Support const generics in derive

Co-Authored-By: Gabriel Smith <yodaldevoid@users.noreply.github.com>

src/libsyntax/ext/build.rs

@@ -38,12 +38,14 @@ pub trait AstBuilder {
                 bindings: Vec<ast::TypeBinding>)
                 -> (ast::QSelf, ast::Path);
 
-    // types
+    // types and consts
     fn ty_mt(&self, ty: P<ast::Ty>, mutbl: ast::Mutability) -> ast::MutTy;
 
     fn ty(&self, span: Span, ty: ast::TyKind) -> P<ast::Ty>;
     fn ty_path(&self, path: ast::Path) -> P<ast::Ty>;
     fn ty_ident(&self, span: Span, idents: ast::Ident) -> P<ast::Ty>;
+    fn anon_const(&self, span: Span, expr: ast::ExprKind) -> ast::AnonConst;
+    fn const_ident(&self, span: Span, idents: ast::Ident) -> ast::AnonConst;
 
     fn ty_rptr(&self, span: Span,
                ty: P<ast::Ty>,
@@ -394,6 +396,22 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         self.ty_path(self.path_ident(span, ident))
     }
 
+    fn anon_const(&self, span: Span, expr: ast::ExprKind) -> ast::AnonConst {
+        ast::AnonConst {
+            id: ast::DUMMY_NODE_ID,
+            value: P(ast::Expr {
+                id: ast::DUMMY_NODE_ID,
+                node: expr,
+                span,
+                attrs: ThinVec::new(),
+            })
+        }
+    }
+
+    fn const_ident(&self, span: Span, ident: ast::Ident) -> ast::AnonConst {
+        self.anon_const(span, ast::ExprKind::Path(None, self.path_ident(span, ident)))
+    }
+
     fn ty_rptr(&self,
                span: Span,
                ty: P<ast::Ty>,

src/libsyntax_ext/deriving/generic/mod.rs

@@ -560,6 +560,7 @@ impl<'a> TraitDef<'a> {
 
                 cx.typaram(self.span, param.ident, vec![], bounds, None)
             }
+            GenericParamKind::Const { .. } => param.clone(),
         }));
 
         // and similarly for where clauses
@@ -657,6 +658,9 @@ impl<'a> TraitDef<'a> {
             GenericParamKind::Type { .. } => {
                 GenericArg::Type(cx.ty_ident(self.span, param.ident))
             }
+            GenericParamKind::Const { .. } => {
+                GenericArg::Const(cx.const_ident(self.span, param.ident))
+            }
         }).collect();
 
         // Create the type of `self`.

src/libsyntax_ext/deriving/generic/ty.rs

@@ -94,7 +94,7 @@ impl<'a> Path<'a> {
     }
 }
 
-/// A type. Supports pointers, Self, and literals
+/// A type. Supports pointers, Self, and literals.
 #[derive(Clone)]
 pub enum Ty<'a> {
     Self_,
@@ -107,6 +107,13 @@ pub enum Ty<'a> {
     Tuple(Vec<Ty<'a>>),
 }
 
+/// A const expression. Supports literals and blocks.
+#[derive(Clone, Eq, PartialEq)]
+pub enum Const {
+    Literal,
+    Block,
+}
+
 pub fn borrowed_ptrty<'r>() -> PtrTy<'r> {
     Borrowed(None, ast::Mutability::Immutable)
 }
@@ -180,6 +187,9 @@ impl<'a> Ty<'a> {
                     GenericParamKind::Type { .. } => {
                         GenericArg::Type(cx.ty_ident(span, param.ident))
                     }
+                    GenericParamKind::Const { .. } => {
+                        GenericArg::Const(cx.const_ident(span, param.ident))
+                    }
                 }).collect();
 
                 cx.path_all(span, false, vec![self_ty], params, vec![])