            }
         });
-        self.elaborator.file = old_file;
     }
 
     fn evaluate_method_call(

compiler/noirc_frontend/src/tests.rs

@@ -2373,7 +2373,7 @@
 }
 
 #[test]
 fn underflowing_u8() {
     let src = r#"
         fn main() {
             let _: u8 = -1;
@@ -2411,7 +2411,7 @@
 }
 
 #[test]
 fn underflowing_i8() {
     let src = r#"
         fn main() {
             let _: i8 = -129;
@@ -3556,7 +3556,7 @@
 }
 
 #[test]
 fn arithmetic_generics_canonicalization_deduplication_regression() {
     let source = r#"
         struct ArrData<let N: u32> {
             a: [Field; N],
@@ -3725,3 +3725,26 @@
     println!("{errors:?}");
     assert_eq!(errors.len(), 0);
 }
+
+#[test]
+fn macro_result_type_mismatch() {
+    let src = r#"
+        fn main() {
+            comptime {
+                let x = unquote!(quote { "test" });
+                let _: Field = x;
+            }
+        }
+
+        comptime fn unquote(q: Quoted) -> Quoted {
+            q
+        }
+    "#;
+
+    let errors = get_program_errors(src);
+    assert_eq!(errors.len(), 1);
+    assert!(matches!(
+        errors[0].0,
+        CompilationError::TypeError(TypeCheckError::TypeMismatch { .. })
+    ));
+}

test_programs/compile_success_empty/comptime_change_type_each_iteration/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "comptime_change_type_each_iteration"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.34.0"
+
+[dependencies]

test_programs/compile_success_empty/comptime_change_type_each_iteration/src/main.nr

@@ -0,0 +1,19 @@
+fn main() {
+    comptime
+    {
+        for i in 9..11 {
+            // Lengths are different on each iteration:
+            // foo9, foo10
+            let name = f"foo{i}".as_ctstring().as_quoted_str!();
+
+            // So to call `from_signature` we need to delay the type check
+            // by quoting the function call so that we re-typecheck on each iteration
+            let hash = std::meta::unquote!(quote { from_signature($name) });
+            assert(hash > 3);
+        }
+    }
+}
+
+fn from_signature<let N: u32>(_signature: str<N>) -> u32 {
+    N
+}