Cleanup: Use rustc's same_types for our same_tys

This delegates our `same_tys` to [ty::TyS::same_type][same_type] to
remove some duplication.

Our `same_tys` was introduced 4 years ago in #730.

Before, `same_tys` was building an inference context to be able to call
`can_eq` to compare the types. The `rustc-dev-guide` has some more details
about `can_eq` in [Type Inference -> Trying equality][try_eq].

Now, using the rustc function, we are essentially comparing the `DefId`s
of the given types, which also makes more sense, IMO.

I also confirmed that the FIXME is resolved via a bit of `dbg!`, however
no UI tests were affected.

[same_type]: https://github.com/rust-lang/rust/blob/659951c4a0d7450e43f61c61c0e87d0ceae17087/src/librustc_middle/ty/util.rs#L777
[try_eq]: https://rustc-dev-guide.rust-lang.org/type-inference.html#trying-equality

clippy_lints/src/copies.rs

@@ -243,14 +243,13 @@ fn lint_same_fns_in_if_cond(cx: &LateContext<'_, '_>, conds: &[&Expr<'_>]) {
 /// Implementation of `MATCH_SAME_ARMS`.
 fn lint_match_arms<'tcx>(cx: &LateContext<'_, 'tcx>, expr: &Expr<'_>) {
     fn same_bindings<'tcx>(
-        cx: &LateContext<'_, 'tcx>,
         lhs: &FxHashMap<Symbol, Ty<'tcx>>,
         rhs: &FxHashMap<Symbol, Ty<'tcx>>,
     ) -> bool {
         lhs.len() == rhs.len()
             && lhs
                 .iter()
-                .all(|(name, l_ty)| rhs.get(name).map_or(false, |r_ty| same_tys(cx, l_ty, r_ty)))
+                .all(|(name, l_ty)| rhs.get(name).map_or(false, |r_ty| same_tys(l_ty, r_ty)))
     }
 
     if let ExprKind::Match(_, ref arms, MatchSource::Normal) = expr.kind {
@@ -269,7 +268,7 @@ fn lint_match_arms<'tcx>(cx: &LateContext<'_, 'tcx>, expr: &Expr<'_>) {
             (min_index..=max_index).all(|index| arms[index].guard.is_none()) &&
                 SpanlessEq::new(cx).eq_expr(&lhs.body, &rhs.body) &&
                 // all patterns should have the same bindings
-                same_bindings(cx, &bindings(cx, &lhs.pat), &bindings(cx, &rhs.pat))
+                same_bindings(&bindings(cx, &lhs.pat), &bindings(cx, &rhs.pat))
         };
 
         let indexed_arms: Vec<(usize, &Arm<'_>)> = arms.iter().enumerate().collect();

clippy_lints/src/identity_conversion.rs

@@ -55,7 +55,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for IdentityConversion {
                 if match_trait_method(cx, e, &paths::INTO) && &*name.ident.as_str() == "into" {
                     let a = cx.tables.expr_ty(e);
                     let b = cx.tables.expr_ty(&args[0]);
-                    if same_tys(cx, a, b) {
+                    if same_tys(a, b) {
                         let sugg = snippet_with_macro_callsite(cx, args[0].span, "<expr>").to_string();
 
                         span_lint_and_sugg(
@@ -72,7 +72,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for IdentityConversion {
                 if match_trait_method(cx, e, &paths::INTO_ITERATOR) && &*name.ident.as_str() == "into_iter" {
                     let a = cx.tables.expr_ty(e);
                     let b = cx.tables.expr_ty(&args[0]);
-                    if same_tys(cx, a, b) {
+                    if same_tys(a, b) {
                         let sugg = snippet(cx, args[0].span, "<expr>").into_owned();
                         span_lint_and_sugg(
                             cx,
@@ -93,7 +93,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for IdentityConversion {
                         if match_def_path(cx, def_id, &paths::FROM_FROM) {
                             let a = cx.tables.expr_ty(e);
                             let b = cx.tables.expr_ty(&args[0]);
-                            if same_tys(cx, a, b) {
+                            if same_tys(a, b) {
                                 let sugg = snippet(cx, args[0].span.source_callsite(), "<expr>").into_owned();
                                 let sugg_msg =
                                     format!("consider removing `{}()`", snippet(cx, path.span, "From::from"));

