refactor 'Output = $ty' & reduce rustc dep

src/librustc/traits/project.rs

@@ -17,7 +17,6 @@
 use crate::ty::fold::{TypeFoldable, TypeFolder};
 use crate::ty::subst::{InternalSubsts, Subst};
 use crate::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt};
-use crate::util::common::FN_OUTPUT_NAME;
 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
 use rustc_hir::def_id::DefId;
 use rustc_macros::HashStable;
@@ -1364,7 +1363,7 @@ fn confirm_callable_candidate<'cx, 'tcx>(
         projection_ty: ty::ProjectionTy::from_ref_and_name(
             tcx,
             trait_ref,
-            Ident::with_dummy_span(FN_OUTPUT_NAME),
+            Ident::with_dummy_span(rustc_hir::FN_OUTPUT_NAME),
         ),
         ty: ret_type,
     });

src/librustc/util/common.rs

@@ -5,14 +5,9 @@
 use std::fmt::Debug;
 use std::time::{Duration, Instant};
 
-use rustc_span::symbol::{sym, Symbol};
-
 #[cfg(test)]
 mod tests;
 
-// The name of the associated type for `Fn` return types.
-pub const FN_OUTPUT_NAME: Symbol = sym::Output;
-
 pub use errors::ErrorReported;
 
 pub fn to_readable_str(mut val: usize) -> String {

src/librustc_ast_lowering/lib.rs

@@ -41,7 +41,6 @@
 use rustc::lint::builtin;
 use rustc::middle::cstore::CrateStore;
 use rustc::util::captures::Captures;
-use rustc::util::common::FN_OUTPUT_NAME;
 use rustc::{bug, span_bug};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::sync::Lrc;
@@ -1978,12 +1977,7 @@ fn lower_async_fn_output_type_to_future_bound(
         // "<Output = T>"
         let future_params = self.arena.alloc(hir::GenericArgs {
             args: &[],
-            bindings: arena_vec![self; hir::TypeBinding {
-                ident: Ident::with_dummy_span(FN_OUTPUT_NAME),
-                kind: hir::TypeBindingKind::Equality { ty: output_ty },
-                hir_id: self.next_id(),
-                span,
-            }],
+            bindings: arena_vec![self; self.output_ty_binding(span, output_ty)],
             parenthesized: false,
         });
 

src/librustc_ast_lowering/path.rs

@@ -3,7 +3,6 @@
 
 use rustc::lint::builtin::{self, ELIDED_LIFETIMES_IN_PATHS};
 use rustc::span_bug;
-use rustc::util::common::FN_OUTPUT_NAME;
 use rustc_error_codes::*;
 use rustc_errors::{struct_span_err, Applicability};
 use rustc_hir as hir;
@@ -406,16 +405,22 @@ fn lower_parenthesized_parameter_data(
                 FunctionRetTy::Default(_) => this.arena.alloc(this.ty_tup(span, &[])),
             };
             let args = smallvec![GenericArg::Type(this.ty_tup(span, inputs))];
-            let binding = hir::TypeBinding {
-                hir_id: this.next_id(),
-                ident: Ident::with_dummy_span(FN_OUTPUT_NAME),
-                span: output_ty.span,
-                kind: hir::TypeBindingKind::Equality { ty: output_ty },
-            };
+            let binding = this.output_ty_binding(output_ty.span, output_ty);
             (
                 GenericArgsCtor { args, bindings: arena_vec![this; binding], parenthesized: true },
                 false,
             )
         })
     }
+
+    /// An associated type binding `Output = $ty`.
+    crate fn output_ty_binding(
+        &mut self,
+        span: Span,
+        ty: &'hir hir::Ty<'hir>,
+    ) -> hir::TypeBinding<'hir> {
+        let ident = Ident::with_dummy_span(hir::FN_OUTPUT_NAME);
+        let kind = hir::TypeBindingKind::Equality { ty };
+        hir::TypeBinding { hir_id: self.next_id(), span, ident, kind }
+    }
 }

src/librustc_hir/hir.rs

@@ -1875,6 +1875,9 @@ pub enum ImplItemKind<'hir> {
     OpaqueTy(GenericBounds<'hir>),
 }
 
+// The name of the associated type for `Fn` return types.
+pub const FN_OUTPUT_NAME: Symbol = sym::Output;
+
 /// Bind a type to an associated type (i.e., `A = Foo`).
 ///
 /// Bindings like `A: Debug` are represented as a special type `A =