Add linker_script option to rust_binary (#208)

This explicitly adds a linker_script option to rust_binary as suggested in the bottom of #204.

It's fundamentally identical to adding the ldscript to data and then adding --codegen=link-args=-T$(location //:ldscript) to rustc_flags, but now without the hassle of figuring that out yourself.

Author: m3rcuriel

rust/private/rust.bzl

@@ -354,9 +354,19 @@ INFO: Elapsed time: 1.245s, Critical Path: 1.01s
 """,
 )
 
+_rust_binary_attrs = {
+    "linker_script": attr.label(
+        doc = _tidy("""
+            Link script to forward into linker via rustc options.
+        """),
+        cfg = "host",
+        allow_single_file = True,
+    ),
+}
+
 rust_binary = rule(
     _rust_binary_impl,
-    attrs = _rust_common_attrs,
+    attrs = dict(_rust_common_attrs.items() + _rust_binary_attrs.items()),
     executable = True,
     fragments = ["cpp"],
     host_fragments = ["cpp"],

rust/private/rustc.bzl

@@ -182,12 +182,16 @@ def rustc_compile_action(
         toolchain,
     )
 
+    linker_script = getattr(ctx.file, "linker_script") if hasattr(ctx.file, "linker_script") \
+            else None
+
     compile_inputs = depset(
         crate_info.srcs +
         getattr(ctx.files, "data", []) +
         dep_info.transitive_libs +
         [toolchain.rustc] +
-        toolchain.crosstool_files,
+        toolchain.crosstool_files +
+        ([] if linker_script == None else [linker_script]),
         transitive = [
             toolchain.rustc_lib.files,
             toolchain.rust_lib.files,
@@ -214,6 +218,8 @@ def rustc_compile_action(
     args.add("--target=" + toolchain.target_triple)
     if hasattr(ctx.attr, "crate_features"):
         args.add_all(getattr(ctx.attr, "crate_features"), before_each = "--cfg", format_each = 'feature="%s"')
+    if hasattr(ctx.attr, "linker_script") and linker_script != None:
+        args.add(linker_script.path, format = "--codegen=link-arg=-T%s")
     args.add_all(rust_flags)
     args.add_all(getattr(ctx.attr, "rustc_flags", []))
     add_edition_flags(args, crate_info)