Add -Z allow_features=... flag

src/librustc/session/config.rs

@@ -1440,6 +1440,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
     merge_functions: Option<MergeFunctions> = (None, parse_merge_functions, [TRACKED],
         "control the operation of the MergeFunctions LLVM pass, taking
          the same values as the target option of the same name"),
+    allow_features: Option<Vec<String>> = (None, parse_opt_comma_list, [TRACKED],
+        "only allow the listed language features to be enabled in code (space separated)"),
 }
 
 pub fn default_lib_output() -> CrateType {
@@ -3286,6 +3288,10 @@ mod tests {
         opts = reference.clone();
         opts.debugging_opts.merge_functions = Some(MergeFunctions::Disabled);
         assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
+
+        opts = reference.clone();
+        opts.debugging_opts.allow_features = Some(vec![String::from("lang_items")]);
+        assert!(reference.dep_tracking_hash() != opts.dep_tracking_hash());
     }
 
     #[test]

src/librustc_interface/passes.rs

@@ -243,6 +243,7 @@ pub fn register_plugins<'a>(
         krate,
         &sess.parse_sess,
         sess.edition(),
+        &sess.opts.debugging_opts.allow_features,
     );
     // these need to be set "early" so that expansion sees `quote` if enabled.
     sess.init_features(features);

src/libsyntax/config.rs

@@ -24,8 +24,8 @@ 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, edition: Edition)
+pub fn features(mut krate: ast::Crate, sess: &ParseSess, edition: Edition,
-                -> (ast::Crate, Features) {
+                allow_features: &Option<Vec<String>>) -> (ast::Crate, Features) {
     let features;
     {
         let mut strip_unconfigured = StripUnconfigured {
@@ -43,7 +43,7 @@ pub fn features(mut krate: ast::Crate, sess: &ParseSess, edition: Edition)
             return (krate, Features::new());
         }
 
-        features = get_features(&sess.span_diagnostic, &krate.attrs, edition);
+        features = get_features(&sess.span_diagnostic, &krate.attrs, edition, allow_features);
 
         // Avoid reconfiguring malformed `cfg_attr`s
         if err_count == sess.span_diagnostic.err_count() {

src/libsyntax/diagnostic_list.rs

@@ -378,6 +378,21 @@ Erroneous code example:
 
 "##,
 
+E0725: r##"
+A feature attribute named a feature that was disallowed in the compiler
+command line flags.
+
+Erroneous code example:
+
+```ignore (can't specify compiler flags from doctests)
+#![feature(never_type)] // error: the feature `never_type` is not in
+                        // the list of allowed features
+```
+
+Delete the offending feature attribute, or add it to the list of allowed
+features in the `-Z allow_features` flag.
+"##,
+
 }
 
 register_diagnostics! {

src/libsyntax/feature_gate.rs

@@ -2008,7 +2008,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 }
 
 pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
-                    crate_edition: Edition) -> Features {
+                    crate_edition: Edition, allow_features: &Option<Vec<String>>) -> Features {
     fn feature_removed(span_handler: &Handler, span: Span, reason: Option<&str>) {
         let mut err = struct_span_err!(span_handler, span, E0557, "feature has been removed");
         if let Some(reason) = reason {
@@ -2127,6 +2127,15 @@ pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
             }
 
             if let Some((.., set)) = ACTIVE_FEATURES.iter().find(|f| name == f.0) {
+                if let Some(allowed) = allow_features.as_ref() {
+                    if allowed.iter().find(|f| *f == name.as_str()).is_none() {
+                        span_err!(span_handler, mi.span, E0725,
+                                  "the feature `{}` is not in the list of allowed features",
+                                  name);
+                        continue;
+                    }
+                }
+
                 set(&mut features, mi.span);
                 features.declared_lang_features.push((name, mi.span, None));
                 continue

src/test/ui/feature-gate/allow-features-empty.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Z allow_features=
+// Note: This test uses rustc internal flags because they will never stabilize.
+
+#![feature(rustc_diagnostic_macros)] //~ ERROR
+
+#![feature(rustc_const_unstable)] //~ ERROR
+
+#![feature(lang_items)] //~ ERROR
+
+fn main() {}

src/test/ui/feature-gate/allow-features-empty.stderr

@@ -0,0 +1,21 @@
+error[E0725]: the feature `rustc_diagnostic_macros` is not in the list of allowed features
+  --> $DIR/allow-features-empty.rs:4:12
+   |
+LL | #![feature(rustc_diagnostic_macros)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0725]: the feature `rustc_const_unstable` is not in the list of allowed features
+  --> $DIR/allow-features-empty.rs:6:12
+   |
+LL | #![feature(rustc_const_unstable)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+
+error[E0725]: the feature `lang_items` is not in the list of allowed features
+  --> $DIR/allow-features-empty.rs:8:12
+   |
+LL | #![feature(lang_items)]
+   |            ^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0725`.

src/test/ui/feature-gate/allow-features.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Z allow_features=rustc_diagnostic_macros,lang_items
+// Note: This test uses rustc internal flags because they will never stabilize.
+
+#![feature(rustc_diagnostic_macros)]
+
+#![feature(rustc_const_unstable)] //~ ERROR
+
+#![feature(lang_items)]
+
+fn main() {}

src/test/ui/feature-gate/allow-features.stderr

@@ -0,0 +1,9 @@
+error[E0725]: the feature `rustc_const_unstable` is not in the list of allowed features
+  --> $DIR/allow-features.rs:6:12
+   |
+LL | #![feature(rustc_const_unstable)]
+   |            ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0725`.