Auto merge of #9386 - smoelius:further-enhance-needless-borrow, r=Jarcho

Further enhance `needless_borrow`, mildly refactor `redundant_clone`

This PR does the following:
* Moves some code from `redundant_clone` into a new `clippy_utils` module called `mir`, and wraps that code in a function called `dropped_without_further_use`.
* Relaxes the "is copyable" condition condition from #9136 by also suggesting to remove borrows from values dropped without further use. The changes involve the just mentioned function.
* Separates `redundant_clone` into modules.

Strictly speaking, the last bullet is independent of the others. `redundant_clone` is somewhat hairy, IMO. Separating it into modules makes it slightly less so, by helping to delineate what depends upon what.

I've tried to break everything up into digestible commits.

r? `@Jarcho`

(`@Jarcho` I hope you don't mind.)

changelog: continuation of #9136

22 files changed, 879 insertions, 504 deletions

diff --git a/clippy_dev/src/serve.rs b/clippy_dev/src/serve.rs
index 2e0794f12fa..535c25e69f1 100644
--- a/clippy_dev/src/serve.rs
+++ b/clippy_dev/src/serve.rs
@@ -49,7 +49,7 @@ fn mtime(path: impl AsRef<Path>) -> SystemTime {
             .into_iter()
             .flatten()
             .flatten()
-            .map(|entry| mtime(&entry.path()))
+            .map(|entry| mtime(entry.path()))
             .max()
             .unwrap_or(SystemTime::UNIX_EPOCH)
     } else {
diff --git a/clippy_dev/src/update_lints.rs b/clippy_dev/src/update_lints.rs
index 0eb443167ec..8d1bfacd1dc 100644
--- a/clippy_dev/src/update_lints.rs
+++ b/clippy_dev/src/update_lints.rs
@@ -128,7 +128,7 @@ fn generate_lint_files(
     for (lint_group, lints) in Lint::by_lint_group(usable_lints.into_iter().chain(internal_lints)) {
         let content = gen_lint_group_list(&lint_group, lints.iter());
         process_file(
-            &format!("clippy_lints/src/lib.register_{lint_group}.rs"),
+            format!("clippy_lints/src/lib.register_{lint_group}.rs"),
             update_mode,
             &content,
         );
diff --git a/clippy_lints/src/dereference.rs b/clippy_lints/src/dereference.rs
index 3cd8f236e7a..45ee2fce4e4 100644
--- a/clippy_lints/src/dereference.rs
+++ b/clippy_lints/src/dereference.rs
@@ -1,4 +1,5 @@
 use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_hir_and_then};
+use clippy_utils::mir::{enclosing_mir, expr_local, local_assignments, used_exactly_once, PossibleBorrowerMap};
 use clippy_utils::source::{snippet_with_applicability, snippet_with_context};
 use clippy_utils::sugg::has_enclosing_paren;
 use clippy_utils::ty::{expr_sig, is_copy, peel_mid_ty_refs, ty_sig, variant_of_res};
@@ -11,13 +12,16 @@ use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::Applicability;
 use rustc_hir::intravisit::{walk_ty, Visitor};
 use rustc_hir::{
-    self as hir, def_id::DefId, BindingAnnotation, Body, BodyId, BorrowKind, Closure, Expr, ExprKind, FnRetTy,
-    GenericArg, HirId, ImplItem, ImplItemKind, Item, ItemKind, Local, MatchSource, Mutability, Node, Pat, PatKind,
-    Path, QPath, TraitItem, TraitItemKind, TyKind, UnOp,
+    self as hir,
+    def_id::{DefId, LocalDefId},
+    BindingAnnotation, Body, BodyId, BorrowKind, Closure, Expr, ExprKind, FnRetTy, GenericArg, HirId, ImplItem,
+    ImplItemKind, Item, ItemKind, Local, MatchSource, Mutability, Node, Pat, PatKind, Path, QPath, TraitItem,
+    TraitItemKind, TyKind, UnOp,
 };
 use rustc_index::bit_set::BitSet;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::mir::{Rvalue, StatementKind};
 use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
 use rustc_middle::ty::{
     self, Binder, BoundVariableKind, EarlyBinder, FnSig, GenericArgKind, List, ParamTy, PredicateKind,
@@ -141,7 +145,7 @@ declare_clippy_lint! {
     "dereferencing when the compiler would automatically dereference"
 }
 
