Auto merge of #9743 - smoelius:improve-needless-lifetimes, r=Alexendoo

Improve `needless_lifetimes`

This PR makes the following improvements to `needless_lifetimes`.

* It fixes the following false negative, where `foo` is flagged but `bar` is not:
  ```rust
    fn foo<'a>(x: &'a u8, y: &'_ u8) {}

    fn bar<'a>(x: &'a u8, y: &'_ u8, z: &'_ u8) {}
  ```
* It flags more cases, generally. Previously, `needless_borrow` required *all* lifetimes to be used only once. With the changes, individual lifetimes are flagged for being used only once, even if not all lifetimes are.
* Finally, it tries to produce more clear error messages.

changelog: fix `needless_lifetimes` false negative involving functions with multiple unnamed lifetimes
changelog: in `needless_lifetimes`, flag individual lifetimes used only once, rather than require all lifetimes to be used only once
changelog: in `needless_lifetimes`, emit "replace with `'_`" warnings only when applicable, and point to a generic argument

clippy_lints/src/booleans.rs

@@ -481,7 +481,7 @@ impl<'a, 'tcx> Visitor<'tcx> for NonminimalBoolVisitor<'a, 'tcx> {
     }
 }
 
-fn implements_ord<'tcx>(cx: &LateContext<'tcx>, expr: &Expr<'_>) -> bool {
+fn implements_ord(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
     let ty = cx.typeck_results().expr_ty(expr);
     cx.tcx
         .get_diagnostic_item(sym::Ord)

clippy_lints/src/dereference.rs

@@ -1232,8 +1232,8 @@ fn is_mixed_projection_predicate<'tcx>(
     }
 }
 
-fn referent_used_exactly_once<'a, 'tcx>(
-    cx: &'a LateContext<'tcx>,
+fn referent_used_exactly_once<'tcx>(
+    cx: &LateContext<'tcx>,
     possible_borrowers: &mut Vec<(LocalDefId, PossibleBorrowerMap<'tcx, 'tcx>)>,
     reference: &Expr<'tcx>,
 ) -> bool {

clippy_lints/src/doc.rs

@@ -336,8 +336,8 @@ impl<'tcx> LateLintPass<'tcx> for DocMarkdown {
     }
 }
 
-fn lint_for_missing_headers<'tcx>(
-    cx: &LateContext<'tcx>,
+fn lint_for_missing_headers(
+    cx: &LateContext<'_>,
     def_id: LocalDefId,
     span: impl Into<MultiSpan> + Copy,
     sig: &hir::FnSig<'_>,
@@ -467,7 +467,7 @@ struct DocHeaders {
     panics: bool,
 }
 
-fn check_attrs<'a>(cx: &LateContext<'_>, valid_idents: &FxHashSet<String>, attrs: &'a [Attribute]) -> DocHeaders {
+fn check_attrs(cx: &LateContext<'_>, valid_idents: &FxHashSet<String>, attrs: &[Attribute]) -> DocHeaders {
     use pulldown_cmark::{BrokenLink, CowStr, Options};
     /// We don't want the parser to choke on intra doc links. Since we don't
     /// actually care about rendering them, just pretend that all broken links are

clippy_lints/src/fallible_impl_from.rs

@@ -64,7 +64,7 @@ impl<'tcx> LateLintPass<'tcx> for FallibleImplFrom {
     }
 }
 
-fn lint_impl_body<'tcx>(cx: &LateContext<'tcx>, impl_span: Span, impl_items: &[hir::ImplItemRef]) {
+fn lint_impl_body(cx: &LateContext<'_>, impl_span: Span, impl_items: &[hir::ImplItemRef]) {
     use rustc_hir::intravisit::{self, Visitor};
     use rustc_hir::{Expr, ImplItemKind};
 

clippy_lints/src/implicit_hasher.rs

@@ -66,8 +66,8 @@ impl<'tcx> LateLintPass<'tcx> for ImplicitHasher {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
         use rustc_span::BytePos;
 
