move `is_ty_alias` to `clippy_utils`

clippy_lints/src/useless_conversion.rs

@@ -1,12 +1,12 @@
 use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg};
+use clippy_utils::is_ty_alias;
 use clippy_utils::source::{snippet, snippet_with_context};
 use clippy_utils::sugg::Sugg;
 use clippy_utils::ty::{is_copy, is_type_diagnostic_item, same_type_and_consts};
 use clippy_utils::{get_parent_expr, is_trait_method, match_def_path, path_to_local, paths};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
-use rustc_hir::def::{DefKind, Res};
-use rustc_hir::{BindingAnnotation, Expr, ExprKind, HirId, MatchSource, Node, PatKind, QPath, TyKind};
+use rustc_hir::{BindingAnnotation, Expr, ExprKind, HirId, MatchSource, Node, PatKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty;
 use rustc_session::{declare_tool_lint, impl_lint_pass};
@@ -197,12 +197,3 @@ impl<'tcx> LateLintPass<'tcx> for UselessConversion {
         }
     }
 }
-
-/// `cx.qpath_res` seems to return `AssocFn` so we do this instead
-fn is_ty_alias(qpath: &QPath<'_>) -> bool {
-    match *qpath {
-        QPath::Resolved(_, path) => matches!(path.res, Res::Def(DefKind::TyAlias, ..)),
-        QPath::TypeRelative(ty, _) if let TyKind::Path(qpath) = ty.kind => is_ty_alias(&qpath),
-        _ => false,
-    }
-}

clippy_utils/src/lib.rs

@@ -1,5 +1,6 @@
 #![feature(array_chunks)]
 #![feature(box_patterns)]
+#![feature(if_let_guard)]
 #![feature(let_chains)]
 #![feature(lint_reasons)]
 #![feature(never_type)]
@@ -282,6 +283,15 @@ pub fn is_wild(pat: &Pat<'_>) -> bool {
     matches!(pat.kind, PatKind::Wild)
 }
 
+/// Checks if the given `QPath` belongs to a type alias.
+pub fn is_ty_alias(qpath: &QPath<'_>) -> bool {
+    match *qpath {
+        QPath::Resolved(_, path) => matches!(path.res, Res::Def(DefKind::TyAlias, ..)),
+        QPath::TypeRelative(ty, _) if let TyKind::Path(qpath) = ty.kind => { is_ty_alias(&qpath) },
+        _ => false,
+    }
+}
+
 /// Checks if the method call given in `expr` belongs to the given trait.
 /// This is a deprecated function, consider using [`is_trait_method`].
 pub fn match_trait_method(cx: &LateContext<'_>, expr: &Expr<'_>, path: &[&str]) -> bool {