bug fix(semantic): Support struct-update (..base) syntax in const expressions. - #10136

Summary

Adds support for struct update syntax (Foo { field: value, ..base }) in constant expressions. Previously, ConstantEvaluateContext only handled struct constructors with no base struct (base_struct: None), causing struct update syntax to be unsupported in const contexts. Now, the base struct expression is validated and evaluated, and its member values are used as fallbacks for any fields not explicitly specified in the constructor.

Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

Why is this change needed?

Struct update syntax (Point { y: 20, ..BASE }) was not usable in const declarations. The constant evaluator would silently skip or fail on any struct constructor that included a base struct, leaving users unable to derive one constant struct from another using this syntax.

What was the behavior or documentation before?

Struct update syntax in const expressions was not supported. The evaluator only matched ExprStructCtor with base_struct: None, so any use of ..base in a constant struct constructor would not be evaluated correctly.

What is the behavior or documentation after?

Struct update syntax is now valid in const expressions. The base struct is validated and evaluated, and its member values fill in any fields not explicitly provided. Chained updates (e.g., deriving a constant from another already-updated constant) are also supported, as demonstrated by the new test:

const BASE: Point = Point { x: 1, y: 2, z: 3 };
const UPDATED: Point = Point { y: 20, ..BASE };
assert_eq!(UPDATED, Point { x: 1, y: 20, z: 3 });
const FROM_UPDATED: Point = Point { x: 10, z: 30, ..UPDATED };
assert_eq!(FROM_UPDATED, Point { x: 10, y: 20, z: 30 });

Related issue or discussion (if any)

Additional context

The base struct's evaluated ConstValue::Struct values are indexed by member order position and used as fallbacks via .or_else(|| Some(base_members?[index])) when a field is not explicitly listed in the constructor.