Rust 1.81 and later support elision with explicit self types

Commit 9ef6e21 introduced a check to
ensure that Clippy doesn't consider a lifetime present in an explicit
self type as being the default for an elided output lifetime. For
example, elision did not work in the case like:

```rust
  fn func(self: &Rc<Self>, &str) -> &str { … }
```

Since Rust 1.81.0, the lifetime in the self type is now considered
the default for elision. Elision should then be suggested when
appropriate.

Author: samueltardieu

clippy_lints/src/lib.rs

@@ -681,7 +681,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(|_| Box::new(unit_types::UnitTypes));
     store.register_late_pass(move |_| Box::new(loops::Loops::new(conf)));
     store.register_late_pass(|_| Box::<main_recursion::MainRecursion>::default());
-    store.register_late_pass(|_| Box::new(lifetimes::Lifetimes));
+    store.register_late_pass(move |_| Box::new(lifetimes::Lifetimes::new(conf)));
     store.register_late_pass(|_| Box::new(entry::HashMapPass));
     store.register_late_pass(|_| Box::new(minmax::MinMaxPass));
     store.register_late_pass(|_| Box::new(zero_div_zero::ZeroDiv));

clippy_lints/src/lifetimes.rs

@@ -1,4 +1,6 @@
+use clippy_config::Conf;
 use clippy_utils::diagnostics::{span_lint, span_lint_and_then};
+use clippy_utils::msrvs::{self, Msrv};
 use clippy_utils::trait_ref_of_method;
 use itertools::Itertools;
 use rustc_ast::visit::{try_visit, walk_list};
@@ -20,7 +22,7 @@ use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::hir::map::Map;
 use rustc_middle::hir::nested_filter as middle_nested_filter;
 use rustc_middle::lint::in_external_macro;
-use rustc_session::declare_lint_pass;
+use rustc_session::impl_lint_pass;
 use rustc_span::Span;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::symbol::{Ident, kw};
@@ -91,7 +93,19 @@ declare_clippy_lint! {
     "unused lifetimes in function definitions"
 }
 
-declare_lint_pass!(Lifetimes => [NEEDLESS_LIFETIMES, EXTRA_UNUSED_LIFETIMES]);
+pub struct Lifetimes {
+    msrv: Msrv,
+}
+
+impl Lifetimes {
+    pub fn new(conf: &'static Conf) -> Self {
+        Self {
+            msrv: conf.msrv.clone(),
+        }
+    }
+}
+
+impl_lint_pass!(Lifetimes => [NEEDLESS_LIFETIMES, EXTRA_UNUSED_LIFETIMES]);
 
 impl<'tcx> LateLintPass<'tcx> for Lifetimes {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
@@ -102,7 +116,7 @@ impl<'tcx> LateLintPass<'tcx> for Lifetimes {
             ..
         } = item.kind
         {
-            check_fn_inner(cx, sig, Some(id), None, generics, item.span, true);
+            check_fn_inner(cx, sig, Some(id), None, generics, item.span, true, &self.msrv);
         } else if let ItemKind::Impl(impl_) = item.kind {
             if !item.span.from_expansion() {
                 report_extra_impl_lifetimes(cx, impl_);
@@ -121,6 +135,7 @@ impl<'tcx> LateLintPass<'tcx> for Lifetimes {
                 item.generics,
                 item.span,
                 report_extra_lifetimes,
+                &self.msrv,
             );
         }
     }
@@ -131,11 +146,14 @@ impl<'tcx> LateLintPass<'tcx> for Lifetimes {
                 TraitFn::Required(sig) => (None, Some(sig)),
                 TraitFn::Provided(id) => (Some(id), None),
             };
-            check_fn_inner(cx, sig, body, trait_sig, item.generics, item.span, true);
+            check_fn_inner(cx, sig, body, trait_sig, item.generics, item.span, true, &self.msrv);
         }
     }
+
+    extract_msrv_attr!(LateContext);
 }
 
