Recognize edition = "required" idiom

rust/private/rust.bzl

@@ -106,7 +106,7 @@ def _determine_lib_name(name, crate_type, toolchain, lib_hash = None):
         extension = extension,
     )
 
-def get_edition(attr, toolchain):
+def get_edition(ctx, toolchain):
     """Returns the Rust edition from either the current rule's attirbutes or the current `rust_toolchain`
 
     Args:
@@ -116,8 +116,10 @@ def get_edition(attr, toolchain):
     Returns:
         str: The target Rust edition
     """
-    if getattr(attr, "edition"):
-        return attr.edition
+    if getattr(ctx.attr, "edition"):
+        return ctx.attr.edition
+    elif toolchain.default_edition == "required":
+        fail("Attribute `edition` is required for {}.".format(ctx.label))
     else:
         return toolchain.default_edition
 
@@ -280,7 +282,7 @@ def _rust_library_common(ctx, crate_type):
             proc_macro_deps = depset(proc_macro_deps),
             aliases = ctx.attr.aliases,
             output = rust_lib,
-            edition = get_edition(ctx.attr, toolchain),
+            edition = get_edition(ctx, toolchain),
             rustc_env = ctx.attr.rustc_env,
             is_test = False,
             compile_data = depset(ctx.files.compile_data),
@@ -320,7 +322,7 @@ def _rust_binary_impl(ctx):
             proc_macro_deps = depset(proc_macro_deps),
             aliases = ctx.attr.aliases,
             output = output,
-            edition = get_edition(ctx.attr, toolchain),
+            edition = get_edition(ctx, toolchain),
             rustc_env = ctx.attr.rustc_env,
             is_test = False,
             compile_data = depset(ctx.files.compile_data),
@@ -386,7 +388,7 @@ def _rust_test_common(ctx, toolchain, output):
             proc_macro_deps = depset(proc_macro_deps),
             aliases = ctx.attr.aliases,
             output = output,
-            edition = get_edition(ctx.attr, toolchain),
+            edition = get_edition(ctx, toolchain),
             rustc_env = ctx.attr.rustc_env,
             is_test = True,
             compile_data = depset(ctx.files.compile_data),