-impl_lint_pass!(Dereferencing => [
+impl_lint_pass!(Dereferencing<'_> => [
     EXPLICIT_DEREF_METHODS,
     NEEDLESS_BORROW,
     REF_BINDING_TO_REFERENCE,
@@ -149,7 +153,7 @@ impl_lint_pass!(Dereferencing => [
 ]);
 
 #[derive(Default)]
-pub struct Dereferencing {
+pub struct Dereferencing<'tcx> {
     state: Option<(State, StateData)>,
 
     // While parsing a `deref` method call in ufcs form, the path to the function is itself an
@@ -170,11 +174,16 @@ pub struct Dereferencing {
     /// e.g. `m!(x) | Foo::Bar(ref x)`
     ref_locals: FxIndexMap<HirId, Option<RefPat>>,
 
+    /// Stack of (body owner, `PossibleBorrowerMap`) pairs. Used by
+    /// `needless_borrow_impl_arg_position` to determine when a borrowed expression can instead
+    /// be moved.
+    possible_borrowers: Vec<(LocalDefId, PossibleBorrowerMap<'tcx, 'tcx>)>,
+
     // `IntoIterator` for arrays requires Rust 1.53.
     msrv: Option<RustcVersion>,
 }
 
-impl Dereferencing {
+impl<'tcx> Dereferencing<'tcx> {
     #[must_use]
     pub fn new(msrv: Option<RustcVersion>) -> Self {
         Self {
@@ -244,7 +253,7 @@ struct RefPat {
     hir_id: HirId,
 }
 
-impl<'tcx> LateLintPass<'tcx> for Dereferencing {
+impl<'tcx> LateLintPass<'tcx> for Dereferencing<'tcx> {
     #[expect(clippy::too_many_lines)]
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
         // Skip path expressions from deref calls. e.g. `Deref::deref(e)`
@@ -278,7 +287,7 @@ impl<'tcx> LateLintPass<'tcx> for Dereferencing {
         match (self.state.take(), kind) {
             (None, kind) => {
                 let expr_ty = typeck.expr_ty(expr);
-                let (position, adjustments) = walk_parents(cx, expr, self.msrv);
+                let (position, adjustments) = walk_parents(cx, &mut self.possible_borrowers, expr, self.msrv);
                 match kind {
                     RefOp::Deref => {
                         if let Position::FieldAccess {
@@ -550,6 +559,12 @@ impl<'tcx> LateLintPass<'tcx> for Dereferencing {
     }
 
     fn check_body_post(&mut self, cx: &LateContext<'tcx>, body: &'tcx Body<'_>) {
+        if self.possible_borrowers.last().map_or(false, |&(local_def_id, _)| {
+            local_def_id == cx.tcx.hir().body_owner_def_id(body.id())
+        }) {
+            self.possible_borrowers.pop();
+        }
+
         if Some(body.id()) == self.current_body {
             for pat in self.ref_locals.drain(..).filter_map(|(_, x)| x) {
                 let replacements = pat.replacements;
@@ -682,6 +697,7 @@ impl Position {
 #[expect(clippy::too_many_lines)]
 fn walk_parents<'tcx>(
     cx: &LateContext<'tcx>,
+    possible_borrowers: &mut Vec<(LocalDefId, PossibleBorrowerMap<'tcx, 'tcx>)>,
     e: &'tcx Expr<'_>,
     msrv: Option<RustcVersion>,
 ) -> (Position, &'tcx [Adjustment<'tcx>]) {
@@ -796,7 +812,16 @@ fn walk_parents<'tcx>(
                             Some(hir_ty) => binding_ty_auto_deref_stability(cx, hir_ty, precedence, ty.bound_vars()),
                             None => {
                                 if let ty::Param(param_ty) = ty.skip_binder().kind() {
-                                    needless_borrow_impl_arg_position(cx, parent, i, *param_ty, e, precedence, msrv)
+                                    needless_borrow_impl_arg_position(
+                                        cx,
+                                        possible_borrowers,
+                                        parent,
+                                        i,
+                                        *param_ty,
+                                        e,
+                                        precedence,
+                                        msrv,
+                                    )
                                 } else {
                                     ty_auto_deref_stability(cx, cx.tcx.erase_late_bound_regions(ty), precedence)
                                         .position_for_arg()
@@ -844,7 +869,16 @@ fn walk_parents<'tcx>(
                     args.iter().position(|arg| arg.hir_id == child_id).map(|i| {
                         let ty = cx.tcx.fn_sig(id).skip_binder().inputs()[i + 1];
                         if let ty::Param(param_ty) = ty.kind() {
-                            needless_borrow_impl_arg_position(cx, parent, i + 1, *param_ty, e, precedence, msrv)
+                            needless_borrow_impl_arg_position(
+                                cx,
+                                possible_borrowers,
+                                parent,
+                                i + 1,
+                                *param_ty,
+                                e,
+                                precedence,
+                                msrv,
+                            )
                         } else {
                             ty_auto_deref_stability(
                                 cx,
@@ -1018,8 +1052,10 @@ fn ty_contains_infer(ty: &hir::Ty<'_>) -> bool {
 // If the conditions are met, returns `Some(Position::ImplArg(..))`; otherwise, returns `None`.
 //   The "is copyable" condition is to avoid the case where removing the `&` means `e` would have to
 // be moved, but it cannot be.
+#[expect(clippy::too_many_arguments)]
 fn needless_borrow_impl_arg_position<'tcx>(
     cx: &LateContext<'tcx>,
+    possible_borrowers: &mut Vec<(LocalDefId, PossibleBorrowerMap<'tcx, 'tcx>)>,
     parent: &Expr<'tcx>,
     arg_index: usize,
     param_ty: ParamTy,
@@ -1082,10 +1118,13 @@ fn needless_borrow_impl_arg_position<'tcx>(
     // elements are modified each time `check_referent` is called.
     let mut substs_with_referent_ty = substs_with_expr_ty.to_vec();
 
