Deny #[cfg] and #[cfg_attr] on generic parameters.

4 files changed, 40 insertions, 3 deletions

diff --git a/src/libsyntax/attr.rs b/src/libsyntax/attr.rs
index ace9904e0c0..c70f5749202 100644
--- a/src/libsyntax/attr.rs
+++ b/src/libsyntax/attr.rs
@@ -17,7 +17,7 @@ pub use self::IntType::*;
 use ast;
 use ast::{AttrId, Attribute, Name, Ident, Path, PathSegment};
 use ast::{MetaItem, MetaItemKind, NestedMetaItem, NestedMetaItemKind};
-use ast::{Lit, LitKind, Expr, ExprKind, Item, Local, Stmt, StmtKind};
+use ast::{Lit, LitKind, Expr, ExprKind, Item, Local, Stmt, StmtKind, GenericParam};
 use codemap::{BytePos, Spanned, respan, dummy_spanned};
 use syntax_pos::Span;
 use errors::Handler;
@@ -1438,6 +1438,22 @@ impl HasAttrs for Stmt {
     }
 }
 
+impl HasAttrs for GenericParam {
+    fn attrs(&self) -> &[ast::Attribute] {
+        match self {
+            GenericParam::Lifetime(lifetime) => lifetime.attrs(),
+            GenericParam::Type(ty) => ty.attrs(),
+        }
+    }
+
+    fn map_attrs<F: FnOnce(Vec<Attribute>) -> Vec<Attribute>>(self, f: F) -> Self {
+        match self {
+            GenericParam::Lifetime(lifetime) => GenericParam::Lifetime(lifetime.map_attrs(f)),
+            GenericParam::Type(ty) => GenericParam::Type(ty.map_attrs(f)),
+        }
+    }
+}
+
 macro_rules! derive_has_attrs {
     ($($ty:path),*) => { $(
         impl HasAttrs for $ty {
@@ -1457,5 +1473,5 @@ macro_rules! derive_has_attrs {
 
 derive_has_attrs! {
     Item, Expr, Local, ast::ForeignItem, ast::StructField, ast::ImplItem, ast::TraitItem, ast::Arm,
-    ast::Field, ast::FieldPat, ast::Variant_
+    ast::Field, ast::FieldPat, ast::Variant_, ast::LifetimeDef, ast::TyParam
 }
diff --git a/src/libsyntax/config.rs b/src/libsyntax/config.rs
index 36911683a0e..33643789139 100644
--- a/src/libsyntax/config.rs
+++ b/src/libsyntax/config.rs
@@ -278,6 +278,22 @@ impl<'a> StripUnconfigured<'a> {
             pattern
         })
     }
+
+    // deny #[cfg] on generic parameters until we decide what to do with it.
+    // see issue #51279.
+    pub fn disallow_cfg_on_generic_param(&mut self, param: &ast::GenericParam) {
+        for attr in param.attrs() {
+            let offending_attr = if attr.check_name("cfg") {
+                "cfg"
+            } else if attr.check_name("cfg_attr") {
+                "cfg_attr"
+            } else {
+                continue;
+            };
+            let msg = format!("#[{}] cannot be applied on a generic parameter", offending_attr);
+            self.sess.span_diagnostic.span_err(attr.span, &msg);
+        }
+    }
 }
 
 impl<'a> fold::Folder for StripUnconfigured<'a> {
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 584b9455a93..8fee4881cca 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -1350,6 +1350,11 @@ impl<'a, 'b> Folder for InvocationCollector<'a, 'b> {
         }
     }
 
+    fn fold_generic_param(&mut self, param: ast::GenericParam) -> ast::GenericParam {
+        self.cfg.disallow_cfg_on_generic_param(&param);
+        noop_fold_generic_param(param, self)
+    }
+
     fn fold_attribute(&mut self, at: ast::Attribute) -> Option<ast::Attribute> {
         // turn `#[doc(include="filename")]` attributes into `#[doc(include(file="filename",
         // contents="file contents")]` attributes
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 9838a11372b..7f8c361525f 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -586,7 +586,7 @@ declare_features! (
     // allow `'_` placeholder lifetimes
     (accepted, underscore_lifetimes, "1.26.0", Some(44524), None),
     // Allows attributes on lifetime/type formal parameters in generics (RFC 1327)
-    (accepted, generic_param_attrs, "1.26.0", Some(48848), None),
+    (accepted, generic_param_attrs, "1.27.0", Some(48848), None),
     // Allows cfg(target_feature = "...").
     (accepted, cfg_target_feature, "1.27.0", Some(29717), None),
     // Allows #[target_feature(...)]