use linking_context for additional_link_libraries (#73)

Instead of assuming that targets in `additional_link_libraries` always
provide a single file, use `linking_context` to obtain additional link
flags. Additionally, ensure that `additional_linker_inputs` are
available in the transitioned library.

Author: oliverlee

MODULE.bazel.lock

@@ -50,6 +50,8 @@ arm_none_eabi_toolchain(
 cc_library(
     name = "start",
     srcs = ["start.c"],
+    additional_linker_inputs = ["link.ld"],
+    linkopts = ["-T $(location :link.ld)"],
 )
 
 # Cortex-M4 toolchain that always links `:start`

examples/bzlmod/custom/toolchain/BUILD

@@ -44,12 +44,21 @@ def _default_linker_flags(_ctx):
         "-no-canonical-prefixes",
     ]
 
-def _additional_link_library_paths(ctx):
-    """Extract library paths from CcInfo providers."""
-    return [
-        target[DefaultInfo].files.to_list()[0].path
-        for target in ctx.attr.additional_link_libraries
-    ]
+def _additional_link_library_flags(ctx):
+    """Extract link flags from CcInfo providers."""
+    flags = []
+
+    for link in ctx.attr.additional_link_libraries:
+        for input in link[CcInfo].linking_context.linker_inputs.to_list():
+            flags.extend(input.user_link_flags)
+
+            for library in input.libraries:
+                if library.static_library:
+                    flags.append(library.static_library.path)
+                elif library.dynamic_library:
+                    flags.append(library.dynamic_library.path)
+
+    return flags
 
 def _impl(ctx):
     default_compiler_flags = _default_compiler_flags(ctx)
@@ -140,7 +149,7 @@ def _impl(ctx):
                 actions = [ACTION_NAMES.cpp_link_executable],
                 flag_groups = [
                     flag_group(
-                        flags = _additional_link_library_paths(ctx),
+                        flags = _additional_link_library_flags(ctx),
                     ),
                 ] if ctx.attr.additional_link_libraries else [],
             ),

examples/bzlmod/custom/toolchain/link.ld

@@ -14,9 +14,23 @@ _toolchain_transition = transition(
 )
 
 def _toolchain_transition_library_impl(ctx):
+    default_info = ctx.attr.src[DefaultInfo]
+    cc_info =  ctx.attr.src[CcInfo]
+
+    linker_input_files = []
+    for linker_input in cc_info.linking_context.linker_inputs.to_list():
+        linker_input_files.extend(linker_input.additional_inputs)
+
     return [
-        ctx.attr.src[DefaultInfo],
-        ctx.attr.src[CcInfo],
+        DefaultInfo(
+            files = depset(
+                transitive = [
+                    default_info.files,
+                    depset(linker_input_files),
+                ],
+            ),
+        ),
+        cc_info,
     ]
 
 toolchain_transition_library = rule(