-        fn suggestion<'tcx>(
-            cx: &LateContext<'tcx>,
+        fn suggestion(
+            cx: &LateContext<'_>,
             diag: &mut Diagnostic,
             generics_span: Span,
             generics_suggestion_span: Span,

clippy_lints/src/index_refutable_slice.rs

@@ -207,8 +207,8 @@ impl SliceLintInformation {
     }
 }
 
-fn filter_lintable_slices<'a, 'tcx>(
-    cx: &'a LateContext<'tcx>,
+fn filter_lintable_slices<'tcx>(
+    cx: &LateContext<'tcx>,
     slice_lint_info: FxIndexMap<hir::HirId, SliceLintInformation>,
     max_suggested_slice: u64,
     scope: &'tcx hir::Expr<'tcx>,

clippy_lints/src/indexing_slicing.rs

@@ -171,11 +171,7 @@ impl<'tcx> LateLintPass<'tcx> for IndexingSlicing {
 
 /// Returns a tuple of options with the start and end (exclusive) values of
 /// the range. If the start or end is not constant, None is returned.
-fn to_const_range<'tcx>(
-    cx: &LateContext<'tcx>,
-    range: higher::Range<'_>,
-    array_size: u128,
-) -> (Option<u128>, Option<u128>) {
+fn to_const_range(cx: &LateContext<'_>, range: higher::Range<'_>, array_size: u128) -> (Option<u128>, Option<u128>) {
     let s = range
         .start
         .map(|expr| constant(cx, cx.typeck_results(), expr).map(|(c, _)| c));

clippy_lints/src/invalid_upcast_comparisons.rs

@@ -38,7 +38,7 @@ declare_clippy_lint! {
 
 declare_lint_pass!(InvalidUpcastComparisons => [INVALID_UPCAST_COMPARISONS]);
 
-fn numeric_cast_precast_bounds<'a>(cx: &LateContext<'_>, expr: &'a Expr<'_>) -> Option<(FullInt, FullInt)> {
+fn numeric_cast_precast_bounds(cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<(FullInt, FullInt)> {
     if let ExprKind::Cast(cast_exp, _) = expr.kind {
         let pre_cast_ty = cx.typeck_results().expr_ty(cast_exp);
         let cast_ty = cx.typeck_results().expr_ty(expr);

clippy_lints/src/lifetimes.rs

@@ -1,4 +1,4 @@
-use clippy_utils::diagnostics::span_lint;
+use clippy_utils::diagnostics::{span_lint, span_lint_and_then};
 use clippy_utils::trait_ref_of_method;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::intravisit::nested_filter::{self as hir_nested_filter, NestedFilter};
@@ -151,6 +151,7 @@ fn check_fn_inner<'tcx>(
         .params
         .iter()
         .filter(|param| matches!(param.kind, GenericParamKind::Type { .. }));
+
     for typ in types {
         for pred in generics.bounds_for_param(cx.tcx.hir().local_def_id(typ.hir_id)) {
             if pred.origin == PredicateOrigin::WhereClause {
@@ -187,15 +188,30 @@ fn check_fn_inner<'tcx>(
             }
         }
     }
-    if could_use_elision(cx, decl, body, trait_sig, generics.params) {
-        span_lint(
+
+    if let Some(elidable_lts) = could_use_elision(cx, decl, body, trait_sig, generics.params) {
+        let lts = elidable_lts
+            .iter()
+            // In principle, the result of the call to `Node::ident` could be `unwrap`ped, as `DefId` should refer to a
+            // `Node::GenericParam`.
+            .filter_map(|&(def_id, _)| cx.tcx.hir().get_by_def_id(def_id).ident())
+            .map(|ident| ident.to_string())
+            .collect::<Vec<_>>()
+            .join(", ");
+
+        span_lint_and_then(
             cx,
             NEEDLESS_LIFETIMES,
             span.with_hi(decl.output.span().hi()),
-            "explicit lifetimes given in parameter types where they could be elided \
-             (or replaced with `'_` if needed by type declaration)",
+            &format!("the following explicit lifetimes could be elided: {lts}"),
+            |diag| {
+                if let Some(span) = elidable_lts.iter().find_map(|&(_, span)| span) {
+                    diag.span_help(span, "replace with `'_` in generic arguments such as here");
+                }
+            },
         );
     }
+
     if report_extra_lifetimes {
         self::report_extra_lifetimes(cx, decl, generics);
     }
@@ -226,7 +242,7 @@ fn could_use_elision<'tcx>(
     body: Option<BodyId>,
     trait_sig: Option<&[Ident]>,
     named_generics: &'tcx [GenericParam<'_>],
-) -> bool {
+) -> Option<Vec<(LocalDefId, Option<Span>)>> {
     // There are two scenarios where elision works:
     // * no output references, all input references have different LT
     // * output references, exactly one input reference with same LT
@@ -253,15 +269,15 @@ fn could_use_elision<'tcx>(
     }
 
