review comments

src/librustc_hir/hir.rs

@@ -437,32 +437,6 @@ pub fn bounds_span(&self) -> Option<Span> {
     }
 }
 
-pub trait NextTypeParamName {
-    fn next_type_param_name(&self, name: Option<&str>) -> String;
-}
-
-impl NextTypeParamName for &[GenericParam<'_>] {
-    fn next_type_param_name(&self, name: Option<&str>) -> String {
-        // This is the whitelist of possible parameter names that we might suggest.
-        let name = name.and_then(|n| n.chars().next()).map(|c| c.to_string().to_uppercase());
-        let name = name.as_ref().map(|s| s.as_str());
-        let possible_names = [name.unwrap_or("T"), "T", "U", "V", "X", "Y", "Z", "A", "B", "C"];
-        let used_names = self
-            .iter()
-            .filter_map(|p| match p.name {
-                ParamName::Plain(ident) => Some(ident.name),
-                _ => None,
-            })
-            .collect::<Vec<_>>();
-
-        possible_names
-            .iter()
-            .find(|n| !used_names.contains(&Symbol::intern(n)))
-            .unwrap_or(&"ParamName")
-            .to_string()
-    }
-}
-
 #[derive(Default)]
 pub struct GenericParamCount {
     pub lifetimes: usize,

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

@@ -10,12 +10,12 @@
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::Visitor;
-use rustc_hir::{NextTypeParamName, Node};
+use rustc_hir::Node;
 use rustc_middle::ty::TypeckTables;
 use rustc_middle::ty::{
     self, AdtKind, DefIdTree, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
 };
-use rustc_span::symbol::{kw, sym};
+use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_span::{MultiSpan, Span, DUMMY_SP};
 use std::fmt;
 
@@ -249,6 +249,8 @@ fn suggest_restriction(
         sugg.extend(ty_spans.into_iter().map(|s| (s, type_param_name.to_string())));
 
         // Suggest `fn foo<T: Trait>(t: T) where <T as Trait>::A: Bound`.
+        // FIXME: once `#![feature(associated_type_bounds)]` is stabilized, we should suggest
+        // `fn foo(t: impl Trait<A: Bound>)` instead.
         err.multipart_suggestion(
             "introduce a type parameter with a trait bound instead of using `impl Trait`",
             sugg,
@@ -1702,3 +1704,29 @@ fn visit_body(&mut self, body: &'v hir::Body<'v>) {
         hir::intravisit::walk_body(self, body);
     }
 }
+
+pub trait NextTypeParamName {
+    fn next_type_param_name(&self, name: Option<&str>) -> String;
+}
+
+impl NextTypeParamName for &[hir::GenericParam<'_>] {
+    fn next_type_param_name(&self, name: Option<&str>) -> String {
+        // This is the whitelist of possible parameter names that we might suggest.
+        let name = name.and_then(|n| n.chars().next()).map(|c| c.to_string().to_uppercase());
+        let name = name.as_ref().map(|s| s.as_str());
+        let possible_names = [name.unwrap_or("T"), "T", "U", "V", "X", "Y", "Z", "A", "B", "C"];
+        let used_names = self
+            .iter()
+            .filter_map(|p| match p.name {
+                hir::ParamName::Plain(ident) => Some(ident.name),
+                _ => None,
+            })
+            .collect::<Vec<_>>();
+
+        possible_names
+            .iter()
+            .find(|n| !used_names.contains(&Symbol::intern(n)))
+            .unwrap_or(&"ParamName")
+            .to_string()
+    }
+}

src/librustc_typeck/collect.rs

@@ -29,7 +29,7 @@
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc_hir::weak_lang_items;
-use rustc_hir::{GenericParamKind, NextTypeParamName, Node, Unsafety};
+use rustc_hir::{GenericParamKind, Node, Unsafety};
 use rustc_middle::hir::map::blocks::FnLikeNode;
 use rustc_middle::hir::map::Map;
 use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs};
@@ -45,6 +45,7 @@
 use rustc_span::symbol::{kw, sym, Symbol};
 use rustc_span::{Span, DUMMY_SP};
 use rustc_target::spec::abi;
+use rustc_trait_selection::traits::error_reporting::suggestions::NextTypeParamName;
 
 mod type_of;