-    let mut check_referent = |referent| {
+    let mut check_reference_and_referent = |reference, referent| {
         let referent_ty = cx.typeck_results().expr_ty(referent);
 
-        if !is_copy(cx, referent_ty) {
+        if !is_copy(cx, referent_ty)
+            && (referent_ty.has_significant_drop(cx.tcx, cx.param_env)
+                || !referent_used_exactly_once(cx, possible_borrowers, reference))
+        {
             return false;
         }
 
@@ -1127,7 +1166,7 @@ fn needless_borrow_impl_arg_position<'tcx>(
 
     let mut needless_borrow = false;
     while let ExprKind::AddrOf(_, _, referent) = expr.kind {
-        if !check_referent(referent) {
+        if !check_reference_and_referent(expr, referent) {
             break;
         }
         expr = referent;
@@ -1155,6 +1194,36 @@ fn has_ref_mut_self_method(cx: &LateContext<'_>, trait_def_id: DefId) -> bool {
         })
 }
 
+fn referent_used_exactly_once<'a, 'tcx>(
+    cx: &'a LateContext<'tcx>,
+    possible_borrowers: &mut Vec<(LocalDefId, PossibleBorrowerMap<'tcx, 'tcx>)>,
+    reference: &Expr<'tcx>,
+) -> bool {
+    let mir = enclosing_mir(cx.tcx, reference.hir_id);
+    if let Some(local) = expr_local(cx.tcx, reference)
+        && let [location] = *local_assignments(mir, local).as_slice()
+        && let StatementKind::Assign(box (_, Rvalue::Ref(_, _, place))) =
+            mir.basic_blocks[location.block].statements[location.statement_index].kind
+        && !place.has_deref()
+    {
+        let body_owner_local_def_id = cx.tcx.hir().enclosing_body_owner(reference.hir_id);
+        if possible_borrowers
+            .last()
+            .map_or(true, |&(local_def_id, _)| local_def_id != body_owner_local_def_id)
+        {
+            possible_borrowers.push((body_owner_local_def_id, PossibleBorrowerMap::new(cx, mir)));
+        }
+        let possible_borrower = &mut possible_borrowers.last_mut().unwrap().1;
+        // If `only_borrowers` were used here, the `copyable_iterator::warn` test would fail. The reason is
+        // that `PossibleBorrowerVisitor::visit_terminator` considers `place.local` a possible borrower of
+        // itself. See the comment in that method for an explanation as to why.
+        possible_borrower.bounded_borrowers(&[local], &[local, place.local], place.local, location)
+            && used_exactly_once(mir, place.local).unwrap_or(false)
+    } else {
+        false
+    }
+}
+
 // Iteratively replaces `param_ty` with `new_ty` in `substs`, and similarly for each resulting
 // projected type that is a type parameter. Returns `false` if replacing the types would have an
 // effect on the function signature beyond substituting `new_ty` for `param_ty`.
@@ -1439,8 +1508,8 @@ fn report<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, state: State, data
     }
 }
 
