Add edition support to rules and toolchain. (#157)

examples/hello_runfiles/BUILD

@@ -9,6 +9,7 @@ load(
 
 rust_binary(
     name = "hello_runfiles",
+    edition = "2018",
     srcs = ["hello_runfiles.rs"],
     data = ["hello_runfiles.rs"],  # Yes, we're being cute.
     deps = ["@io_bazel_rules_rust//tools/runfiles"],

examples/hello_runfiles/hello_runfiles.rs

@@ -1,5 +1,3 @@
-extern crate runfiles;
-
 use std::io::prelude::*;
 use std::fs::File;
 
@@ -13,6 +11,5 @@ fn main() {
     let mut buffer = String::new();
     f.read_to_string(&mut buffer).unwrap();
 
-    assert_eq!(buffer.len(), 427);
-    println!("This program's source is:\n```\n{}\n```", buffer);
+    println!("This program's source is {} characters long.", buffer.len());
 }

rust/private/rust.bzl

@@ -183,6 +183,9 @@ _rust_common_attrs = {
     ),
     "deps": attr.label_list(),
     "crate_features": attr.string_list(),
+    "edition": attr.string(
+        doc="The rust edition to use for this crate.",
+    ),
     "rustc_flags": attr.string_list(),
     "version": attr.string(default = "0.0.0"),
     "out_dir_tar": attr.label(

rust/private/rustc.bzl

@@ -217,6 +217,7 @@ def rustc_compile_action(
     if hasattr(ctx.attr, "crate_features"):
         args.add_all(getattr(ctx.attr, "crate_features"), before_each = "--cfg", format_each = 'feature="%s"')
     args.add_all(rust_flags)
+    args.add("--edition", _get_edition(ctx, toolchain))
     args.add_all(getattr(ctx.attr, "rustc_flags", []))
 
     # Link!
@@ -263,6 +264,12 @@ def rustc_compile_action(
         ),
     ]
 
+def _get_edition(ctx, toolchain):
+    if getattr(ctx.attr, "edition"):
+        return ctx.attr.edition
+    else:
+        return toolchain.default_edition
+
 def _create_out_dir_action(ctx):
     tar_file = getattr(ctx.file, "out_dir_tar", None)
     if not tar_file:

rust/toolchain.bzl

@@ -25,6 +25,7 @@ def _rust_toolchain_impl(ctx):
         target_triple = ctx.attr.target_triple,
         exec_triple = ctx.attr.exec_triple,
         os = ctx.attr.os,
+        default_edition = ctx.attr.default_edition,
         compilation_mode_opts = compilation_mode_opts,
         crosstool_files = ctx.files._crosstool,
     )
@@ -40,6 +41,10 @@ rust_toolchain = rule(
         "staticlib_ext": attr.string(mandatory = True),
         "dylib_ext": attr.string(mandatory = True),
         "os": attr.string(mandatory = True),
+        "default_edition": attr.string(
+            doc = "The edition to use for rust_* rules that don't specify an edition.",
+            default = "2015",
+        ),
         "exec_triple": attr.string(),
         "target_triple": attr.string(),
         "_crosstool": attr.label(