Improve extra_exec_rustc_flags and support implicit binaries

Author: dzbarsky

rs/experimental/toolchains/declare_rustc_toolchains.bzl

@@ -10,6 +10,12 @@ def _channel(version):
         return "beta"
     return "stable"
 
+def _rustc_flags_to_select(rustc_flags_by_triple):
+    return select(
+        {"@rules_rs//rs/experimental/platforms/config:" + triple: flags for triple, flags in rustc_flags_by_triple.items()} |
+        {"//conditions:default": []},
+    )
+
 def declare_rustc_toolchains(
         *,
         version,
@@ -98,11 +104,8 @@ def declare_rustc_toolchains(
                 "//conditions:default": [],
             }),
             default_edition = edition,
-            extra_exec_rustc_flags = extra_exec_rustc_flags.get(triple),
-            extra_rustc_flags = select(
-                {"@rules_rs//rs/experimental/platforms/config:" + triple: flags for triple, flags in extra_rustc_flags.items()} |
-                {"//conditions:default": []},
-            ),
+            extra_exec_rustc_flags = _rustc_flags_to_select(extra_exec_rustc_flags),
+            extra_rustc_flags = _rustc_flags_to_select(extra_rustc_flags),
             exec_triple = triple,
             target_triple = select(target_triple_select),
             visibility = ["//visibility:public"],

rs/private/repository_utils.bzl

@@ -158,6 +158,11 @@ rust_crate(
 
     binaries = {bin["name"]: bin["path"] for bin in cargo_toml.get("bin", []) if bin["name"] in rctx.attr.gen_binaries}
 
+    implicit_binary_name = package["name"]
+    implicit_binary_path = "src/main.rs"
+    if implicit_binary_name in rctx.attr.gen_binaries and implicit_binary_name not in binaries and rctx.path(implicit_binary_path).exists:
+        binaries[implicit_binary_name] = implicit_binary_path
+
     return build_content.format(
         name = repr(name),
         hub_name = rctx.attr.hub_name,