     if input_visitor.abort() || output_visitor.abort() {
-        return false;
+        return None;
     }
 
     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 false;
+            return None;
         }
     }
 
@@ -270,22 +286,22 @@ fn could_use_elision<'tcx>(
 
         let first_ident = body.params.first().and_then(|param| param.pat.simple_ident());
         if explicit_self_type(cx, func, first_ident) {
-            return false;
+            return None;
         }
 
         let mut checker = BodyLifetimeChecker {
             lifetimes_used_in_body: false,
         };
         checker.visit_expr(body.value);
         if checker.lifetimes_used_in_body {
-            return false;
+            return None;
         }
     }
 
     // check for lifetimes from higher scopes
     for lt in input_lts.iter().chain(output_lts.iter()) {
         if !allowed_lts.contains(lt) {
-            return false;
+            return None;
         }
     }
 
@@ -301,48 +317,45 @@ fn could_use_elision<'tcx>(
         for lt in input_visitor.nested_elision_site_lts {
             if let RefLt::Named(def_id) = lt {
                 if allowed_lts.contains(&cx.tcx.item_name(def_id.to_def_id())) {
-                    return false;
+                    return None;
                 }
             }
         }
         for lt in output_visitor.nested_elision_site_lts {
             if let RefLt::Named(def_id) = lt {
                 if allowed_lts.contains(&cx.tcx.item_name(def_id.to_def_id())) {
-                    return false;
+                    return None;
                 }
             }
         }
     }
 
-    // no input lifetimes? easy case!
-    if input_lts.is_empty() {
-        false
-    } else if output_lts.is_empty() {
-        // no output lifetimes, check distinctness of input lifetimes
+    // A lifetime can be newly elided if:
+    // - It occurs only once among the inputs.
+    // - If there are multiple input lifetimes, then the newly elided lifetime does not occur among the
+    //   outputs (because eliding such an lifetime would create an ambiguity).
+    let elidable_lts = named_lifetime_occurrences(&input_lts)
+        .into_iter()
+        .filter_map(|(def_id, occurrences)| {
+            if occurrences == 1 && (input_lts.len() == 1 || !output_lts.contains(&RefLt::Named(def_id))) {
+                Some((
+                    def_id,
+                    input_visitor
+                        .lifetime_generic_arg_spans
+                        .get(&def_id)
+                        .or_else(|| output_visitor.lifetime_generic_arg_spans.get(&def_id))
+                        .copied(),
+                ))
+            } else {
+                None
+            }
+        })
+        .collect::<Vec<_>>();
 
