Auto merge of #49252 - Manishearth:easy-feature-flag, r=nikomatsakis

Easy edition feature flag

We no longer gate features on epochs; instead we have a `#![feature(rust_2018_preview)]` that flips on a bunch of features (currently dyn_trait).

Based on #49001 to avoid merge conflicts

r? @nikomatsakis

src/libsyntax/edition.rs

@@ -55,6 +55,13 @@ pub fn lint_name(&self) -> &'static str {
             Edition::Edition2018 => "edition_2018",
         }
     }
+
+    pub fn feature_name(&self) -> &'static str {
+        match *self {
+            Edition::Edition2015 => "rust_2015_preview",
+            Edition::Edition2018 => "rust_2018_preview",
+        }
+    }
 }
 
 impl FromStr for Edition {

src/libsyntax/feature_gate.rs

@@ -28,7 +28,7 @@
 use abi::Abi;
 use ast::{self, NodeId, PatKind, RangeEnd};
 use attr;
-use edition::Edition;
+use edition::{ALL_EDITIONS, Edition};
 use codemap::Spanned;
 use syntax_pos::{Span, DUMMY_SP};
 use errors::{DiagnosticBuilder, Handler, FatalError};
@@ -1800,21 +1800,15 @@ fn visit_generic_param(&mut self, param: &'a ast::GenericParam) {
 }
 
 pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
-                    edition: Edition) -> Features {
+                    crate_edition: Edition) -> Features {
+    fn feature_removed(span_handler: &Handler, span: Span) {
+        span_err!(span_handler, span, E0557, "feature has been removed");
+    }
+
     let mut features = Features::new();
 
     let mut feature_checker = FeatureChecker::default();
 
-    for &(.., f_edition, set) in ACTIVE_FEATURES.iter() {
-        if let Some(f_edition) = f_edition {
-            if edition >= f_edition {
-                // FIXME(Manishearth) there is currently no way to set
-                // lang features by edition
-                set(&mut features, DUMMY_SP);
-            }
-        }
-    }
-
     for attr in krate_attrs {
         if !attr.check_name("feature") {
             continue
@@ -1827,6 +1821,7 @@ pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
             }
             Some(list) => {
                 for mi in list {
+
                     let name = if let Some(word) = mi.word() {
                         word.name()
                     } else {
@@ -1844,11 +1839,26 @@ pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute],
                             .find(|& &(n, _, _)| name == n)
                         .or_else(|| STABLE_REMOVED_FEATURES.iter()
                             .find(|& &(n, _, _)| name == n)) {
-                        span_err!(span_handler, mi.span, E0557, "feature has been removed");
+                        feature_removed(span_handler, mi.span);
                     }
                     else if let Some(&(_, _, _)) = ACCEPTED_FEATURES.iter()
                         .find(|& &(n, _, _)| name == n) {
                         features.declared_stable_lang_features.push((name, mi.span));
+                    } else if let Some(&edition) = ALL_EDITIONS.iter()
+                                                              .find(|e| name == e.feature_name()) {
+                        if edition <= crate_edition {
+                            feature_removed(span_handler, mi.span);
+                        } else {
+                            for &(.., f_edition, set) in ACTIVE_FEATURES.iter() {
+                                if let Some(f_edition) = f_edition {
+                                    if edition >= f_edition {
+                                        // FIXME(Manishearth) there is currently no way to set
+                                        // lib features by edition
+                                        set(&mut features, DUMMY_SP);
+                                    }
+                                }
+                            }
+                        }
                     } else {
                         features.declared_lib_features.push((name, mi.span));
                     }

src/test/run-pass/epoch-gate-feature.rs

@@ -11,7 +11,7 @@
 // Checks if the correct registers are being used to pass arguments
 // when the sysv64 ABI is specified.
 
-// compile-flags: -Zedition=2018
+#![feature(rust_2018_preview)]
 
 pub trait Foo {}