Deeply immutable instances and types

The Dart VM has a concept of deeply immutable instances.

Deeply immutable instances can be shared across isolates within the same group.

Deeply immutable types

A deeply immutable type is a type for which all instances that have this type are deeply immutable.

This is useful for static checks on classes annotated @pragma('vm:deeply-immutable'). If an instance field has a deeply immutable type, it doesn't require any runtime checks for the values it gets. Instance fields of function types are also allowed in @pragma('vm:deeply-immutable') classes, but runtime checks will ensure that only immutable values are captured by the inititalizing values.

A list of immutable types:

• bool
• double
• int
• Null
• String
• Float32x4
• Float64x2
• Int32x4
• Pointer
• classes annotated with @pragma('vm:deeply-immutable')
• type parameters bound by a deeply immutable type

Deeply immutable instances without a deeply immutable type

In addition to instances from deeply immutable types, instances can also be deeply immutable while their type is not deeply immutable:

• SendPort (implemented externally package:isolate, so cannot be final https://github.com/dart-lang/sdk/issues/54885#issuecomment-1967329435)
• Capability (has SendPort as subtype so cannot be final)
• RegExp (can be implemented externally, not final)
• StackTrace (can be implemented externally, not final)
• Type (can be implemented externally, not final)
• const object (the class can be deeply immutable)
• function types (closures can capture arbritary contents)

This means users cannot mark classes with fields typed with these types as @pragma('vm:deeply-immutable'). Only exception is the function types, which are still allowed, but incur runtime-check.

Shallowly immutable instances

The VM also has shallow immutability.

• unmodifiable typed data views (the backing view might not be immutable)
• closures (the context might not be empty)

Implementation details

Deeply and shallowly immutable instances

The UntaggedObject::ShallowImmutableBit and UntaggedObject::DeeplyImmutableBit track whether an instance is shallowly or deeply immutable at runtime. For shallow immutable objects, the VM needs to know the layout and what to check when to check for to check deep immutability at runtime. During objects lifecycle if the object is inspected, it can get it shallow-immutable bit upgraded to deeply-immutable bit.

Deeply immutable types

The Class::is_deeply_immutable tracks whether all instances of a class are deeply immutable.

This bit can be set in two ways:

1. For recognized classes, in the VM initialization.
2. For classes with a Dart source, with the vm:deeply-immutable pragma.

The vm:deeply-immutable pragma is added to classes of which their type is deeply immutable.

This puts the following compile-time restrictions on these classes:

1. All instance fields must
   1. either have a deeply immutable or function type,
   2. be final, and
   3. be non-late.
2. The class must be final or sealed. This ensures no non-deeply-immutable subtypes are added by external code.
3. All subtypes must be deeply immutable. This ensures 1.1. can be trusted.
4. The super type must be deeply immutable (except for Object).
5. Context captured by closures must be deeply-immutable.

Compile-time restructions are enforced by DeeplyImmutableValidator.

Run-time checks are inserted to ensure that function-type instance fields are initialized with closure that only capture deeply-immutable values.