+#[allow(clippy::too_many_arguments)]
 fn check_fn_inner<'tcx>(
     cx: &LateContext<'tcx>,
     sig: &'tcx FnSig<'_>,
@@ -144,6 +162,7 @@ fn check_fn_inner<'tcx>(
     generics: &'tcx Generics<'_>,
     span: Span,
     report_extra_lifetimes: bool,
+    msrv: &Msrv,
 ) {
     if in_external_macro(cx.sess(), span) || has_where_lifetimes(cx, generics) {
         return;
@@ -195,7 +214,7 @@ fn check_fn_inner<'tcx>(
         }
     }
 
-    if let Some((elidable_lts, usages)) = could_use_elision(cx, sig.decl, body, trait_sig, generics.params) {
+    if let Some((elidable_lts, usages)) = could_use_elision(cx, sig.decl, body, trait_sig, generics.params, msrv) {
         if usages.iter().any(|usage| !usage.ident.span.eq_ctxt(span)) {
             return;
         }
@@ -216,6 +235,7 @@ fn could_use_elision<'tcx>(
     body: Option<BodyId>,
     trait_sig: Option<&[Ident]>,
     named_generics: &'tcx [GenericParam<'_>],
+    msrv: &Msrv,
 ) -> Option<(Vec<LocalDefId>, Vec<Lifetime>)> {
     // There are two scenarios where elision works:
     // * no output references, all input references have different LT
@@ -249,17 +269,17 @@ fn could_use_elision<'tcx>(
     let input_lts = input_visitor.lts;
     let output_lts = output_visitor.lts;
 
-    if let Some(trait_sig) = trait_sig {
-        if explicit_self_type(cx, func, trait_sig.first().copied()) {
-            return None;
-        }
+    if let Some(trait_sig) = trait_sig
+        && non_elidable_self_type(cx, func, trait_sig.first().copied(), msrv)
+    {
+        return None;
     }
 
     if let Some(body_id) = body {
         let body = cx.tcx.hir().body(body_id);
 
         let first_ident = body.params.first().and_then(|param| param.pat.simple_ident());
-        if explicit_self_type(cx, func, first_ident) {
+        if non_elidable_self_type(cx, func, first_ident, msrv) {
             return None;
         }
 
@@ -332,9 +352,15 @@ fn allowed_lts_from(named_generics: &[GenericParam<'_>]) -> FxIndexSet<LocalDefI
         .collect()
 }
 
-// elision doesn't work for explicit self types, see rust-lang/rust#69064
-fn explicit_self_type<'tcx>(cx: &LateContext<'tcx>, func: &FnDecl<'tcx>, ident: Option<Ident>) -> bool {
-    if let Some(ident) = ident
+// elision doesn't work for explicit self types before Rust 1.81, see rust-lang/rust#69064
+fn non_elidable_self_type<'tcx>(
+    cx: &LateContext<'tcx>,
+    func: &FnDecl<'tcx>,
+    ident: Option<Ident>,
+    msrv: &Msrv,
+) -> bool {
+    if !msrv.meets(msrvs::EXPLICIT_SELF_TYPE_ELISION)
+        && let Some(ident) = ident
         && ident.name == kw::SelfLower
         && !func.implicit_self.has_implicit_self()
         && let Some(self_ty) = func.inputs.first()

clippy_utils/src/msrvs.rs

@@ -20,7 +20,7 @@ macro_rules! msrv_aliases {
 msrv_aliases! {
     1,83,0 { CONST_EXTERN_FN, CONST_FLOAT_BITS_CONV, CONST_FLOAT_CLASSIFY }
     1,82,0 { IS_NONE_OR, REPEAT_N, RAW_REF_OP }
-    1,81,0 { LINT_REASONS_STABILIZATION, ERROR_IN_CORE }
+    1,81,0 { LINT_REASONS_STABILIZATION, ERROR_IN_CORE, EXPLICIT_SELF_TYPE_ELISION }
     1,80,0 { BOX_INTO_ITER }
     1,77,0 { C_STR_LITERALS }
     1,76,0 { PTR_FROM_REF, OPTION_RESULT_INSPECT }

