Add the -Zfeature=foo nightly flag to enable features from the CLI

Author: pietroalbini

src/librustc/session/config.rs

@@ -1351,6 +1351,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
           "generate build artifacts that are compatible with linker-based LTO."),
     no_parallel_llvm: bool = (false, parse_bool, [UNTRACKED],
           "don't run LLVM in parallel (while keeping codegen-units and ThinLTO)"),
+    feature: Vec<String> = (Vec::new(), parse_string_push, [TRACKED],
+        "enable the provided nightly feature (can be used multiple times)"),
 }
 
 pub fn default_lib_output() -> CrateType {

src/librustc_driver/driver.rs

@@ -812,6 +812,7 @@ where
     let (mut krate, features) = syntax::config::features(
         krate,
         &sess.parse_sess,
+        &sess.opts.debugging_opts.feature,
         sess.opts.test,
         sess.edition(),
     );

src/libsyntax/config.rs

@@ -27,8 +27,8 @@ pub struct StripUnconfigured<'a> {
 }
 
 // `cfg_attr`-process the crate's attributes and compute the crate's features.
-pub fn features(mut krate: ast::Crate, sess: &ParseSess, should_test: bool, edition: Edition)
-                -> (ast::Crate, Features) {
+pub fn features(mut krate: ast::Crate, sess: &ParseSess, z_features: &[String], should_test: bool,
+                edition: Edition) -> (ast::Crate, Features) {
     let features;
     {
         let mut strip_unconfigured = StripUnconfigured {
@@ -47,7 +47,7 @@ pub fn features(mut krate: ast::Crate, sess: &ParseSess, should_test: bool, edit
             return (krate, Features::new());
         }
 
-        features = get_features(&sess.span_diagnostic, &krate.attrs, edition);
+        features = get_features(&sess.span_diagnostic, &krate.attrs, &z_features, edition);
 
         // Avoid reconfiguring malformed `cfg_attr`s
         if err_count == sess.span_diagnostic.err_count() {

src/libsyntax/feature_gate.rs

@@ -1889,7 +1889,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
     }
 }
 
-pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
+pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute], z_features: &[String],
                     crate_edition: Edition) -> Features {
     let mut features = Features::new();
 
@@ -1937,6 +1937,17 @@ pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
         }
     }
 
+    for z_feature in z_features {
+        set_feature(
+            Symbol::intern(&z_feature),
+            DUMMY_SP,
+            span_handler,
+            &mut features,
+            &mut feature_checker,
+            crate_edition,
+        );
+    }
+
     feature_checker.check(span_handler);
 
     features

src/test/run-pass/z-feature.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// This test checks if an unstable feature is enabled with the -Zfeature=foo
+// flag. If the exact feature used here is causing problems feel free to
+// replace it with another perma-unstable feature.
+
+// compile-flags: -Zfeature=abi_unadjusted
+
+#![allow(dead_code)]
+
+extern "unadjusted" fn foo() {}
+
+fn main() {}