Disallow aspect implementation functions returning struct, disallow strings in `providers` of `attr.label_list`.

[comius]

Motivation

Completely removing support for legacy struct providers

Description

Two loose ends after removing support for legacy struct providers:

• disallow aspect implementation functions returning struct
• disallow strings in providers of attr.label_list.

Incompatible Flag

The cleanup should be small enough not to require incompatible flag

Migration Guide

Use modern providers. Strings in providers of attr.label_list are no-op with modern providers.

In which Bazel LTS version will this incompatible change be enabled?

Bazel 9

Additional Context

No response

TODO List

• Bazel: third_party/grpc/bazel/generate_cc.bzl:153
• Grpc: grpc+/bazel/generate_cc.bzl:175
• rules_apple+/apple/internal/rule_attrs.bzl:672
• Grpc: grpc+/bazel/python_rules.bzl:112,:167,:246,:253
• rules_python/python/private/attr_builders.bzl:88
• aspect:intellij_info.bzl%intellij_info_aspect (returning struct providers)