clippy_lints/src/loops.rs

@@ -1348,7 +1348,7 @@ fn check_for_loop_arg(cx: &LateContext<'_, '_>, pat: &Pat<'_>, arg: &Expr<'_>, e
             } else if method_name == "into_iter" && match_trait_method(cx, arg, &paths::INTO_ITERATOR) {
                 let receiver_ty = cx.tables.expr_ty(&args[0]);
                 let receiver_ty_adjusted = cx.tables.expr_ty_adjusted(&args[0]);
-                if same_tys(cx, receiver_ty, receiver_ty_adjusted) {
+                if same_tys(receiver_ty, receiver_ty_adjusted) {
                     let mut applicability = Applicability::MachineApplicable;
                     let object = snippet_with_applicability(cx, args[0].span, "_", &mut applicability);
                     span_lint_and_sugg(
@@ -1369,7 +1369,7 @@ fn check_for_loop_arg(cx: &LateContext<'_, '_>, pat: &Pat<'_>, arg: &Expr<'_>, e
                             mutbl: Mutability::Not,
                         },
                     );
-                    if same_tys(cx, receiver_ty_adjusted, ref_receiver_ty) {
+                    if same_tys(receiver_ty_adjusted, ref_receiver_ty) {
                         lint_iter_method(cx, args, arg, method_name)
                     }
                 }

clippy_lints/src/methods/mod.rs

@@ -1524,7 +1524,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Methods {
 
             let contains_self_ty = |ty: Ty<'tcx>| {
                 ty.walk().any(|inner| match inner.unpack() {
-                    GenericArgKind::Type(inner_ty) => same_tys(cx, self_ty, inner_ty),
+                    GenericArgKind::Type(inner_ty) => same_tys(self_ty, inner_ty),
 
                     GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => false,
                 })
@@ -1554,7 +1554,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Methods {
                 }
             }
 
-            if name == "new" && !same_tys(cx, ret_ty, self_ty) {
+            if name == "new" && !same_tys(ret_ty, self_ty) {
                 span_lint(
                     cx,
                     NEW_RET_NO_SELF,

clippy_lints/src/new_without_default.rs

@@ -129,7 +129,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NewWithoutDefault {
                             let self_did = cx.tcx.hir().local_def_id(cx.tcx.hir().get_parent_item(id));
                             let self_ty = cx.tcx.type_of(self_did);
                             if_chain! {
-                                if same_tys(cx, self_ty, return_ty(cx, id));
+                                if same_tys(self_ty, return_ty(cx, id));
                                 if let Some(default_trait_id) = get_trait_def_id(cx, &paths::DEFAULT_TRAIT);
                                 then {
                                     if self.impling_types.is_none() {

clippy_lints/src/types.rs

@@ -2462,7 +2462,7 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for ImplicitHasherConstructorVisitor<'a, 'b, 't
             if let ExprKind::Path(QPath::TypeRelative(ref ty, ref method)) = fun.kind;
             if let TyKind::Path(QPath::Resolved(None, ref ty_path)) = ty.kind;
             then {
-                if !same_tys(self.cx, self.target.ty(), self.body.expr_ty(e)) {
+                if !same_tys(self.target.ty(), self.body.expr_ty(e)) {
                     return;
                 }
 

clippy_lints/src/utils/mod.rs

@@ -41,7 +41,7 @@ use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint::{LateContext, Level, Lint, LintContext};
 use rustc_middle::hir::map::Map;
 use rustc_middle::traits;
-use rustc_middle::ty::{self, layout::IntegerExt, subst::GenericArg, Binder, Ty, TyCtxt, TypeFoldable};
+use rustc_middle::ty::{self, layout::IntegerExt, subst::GenericArg, Ty, TyCtxt, TypeFoldable};
 use rustc_span::hygiene::{ExpnKind, MacroKind};
 use rustc_span::source_map::original_sp;
 use rustc_span::symbol::{self, kw, Symbol};
@@ -890,17 +890,8 @@ pub fn return_ty<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, fn_item: hir::HirId) -> T
 }
 
 /// Checks if two types are the same.
-///
-/// This discards any lifetime annotations, too.
-//
-// FIXME: this works correctly for lifetimes bounds (`for <'a> Foo<'a>` ==
-// `for <'b> Foo<'b>`, but not for type parameters).
-pub fn same_tys<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
-    let a = cx.tcx.erase_late_bound_regions(&Binder::bind(a));
-    let b = cx.tcx.erase_late_bound_regions(&Binder::bind(b));
-    cx.tcx
-        .infer_ctxt()
-        .enter(|infcx| infcx.can_eq(cx.param_env, a, b).is_ok())
+pub fn same_tys<'tcx>(a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
+    ty::TyS::same_type(a, b)
 }
 
 /// Returns `true` if the given type is an `unsafe` function.