-        // only unnamed and static, ok
-        let unnamed_and_static = input_lts.iter().all(|lt| *lt == RefLt::Unnamed || *lt == RefLt::Static);
-        if unnamed_and_static {
-            return false;
-        }
-        // we have no output reference, so we only need all distinct lifetimes
-        input_lts.len() == unique_lifetimes(&input_lts)
+    if elidable_lts.is_empty() {
+        None
     } else {
-        // we have output references, so we need one input reference,
-        // and all output lifetimes must be the same
-        if unique_lifetimes(&output_lts) > 1 {
-            return false;
-        }
-        if input_lts.len() == 1 {
-            match (&input_lts[0], &output_lts[0]) {
-                (&RefLt::Named(n1), &RefLt::Named(n2)) if n1 == n2 => true,
-                (&RefLt::Named(_), &RefLt::Unnamed) => true,
-                _ => false, /* already elided, different named lifetimes
-                             * or something static going on */
-            }
-        } else {
-            false
-        }
+        Some(elidable_lts)
     }
 }
 
@@ -358,10 +371,24 @@ fn allowed_lts_from(tcx: TyCtxt<'_>, named_generics: &[GenericParam<'_>]) -> FxH
     allowed_lts
 }
 
-/// Number of unique lifetimes in the given vector.
+/// Number of times each named lifetime occurs in the given slice. Returns a vector to preserve
+/// relative order.
 #[must_use]
-fn unique_lifetimes(lts: &[RefLt]) -> usize {
-    lts.iter().collect::<FxHashSet<_>>().len()
+fn named_lifetime_occurrences(lts: &[RefLt]) -> Vec<(LocalDefId, usize)> {
+    let mut occurrences = Vec::new();
+    for lt in lts {
+        if let &RefLt::Named(curr_def_id) = lt {
+            if let Some(pair) = occurrences
+                .iter_mut()
+                .find(|(prev_def_id, _)| *prev_def_id == curr_def_id)
+            {
+                pair.1 += 1;
+            } else {
+                occurrences.push((curr_def_id, 1));
+            }
+        }
+    }
+    occurrences
 }
 
 const CLOSURE_TRAIT_BOUNDS: [LangItem; 3] = [LangItem::Fn, LangItem::FnMut, LangItem::FnOnce];
@@ -370,6 +397,7 @@ const CLOSURE_TRAIT_BOUNDS: [LangItem; 3] = [LangItem::Fn, LangItem::FnMut, Lang
 struct RefVisitor<'a, 'tcx> {
     cx: &'a LateContext<'tcx>,
     lts: Vec<RefLt>,
+    lifetime_generic_arg_spans: FxHashMap<LocalDefId, Span>,
     nested_elision_site_lts: Vec<RefLt>,
     unelided_trait_object_lifetime: bool,
 }
@@ -379,6 +407,7 @@ impl<'a, 'tcx> RefVisitor<'a, 'tcx> {
         Self {
             cx,
             lts: Vec::new(),
+            lifetime_generic_arg_spans: FxHashMap::default(),
             nested_elision_site_lts: Vec::new(),
             unelided_trait_object_lifetime: false,
         }
@@ -472,6 +501,22 @@ impl<'a, 'tcx> Visitor<'tcx> for RefVisitor<'a, 'tcx> {
             _ => walk_ty(self, ty),
         }
     }
+
+    fn visit_generic_arg(&mut self, generic_arg: &'tcx GenericArg<'tcx>) {
+        if let GenericArg::Lifetime(l) = generic_arg
+            && let LifetimeName::Param(def_id, _) = l.name
+        {
+            self.lifetime_generic_arg_spans.entry(def_id).or_insert(l.span);
+        }
+        // Replace with `walk_generic_arg` if/when https://github.com/rust-lang/rust/pull/103692 lands.
+        // walk_generic_arg(self, generic_arg);
+        match generic_arg {
+            GenericArg::Lifetime(lt) => self.visit_lifetime(lt),
+            GenericArg::Type(ty) => self.visit_ty(ty),
+            GenericArg::Const(ct) => self.visit_anon_const(&ct.value),
+            GenericArg::Infer(inf) => self.visit_infer(inf),
+        }
+    }
 }
 
