incompatible_disallow_struct_provider_syntax: disallow rule implementation functions returning struct

[c-parsons]

Flag: --incompatible_disallow_struct_provider_syntax
Available since: 0.23 (February 2019 release)

Motivation
The motivation for this change is described in #6241

Migration
Rule implementation functions should return a collection of provider instances instead of a struct

# Before
def _rule_impl(ctx):
    return struct(
        foo_info = struct(foo = ‘foo’),
        bar_info = struct(bar = ‘bar’),
    )

my_rule = rule(implementation = _rule_impl)

# After
FooInfo = provider()
BarInfo = provider()
def _rule_impl(ctx):
    return [FooInfo(foo = ‘foo’), BarInfo(bar = ‘bar’)]

my_rule = rule(implementation = _rule_impl)

One corner case to note is specifying files for coverage. This used
to be done with the hardcoded struct field instrumented_files, but should
now be done by using coverage_common.instrumented_files_info(). For example:

# Before
def rule_impl(ctx):
    return struct(
        instrumented_files = struct(
            source_attributes = ["srcs"],
            dependency_attributes = ["deps", "image"],
            extensions = ["js"],
        ),
    )

my_rule = rule(implementation = _rule_impl)

# After
def rule_impl(ctx):
    my_coverage_info = coverage_common.instrumented_files_info(
        ctx,
        source_attributes = ["srcs"],
        dependency_attributes = ["deps", "image"],
        extensions = ["js"])
    return [my_coverage_info]

my_rule = rule(implementation = _rule_impl)