tests/ui/needless_lifetimes.fixed

@@ -6,6 +6,7 @@
     clippy::boxed_local,
     clippy::extra_unused_type_parameters,
     clippy::needless_pass_by_value,
+    clippy::redundant_allocation,
     clippy::unnecessary_wraps,
     dyn_drop,
     clippy::get_first
@@ -443,11 +444,20 @@ mod issue7296 {
         fn implicit_mut(&mut self) -> &() {
             &()
         }
-
-        fn explicit<'a>(self: &'a Arc<Self>) -> &'a () {
+        #[clippy::msrv = "1.81"]
+        fn explicit(self: &Arc<Self>) -> &() {
+            &()
+        }
+        #[clippy::msrv = "1.81"]
+        fn explicit_mut(self: &mut Rc<Self>) -> &() {
             &()
         }
-        fn explicit_mut<'a>(self: &'a mut Rc<Self>) -> &'a () {
+        #[clippy::msrv = "1.80"]
+        fn explicit_older<'a>(self: &'a Arc<Self>) -> &'a () {
+            &()
+        }
+        #[clippy::msrv = "1.80"]
+        fn explicit_mut_older<'a>(self: &'a mut Rc<Self>) -> &'a () {
             &()
         }
 
@@ -462,8 +472,16 @@ mod issue7296 {
             &()
         }
 
-        fn explicit<'a>(self: &'a Arc<Self>) -> &'a ();
-        fn explicit_provided<'a>(self: &'a Arc<Self>) -> &'a () {
+        #[clippy::msrv = "1.81"]
+        fn explicit(self: &Arc<Self>) -> &();
+        #[clippy::msrv = "1.81"]
+        fn explicit_provided(self: &Arc<Self>) -> &() {
+            &()
+        }
+        #[clippy::msrv = "1.80"]
+        fn explicit_older<'a>(self: &'a Arc<Self>) -> &'a ();
+        #[clippy::msrv = "1.80"]
+        fn explicit_provided_older<'a>(self: &'a Arc<Self>) -> &'a () {
             &()
         }
 

tests/ui/needless_lifetimes.rs

@@ -6,6 +6,7 @@
     clippy::boxed_local,
     clippy::extra_unused_type_parameters,
     clippy::needless_pass_by_value,
+    clippy::redundant_allocation,
     clippy::unnecessary_wraps,
     dyn_drop,
     clippy::get_first
@@ -443,13 +444,22 @@ mod issue7296 {
         fn implicit_mut<'a>(&'a mut self) -> &'a () {
             &()
         }
-
+        #[clippy::msrv = "1.81"]
         fn explicit<'a>(self: &'a Arc<Self>) -> &'a () {
             &()
         }
+        #[clippy::msrv = "1.81"]
         fn explicit_mut<'a>(self: &'a mut Rc<Self>) -> &'a () {
             &()
         }
+        #[clippy::msrv = "1.80"]
+        fn explicit_older<'a>(self: &'a Arc<Self>) -> &'a () {
+            &()
+        }
+        #[clippy::msrv = "1.80"]
+        fn explicit_mut_older<'a>(self: &'a mut Rc<Self>) -> &'a () {
+            &()
+        }
 
         fn lifetime_elsewhere<'a>(self: Box<Self>, here: &'a ()) -> &'a () {
             &()
@@ -462,10 +472,18 @@ mod issue7296 {
             &()
         }
 
+        #[clippy::msrv = "1.81"]
         fn explicit<'a>(self: &'a Arc<Self>) -> &'a ();
+        #[clippy::msrv = "1.81"]
         fn explicit_provided<'a>(self: &'a Arc<Self>) -> &'a () {
             &()
         }
+        #[clippy::msrv = "1.80"]
+        fn explicit_older<'a>(self: &'a Arc<Self>) -> &'a ();
+        #[clippy::msrv = "1.80"]
+        fn explicit_provided_older<'a>(self: &'a Arc<Self>) -> &'a () {
+            &()
+        }
 
         fn lifetime_elsewhere<'a>(self: Box<Self>, here: &'a ()) -> &'a ();
         fn lifetime_elsewhere_provided<'a>(self: Box<Self>, here: &'a ()) -> &'a () {