generate

pub fn generate(gpa: Allocator, tree: Ast) Allocator.Error!Zir {
    assert(tree.mode == .zig);

    var arena = std.heap.ArenaAllocator.init(gpa);
    defer arena.deinit();

    var nodes_need_rl = try AstRlAnnotate.annotate(gpa, arena.allocator(), tree);
    defer nodes_need_rl.deinit(gpa);

    var astgen: AstGen = .{
        .gpa = gpa,
        .arena = arena.allocator(),
        .tree = &tree,
        .nodes_need_rl = &nodes_need_rl,
        .src_hasher = undefined, // `structDeclInner` for the root struct will set this
    };
    defer astgen.deinit(gpa);

    // String table index 0 is reserved for `NullTerminatedString.empty`.
    try astgen.string_bytes.append(gpa, 0);

    // We expect at least as many ZIR instructions and extra data items
    // as AST nodes.
    try astgen.instructions.ensureTotalCapacity(gpa, tree.nodes.len);

    // First few indexes of extra are reserved and set at the end.
    const reserved_count = @typeInfo(Zir.ExtraIndex).@"enum".field_names.len;
    try astgen.extra.ensureTotalCapacity(gpa, tree.nodes.len + reserved_count);
    astgen.extra.items.len += reserved_count;

    var top_scope: Scope.Top = .{};

    var gz_instructions: std.ArrayList(Zir.Inst.Index) = .empty;
    var gen_scope: GenZir = .{
        .is_comptime = true,
        .parent = &top_scope.base,
        .decl_node_index = .root,
        .decl_line = 0,
        .astgen = &astgen,
        .instructions = &gz_instructions,
        .instructions_top = 0,
    };
    defer gz_instructions.deinit(gpa);

    // The AST -> ZIR lowering process assumes an AST that does not have any parse errors.
    // Parse errors, or AstGen errors in the root struct, are considered "fatal", so we emit no ZIR.
    const fatal = if (tree.errors.len == 0) fatal: {
        if (AstGen.structDeclInner(
            &gen_scope,
            &gen_scope.base,
            .root,
            tree.containerDeclRoot(),
            .auto,
            .none,
            .parent,
        )) |struct_decl_ref| {
            assert(struct_decl_ref.toIndex().? == .main_struct_inst);
            break :fatal false;
        } else |err| switch (err) {
            error.OutOfMemory => |e| return e,
            error.AnalysisFail => break :fatal true, // Handled via compile_errors below.
        }
    } else fatal: {
        try lowerAstErrors(&astgen);
        break :fatal true;
    };

    const err_index = @backingInt(Zir.ExtraIndex.compile_errors);
    if (astgen.compile_errors.items.len == 0) {
        astgen.extra.items[err_index] = 0;
    } else {
        try astgen.extra.ensureUnusedCapacity(gpa, 1 + astgen.compile_errors.items.len *
            @typeInfo(Zir.Inst.CompileErrors.Item).@"struct".field_names.len);

        astgen.extra.items[err_index] = astgen.addExtraAssumeCapacity(Zir.Inst.CompileErrors{
            .items_len = @intCast(astgen.compile_errors.items.len),
        });

        for (astgen.compile_errors.items) |item| {
            _ = astgen.addExtraAssumeCapacity(item);
        }
    }

    const imports_index = @backingInt(Zir.ExtraIndex.imports);
    if (astgen.imports.count() == 0) {
        astgen.extra.items[imports_index] = 0;
    } else {
        try astgen.extra.ensureUnusedCapacity(gpa, @typeInfo(Zir.Inst.Imports).@"struct".field_names.len +
            astgen.imports.count() * @typeInfo(Zir.Inst.Imports.Item).@"struct".field_names.len);

        astgen.extra.items[imports_index] = astgen.addExtraAssumeCapacity(Zir.Inst.Imports{
            .imports_len = @intCast(astgen.imports.count()),
        });

        var it = astgen.imports.iterator();
        while (it.next()) |entry| {
            _ = astgen.addExtraAssumeCapacity(Zir.Inst.Imports.Item{
                .name = entry.key_ptr.*,
                .token = entry.value_ptr.*,
            });
        }
    }

    try astgen.extra.shrinkToLen(gpa);
    try astgen.string_bytes.shrinkToLen(gpa);

    return .{
        .instructions = if (fatal) .empty else astgen.instructions.toOwnedSlice(),
        .string_bytes = astgen.string_bytes.toOwnedSliceAssert(),
        .extra = astgen.extra.toOwnedSliceAssert(),
    };
}