Add phantom attribute. (#5381)

crates/cairo-lang-defs/src/db.rs

@@ -7,7 +7,7 @@ use cairo_lang_filesystem::ids::{CrateId, Directory, FileId, FileKind, FileLongI
 use cairo_lang_parser::db::ParserGroup;
 use cairo_lang_syntax::attribute::consts::{
     FEATURE_ATTR, FMT_SKIP_ATTR, IMPLICIT_PRECEDENCE_ATTR, INLINE_ATTR, MUST_USE_ATTR,
-    STARKNET_INTERFACE_ATTR, UNSTABLE_ATTR,
+    PHANTOM_ATTR, STARKNET_INTERFACE_ATTR, UNSTABLE_ATTR,
 };
 use cairo_lang_syntax::node::ast::MaybeModuleBody;
 use cairo_lang_syntax::node::db::SyntaxGroup;
@@ -227,6 +227,7 @@ fn allowed_attributes(db: &dyn DefsGroup) -> Arc<OrderedHashSet<String>> {
         MUST_USE_ATTR.into(),
         UNSTABLE_ATTR.into(),
         FEATURE_ATTR.into(),
+        PHANTOM_ATTR.into(),
         IMPLICIT_PRECEDENCE_ATTR.into(),
         FMT_SKIP_ATTR.into(),
         // TODO(orizi): Remove this once `starknet` is removed from corelib.

crates/cairo-lang-semantic/src/diagnostic.rs

@@ -757,6 +757,9 @@ impl DiagnosticEntry for SemanticDiagnostic {
             SemanticDiagnosticKind::SelfMustBeFirst => {
                 "`Self` can only be the first segment of a path.".into()
             }
+            SemanticDiagnosticKind::CannotCreateInstancesOfPhantomTypes => {
+                "Can not create instances of phantom types.".into()
+            }
         }
     }
 
@@ -817,6 +820,7 @@ pub enum SemanticDiagnosticKind {
     },
     UnexpectedGenericArgs,
     UnknownMember,
+    CannotCreateInstancesOfPhantomTypes,
     MemberSpecifiedMoreThanOnce,
     StructBaseStructExpressionNotLast,
     StructBaseStructExpressionNoEffect,

crates/cairo-lang-semantic/src/expr/compute.rs

@@ -13,6 +13,7 @@ use cairo_lang_defs::ids::{
 };
 use cairo_lang_diagnostics::{DiagnosticAdded, Maybe, ToOption};
 use cairo_lang_filesystem::ids::{FileKind, FileLongId, VirtualFile};
+use cairo_lang_syntax::attribute::consts::PHANTOM_ATTR;
 use cairo_lang_syntax::node::ast::{
     BlockOrIf, ExprPtr, PatternListOr, PatternStructParam, UnaryOperator,
 };
@@ -52,6 +53,7 @@ use crate::diagnostic::{
     ElementKind, NotFoundItemType, SemanticDiagnostics, TraitInferenceErrors,
     UnsupportedOutsideOfFunctionFeatureName,
 };
+use crate::items::attribute::SemanticQueryAttrs;
 use crate::items::constant::ConstValue;
 use crate::items::enm::SemanticEnumEx;
 use crate::items::feature_kind::extract_item_allowed_features;
@@ -721,6 +723,9 @@ fn compute_expr_function_call_semantic(
                 return Err(diag_added);
             }
             let concrete_enum_id = concrete_variant.concrete_enum_id;
+            if concrete_enum_id.has_attr(db, PHANTOM_ATTR)? {
+                ctx.diagnostics.report(syntax, CannotCreateInstancesOfPhantomTypes);
+            }
             Ok(semantic::Expr::EnumVariantCtor(semantic::ExprEnumVariantCtor {
                 variant: concrete_variant,
                 value_expr: arg.id,
@@ -1839,6 +1844,10 @@ fn struct_ctor_expr(
             .and_then(|c| try_extract_matches!(c, ConcreteTypeId::Struct))
             .ok_or_else(|| ctx.diagnostics.report(&path, NotAStruct))?;
 
+    if concrete_struct_id.has_attr(db, PHANTOM_ATTR)? {
+        ctx.diagnostics.report(ctor_syntax, CannotCreateInstancesOfPhantomTypes);
+    }
+
     let members = db.concrete_struct_members(concrete_struct_id)?;
     let mut member_exprs: OrderedHashMap<MemberId, Option<ExprId>> = OrderedHashMap::default();
     let mut base_struct = None;

crates/cairo-lang-semantic/src/expr/test.rs

@@ -26,8 +26,10 @@ cairo_lang_test_utils::test_file_test!(
 cairo_lang_test_utils::test_file_test!(
     expr_diagnostics,
     "src/expr/test_data",
+
     {
         assignment: "assignment",
+        attributes: "attributes",
         constant: "constant",
         constructor: "constructor",
         coupon: "coupon",

crates/cairo-lang-semantic/src/expr/test_data/attributes

@@ -0,0 +1,33 @@
+//! > Test instantiation.
+
+//! > test_runner_name
+test_function_diagnostics(expect_diagnostics: true)
+
+//! > function
+fn foo() {
+    MyStruct {};
+    MyEnum::a(3);
+}
+
+//! > function_name
+foo
+
+//! > module_code
+#[phantom]
+struct MyStruct {}
+
+#[phantom]
+enum MyEnum {
+    a: felt252
+}
+
+//! > expected_diagnostics
+error: Can not create instances of phantom types.
+ --> lib.cairo:9:5
+    MyStruct {};
+    ^*********^
+
+error: Can not create instances of phantom types.
+ --> lib.cairo:10:5
+    MyEnum::a(3);
+    ^**********^

crates/cairo-lang-syntax/src/attribute/consts.rs

@@ -27,3 +27,7 @@ pub const IMPLICIT_PRECEDENCE_ATTR: &str = "implicit_precedence";
 /// corelib.
 /// TODO(Gil): Remove this once `starknet` is removed from corelib.
 pub const STARKNET_INTERFACE_ATTR: &str = "starknet::interface";
+
+/// An attribute to define a type as a phantom type, phantom types cannot be created at run time and
+/// are typically used for meta-programming.
+pub const PHANTOM_ATTR: &str = "phantom";