 /// Are any lifetimes mentioned in the `where` clause? If so, we don't try to

clippy_lints/src/loops/mut_range_bound.rs

@@ -52,8 +52,8 @@ fn check_for_mutability(cx: &LateContext<'_>, bound: &Expr<'_>) -> Option<HirId>
     None
 }
 
-fn check_for_mutation<'tcx>(
-    cx: &LateContext<'tcx>,
+fn check_for_mutation(
+    cx: &LateContext<'_>,
     body: &Expr<'_>,
     bound_id_start: Option<HirId>,
     bound_id_end: Option<HirId>,

clippy_lints/src/map_unit_fn.rs

@@ -119,7 +119,7 @@ fn is_unit_expression(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
 /// semicolons, which causes problems when generating a suggestion. Given an
 /// expression that evaluates to '()' or '!', recursively remove useless braces
 /// and semi-colons until is suitable for including in the suggestion template
-fn reduce_unit_expression<'a>(cx: &LateContext<'_>, expr: &'a hir::Expr<'_>) -> Option<Span> {
+fn reduce_unit_expression(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> Option<Span> {
     if !is_unit_expression(cx, expr) {
         return None;
     }

clippy_lints/src/matches/manual_filter.rs

@@ -62,7 +62,7 @@ fn peels_blocks_incl_unsafe<'a>(expr: &'a Expr<'a>) -> &'a Expr<'a> {
 //    <expr>
 // }
 // Returns true if <expr> resolves to `Some(x)`, `false` otherwise
-fn is_some_expr<'tcx>(cx: &LateContext<'_>, target: HirId, ctxt: SyntaxContext, expr: &'tcx Expr<'_>) -> bool {
+fn is_some_expr(cx: &LateContext<'_>, target: HirId, ctxt: SyntaxContext, expr: &Expr<'_>) -> bool {
     if let Some(inner_expr) = peels_blocks_incl_unsafe_opt(expr) {
         // there can be not statements in the block as they would be removed when switching to `.filter`
         if let ExprKind::Call(callee, [arg]) = inner_expr.kind {

clippy_lints/src/matches/significant_drop_in_scrutinee.rs

@@ -83,8 +83,8 @@ fn set_diagnostic<'tcx>(diag: &mut Diagnostic, cx: &LateContext<'tcx>, expr: &'t
 
 /// If the expression is an `ExprKind::Match`, check if the scrutinee has a significant drop that
 /// may have a surprising lifetime.
-fn has_significant_drop_in_scrutinee<'tcx, 'a>(
-    cx: &'a LateContext<'tcx>,
+fn has_significant_drop_in_scrutinee<'tcx>(
+    cx: &LateContext<'tcx>,
     scrutinee: &'tcx Expr<'tcx>,
     source: MatchSource,
 ) -> Option<(Vec<FoundSigDrop>, &'static str)> {
@@ -377,7 +377,7 @@ impl<'a, 'tcx> ArmSigDropHelper<'a, 'tcx> {
     }
 }
 
-fn has_significant_drop_in_arms<'tcx, 'a>(cx: &'a LateContext<'tcx>, arms: &'tcx [Arm<'_>]) -> FxHashSet<Span> {
+fn has_significant_drop_in_arms<'tcx>(cx: &LateContext<'tcx>, arms: &'tcx [Arm<'_>]) -> FxHashSet<Span> {
     let mut helper = ArmSigDropHelper::new(cx);
     for arm in arms {
         helper.visit_expr(arm.body);

clippy_lints/src/matches/single_match.rs

@@ -153,7 +153,7 @@ fn pat_in_candidate_enum<'a>(cx: &LateContext<'a>, ty: Ty<'a>, pat: &Pat<'_>) ->
 }
 
 /// Returns `true` if the given type is an enum we know won't be expanded in the future
-fn in_candidate_enum<'a>(cx: &LateContext<'a>, ty: Ty<'_>) -> bool {
+fn in_candidate_enum(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
     // list of candidate `Enum`s we know will never get any more members
     let candidates = [sym::Cow, sym::Option, sym::Result];