Apply get_lib_name correctly to the C++ runtime libraries

[scentini]

#1500 added an additional for_windows parameter to get_lib_name. I missed the fact that we also pass that function to map_each here: https://github.com/bazelbuild/rules_rust/blob/main/rust/private/rustc.bzl#L1671
and as such, this code does not always work correctly (we don't get to pass the for_windows parameter, and internally at Google it ended up evaluating to True on Linux builds).

I tried to avoid flattening the cc_toolchain.dynamic_runtime_lib and cc_toolchain.static_runtime_lib depsets by using a lambda:

args.add_all(
    cc_toolchain.dynamic_runtime_lib(feature_configuration = feature_configuration),
    map_each = lambda x: get_lib_name(x, for_windows = toolchain.os.startswith("windows)),
    format_each = "-ldylib=%s",
)

However it looks like such usage of lambdas is not allowed:

Error in add_all: to avoid unintended retention of analysis data structures,
the map_each function (declared at ...) must be declared by a top-level def statement

So instead of get_lib_name we now have get_lib_name_default and get_lib_name_for_windows.

[krasimirgg]

LGTM

[scentini]

hlopko@ pointed to me why this didn't work internally, when it does work externally:
Internally we populate the cc_toolchain.{static|dynamic}_runtime_lib fields, while externally they are not populated in the default C++ toolchain configuration for Bazel.

As for why this turned out to be an issue on Linux when get_lib_name had for_windows = False by default:
From args.add_all documentation:

map_each: callable; or None; default = None
A function that converts each item to zero or more strings, which may be further processed before appending. If this param is not provided, the standard conversion is used.
The function is passed either one or two positional arguments: the item to convert, followed by an optional DirectoryExpander. The second argument will be passed only if the supplied function is user-defined (not built-in) and declares more than one parameter.

So we end up passing a DirectoryExpander as the second argument, and as it is not None, for_windows will evaluate to True. -.-