-impl Dereferencing {
-    fn check_local_usage<'tcx>(&mut self, cx: &LateContext<'tcx>, e: &Expr<'tcx>, local: HirId) {
+impl<'tcx> Dereferencing<'tcx> {
+    fn check_local_usage(&mut self, cx: &LateContext<'tcx>, e: &Expr<'tcx>, local: HirId) {
         if let Some(outer_pat) = self.ref_locals.get_mut(&local) {
             if let Some(pat) = outer_pat {
                 // Check for auto-deref
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 2dcefd78763..ebb0f14fef5 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -38,7 +38,6 @@ extern crate rustc_infer;
 extern crate rustc_lexer;
 extern crate rustc_lint;
 extern crate rustc_middle;
-extern crate rustc_mir_dataflow;
 extern crate rustc_parse;
 extern crate rustc_session;
 extern crate rustc_span;
@@ -418,7 +417,7 @@ pub fn register_pre_expansion_lints(store: &mut rustc_lint::LintStore, sess: &Se
 
     let msrv = conf.msrv.as_ref().and_then(|s| {
         parse_msrv(s, None, None).or_else(|| {
-            sess.err(&format!(
+            sess.err(format!(
                 "error reading Clippy's configuration file. `{s}` is not a valid Rust version"
             ));
             None
@@ -434,7 +433,7 @@ fn read_msrv(conf: &Conf, sess: &Session) -> Option<RustcVersion> {
         .and_then(|v| parse_msrv(&v, None, None));
     let clippy_msrv = conf.msrv.as_ref().and_then(|s| {
         parse_msrv(s, None, None).or_else(|| {
-            sess.err(&format!(
+            sess.err(format!(
                 "error reading Clippy's configuration file. `{s}` is not a valid Rust version"
             ));
             None
@@ -445,7 +444,7 @@ fn read_msrv(conf: &Conf, sess: &Session) -> Option<RustcVersion> {
         if let Some(clippy_msrv) = clippy_msrv {
             // if both files have an msrv, let's compare them and emit a warning if they differ
             if clippy_msrv != cargo_msrv {
-                sess.warn(&format!(
+                sess.warn(format!(
                     "the MSRV in `clippy.toml` and `Cargo.toml` differ; using `{clippy_msrv}` from `clippy.toml`"
                 ));
             }
@@ -474,7 +473,7 @@ pub fn read_conf(sess: &Session) -> Conf {
     let TryConf { conf, errors, warnings } = utils::conf::read(&file_name);
     // all conf errors are non-fatal, we just use the default conf in case of error
     for error in errors {
-        sess.err(&format!(
+        sess.err(format!(
             "error reading Clippy's configuration file `{}`: {}",
             file_name.display(),
             format_error(error)
diff --git a/clippy_lints/src/nonstandard_macro_braces.rs b/clippy_lints/src/nonstandard_macro_braces.rs
index 0ca0befc135..6c909e5ed73 100644
--- a/clippy_lints/src/nonstandard_macro_braces.rs
+++ b/clippy_lints/src/nonstandard_macro_braces.rs
@@ -266,7 +266,7 @@ impl<'de> Deserialize<'de> for MacroMatcher {
                         .iter()
                         .find(|b| b.0 == brace)
                         .map(|(o, c)| ((*o).to_owned(), (*c).to_owned()))
-                        .ok_or_else(|| de::Error::custom(&format!("expected one of `(`, `{{`, `[` found `{brace}`")))?,
+                        .ok_or_else(|| de::Error::custom(format!("expected one of `(`, `{{`, `[` found `{brace}`")))?,
                 })
             }
         }
diff --git a/clippy_lints/src/redundant_clone.rs b/clippy_lints/src/redundant_clone.rs
index 9fd86331ec7..aedbe08e3e4 100644
--- a/clippy_lints/src/redundant_clone.rs
+++ b/clippy_lints/src/redundant_clone.rs
@@ -1,25 +1,18 @@
 use clippy_utils::diagnostics::{span_lint_hir, span_lint_hir_and_then};
+use clippy_utils::mir::{visit_local_usage, LocalUsage, PossibleBorrowerMap};
 use clippy_utils::source::snippet_opt;
 use clippy_utils::ty::{has_drop, is_copy, is_type_diagnostic_item, walk_ptrs_ty_depth};
 use clippy_utils::{fn_has_unsatisfiable_preds, match_def_path, paths};
 use if_chain::if_chain;
-use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::Applicability;
 use rustc_hir::intravisit::FnKind;
 use rustc_hir::{def_id, Body, FnDecl, HirId};
-use rustc_index::bit_set::{BitSet, HybridBitSet};
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_middle::mir::{
-    self, traversal,
-    visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor as _},
-    Mutability,
-};
-use rustc_middle::ty::{self, visit::TypeVisitor, Ty};
-use rustc_mir_dataflow::{Analysis, AnalysisDomain, CallReturnPlaces, GenKill, GenKillAnalysis, ResultsCursor};
+use rustc_middle::mir;
+use rustc_middle::ty::{self, Ty};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::source_map::{BytePos, Span};
 use rustc_span::sym;
-use std::ops::ControlFlow;
 
 macro_rules! unwrap_or_continue {
     ($x:expr) => {
@@ -89,21 +82,7 @@ impl<'tcx> LateLintPass<'tcx> for RedundantClone {
 
         let mir = cx.tcx.optimized_mir(def_id.to_def_id());
 
