Scope

const Scope = struct {
    tag: Tag,

    fn cast(base: *Scope, comptime T: type) ?*T {
        if (T == Defer) {
            switch (base.tag) {
                .defer_normal, .defer_error => return @alignCast(@fieldParentPtr("base", base)),
                else => return null,
            }
        }
        if (T == Namespace) {
            switch (base.tag) {
                .namespace => return @alignCast(@fieldParentPtr("base", base)),
                else => return null,
            }
        }
        if (base.tag != T.base_tag)
            return null;

        return @alignCast(@fieldParentPtr("base", base));
    }

    fn parent(base: *Scope) ?*Scope {
        return switch (base.tag) {
            .gen_zir => base.cast(GenZir).?.parent,
            .local_val => base.cast(LocalVal).?.parent,
            .local_ptr => base.cast(LocalPtr).?.parent,
            .defer_normal, .defer_error => base.cast(Defer).?.parent,
            .namespace => base.cast(Namespace).?.parent,
            .top => null,
        };
    }

    fn unwrap(base: *Scope) Unwrapped {
        return switch (base.tag) {
            inline else => |tag| @unionInit(
                Unwrapped,
                @tagName(tag),
                @alignCast(@fieldParentPtr("base", base)),
            ),
        };
    }

    const Unwrapped = union(Tag) {
        gen_zir: *GenZir,
        local_val: *LocalVal,
        local_ptr: *LocalPtr,
        defer_normal: *Defer,
        defer_error: *Defer,
        namespace: *Namespace,
        top: *Top,
    };

    const Tag = enum {
        gen_zir,
        local_val,
        local_ptr,
        defer_normal,
        defer_error,
        namespace,
        top,
    };

    /// The category of identifier. These tag names are user-visible in compile errors.
    const IdCat = enum {
        @"function parameter",
        @"local constant",
        @"local variable",
        @"switch tag capture",
        capture,
    };

    /// This is always a `const` local and importantly the `inst` is a value type, not a pointer.
    /// This structure lives as long as the AST generation of the Block
    /// node that contains the variable.
    const LocalVal = struct {
        const base_tag: Tag = .local_val;
        base: Scope = Scope{ .tag = base_tag },
        /// Parents can be: `LocalVal`, `LocalPtr`, `GenZir`, `Defer`, `Namespace`.
        parent: *Scope,
        gen_zir: *GenZir,
        inst: Zir.Inst.Ref,
        /// Source location of the corresponding variable declaration.
        token_src: Ast.TokenIndex,
        /// Track the first identifier where it is referenced.
        /// .none means never referenced.
        used: Ast.OptionalTokenIndex = .none,
        /// Track the identifier where it is discarded, like this `_ = foo;`.
        /// .none means never discarded.
        discarded: Ast.OptionalTokenIndex = .none,
        is_used_or_discarded: ?*bool = null,
        /// String table index.
        name: Zir.NullTerminatedString,
        id_cat: IdCat,
    };

    /// This could be a `const` or `var` local. It has a pointer instead of a value.
    /// This structure lives as long as the AST generation of the Block
    /// node that contains the variable.
    const LocalPtr = struct {
        const base_tag: Tag = .local_ptr;
        base: Scope = Scope{ .tag = base_tag },
        /// Parents can be: `LocalVal`, `LocalPtr`, `GenZir`, `Defer`, `Namespace`.
        parent: *Scope,
        gen_zir: *GenZir,
        ptr: Zir.Inst.Ref,
        /// Source location of the corresponding variable declaration.
        token_src: Ast.TokenIndex,
        /// Track the first identifier where it is referenced.
        /// .none means never referenced.
        used: Ast.OptionalTokenIndex = .none,
        /// Track the identifier where it is discarded, like this `_ = foo;`.
        /// .none means never discarded.
        discarded: Ast.OptionalTokenIndex = .none,
        /// Whether this value is used as an lvalue after initialization.
        /// If not, we know it can be `const`, so will emit a compile error if it is `var`.
        used_as_lvalue: bool = false,
        /// String table index.
        name: Zir.NullTerminatedString,
        id_cat: IdCat,
        /// true means we find out during Sema whether the value is comptime.
        /// false means it is already known at AstGen the value is runtime-known.
        maybe_comptime: bool,
    };

    const Defer = struct {
        base: Scope,
        /// Parents can be: `LocalVal`, `LocalPtr`, `GenZir`, `Defer`, `Namespace`.
        parent: *Scope,
        index: u32,
        len: u32,
    };

    /// Represents a global scope that has any number of declarations in it.
    /// Each declaration has this as the parent scope.
    const Namespace = struct {
        const base_tag: Tag = .namespace;
        base: Scope = Scope{ .tag = base_tag },

        /// Parents can be: `LocalVal`, `LocalPtr`, `GenZir`, `Defer`, `Namespace`.
        parent: *Scope,
        /// Maps string table index to the source location of declaration,
        /// for the purposes of reporting name shadowing compile errors.
        decls: std.AutoHashMapUnmanaged(Zir.NullTerminatedString, Ast.Node.Index) = .empty,
        node: Ast.Node.Index,
        inst: Zir.Inst.Index,
        maybe_generic: bool,

        /// The astgen scope containing this namespace.
        /// Only valid during astgen.
        declaring_gz: ?*GenZir,

        /// Set of captures used by this namespace.
        captures: std.array_hash_map.Auto(Zir.Inst.Capture, Zir.NullTerminatedString) = .empty,

        fn deinit(self: *Namespace, gpa: Allocator) void {
            self.decls.deinit(gpa);
            self.captures.deinit(gpa);
            self.* = undefined;
        }
    };

    const Top = struct {
        const base_tag: Scope.Tag = .top;
        base: Scope = Scope{ .tag = base_tag },
    };
}