[BUG] cannot specify list extra_rustc_flags in rust_register_toolchains

[nmattia]

It's currently impossible to specify a list of extra_rustc_flags in the toolchain:

rust_register_toolchains(
    ...
    extra_rustc_flags = [
        ...
    ],
)

Even though rust_register_toolchains suggests this is possible

extra_rustc_flags (dict, list, optional): Dictionary of target triples to list of extra flags to pass to rustc in non-exec configuration.

It fails with the following error:

File "/foo/bar/baz/external/rules_rust/rust/repositories.bzl", line 1053, column 50, in rust_repository_set
  extra_rustc_flags = extra_rustc_flags.get(toolchain.target_triple) if extra_rustc_flags != None else None,
Error: 'list' value has no field or method 'get'

The following patch seems to fix it:

@@ -1040,6 +1040,17 @@ def rust_repository_set(
         elif type(extra_target_triples) == "dict":
             target_compatible_with = extra_target_triples.get(toolchain.target_triple)

+
+        # Infer toolchain-specific rustc flags depending on the type (list, dict, optional) of extra_rustc_flags
+        if extra_rustc_flags == None:
+            toolchain_extra_rustc_flags = []
+        elif type(extra_rustc_flags) == "list":
+            toolchain_extra_rustc_flags = extra_rustc_flags
+        elif type(extra_rustc_flags) == "dict":
+            toolchain_extra_rustc_flags = extra_rustc_flags.get(toolchain.target_triple)
+        else:
+            fail("extra_rustc_flags should be a list or a dict")
+
         all_toolchain_names.append(rust_toolchain_repository(
             name = toolchain.name,
             allocator_library = allocator_library,
@@ -1050,7 +1061,7 @@ def rust_repository_set(
             edition = edition,
             exec_triple = exec_triple,
             extra_exec_rustc_flags = extra_exec_rustc_flags,
-            extra_rustc_flags = extra_rustc_flags.get(toolchain.target_triple) if extra_rustc_flags != None else None,
+            extra_rustc_flags = toolchain_extra_rustc_flags,
             opt_level = opt_level.get(toolchain.target_triple) if opt_level != None else None,
             target_settings = target_settings,
             iso_date = toolchain.channel.iso_date,