-        let possible_origin = {
-            let mut vis = PossibleOriginVisitor::new(mir);
-            vis.visit_body(mir);
-            vis.into_map(cx)
-        };
-        let maybe_storage_live_result = MaybeStorageLive
-            .into_engine(cx.tcx, mir)
-            .pass_name("redundant_clone")
-            .iterate_to_fixpoint()
-            .into_results_cursor(mir);
-        let mut possible_borrower = {
-            let mut vis = PossibleBorrowerVisitor::new(cx, mir, possible_origin);
-            vis.visit_body(mir);
-            vis.into_map(cx, maybe_storage_live_result)
-        };
+        let mut possible_borrower = PossibleBorrowerMap::new(cx, mir);
 
         for (bb, bbdata) in mir.basic_blocks.iter_enumerated() {
             let terminator = bbdata.terminator();
@@ -374,403 +353,40 @@ struct CloneUsage {
     /// Whether the clone value is mutated.
     clone_consumed_or_mutated: bool,
 }
-fn visit_clone_usage(cloned: mir::Local, clone: mir::Local, mir: &mir::Body<'_>, bb: mir::BasicBlock) -> CloneUsage {
-    struct V {
-        cloned: mir::Local,
-        clone: mir::Local,
-        result: CloneUsage,
-    }
-    impl<'tcx> mir::visit::Visitor<'tcx> for V {
-        fn visit_basic_block_data(&mut self, block: mir::BasicBlock, data: &mir::BasicBlockData<'tcx>) {
-            let statements = &data.statements;
-            for (statement_index, statement) in statements.iter().enumerate() {
-                self.visit_statement(statement, mir::Location { block, statement_index });
-            }
-
-            self.visit_terminator(
-                data.terminator(),
-                mir::Location {
-                    block,
-                    statement_index: statements.len(),
-                },
-            );
-        }
-
-        fn visit_place(&mut self, place: &mir::Place<'tcx>, ctx: PlaceContext, loc: mir::Location) {
-            let local = place.local;
-
-            if local == self.cloned
-                && !matches!(
-                    ctx,
-                    PlaceContext::MutatingUse(MutatingUseContext::Drop) | PlaceContext::NonUse(_)
-                )
-            {
-                self.result.cloned_used = true;
-                self.result.cloned_consume_or_mutate_loc = self.result.cloned_consume_or_mutate_loc.or_else(|| {
-                    matches!(
-                        ctx,
-                        PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)
-                            | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
-                    )
-                    .then(|| loc)
-                });
-            } else if local == self.clone {
-                match ctx {
-                    PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)
-                    | PlaceContext::MutatingUse(MutatingUseContext::Borrow) => {
-                        self.result.clone_consumed_or_mutated = true;
-                    },
-                    _ => {},
-                }
-            }
-        }
-    }
-
-    let init = CloneUsage {
-        cloned_used: false,
-        cloned_consume_or_mutate_loc: None,
-        // Consider non-temporary clones consumed.
-        // TODO: Actually check for mutation of non-temporaries.
-        clone_consumed_or_mutated: mir.local_kind(clone) != mir::LocalKind::Temp,
-    };
-    traversal::ReversePostorder::new(mir, bb)
-        .skip(1)
-        .fold(init, |usage, (tbb, tdata)| {
-            // Short-circuit
-            if (usage.cloned_used && usage.clone_consumed_or_mutated) ||
-                // Give up on loops
-                tdata.terminator().successors().any(|s| s == bb)
-            {
-                return CloneUsage {
-                    cloned_used: true,
-                    clone_consumed_or_mutated: true,
-                    ..usage
-                };
-            }
-
-            let mut v = V {
-                cloned,
-                clone,
-                result: usage,
-            };
-            v.visit_basic_block_data(tbb, tdata);
-            v.result
-        })
-}
-
-/// Determines liveness of each local purely based on `StorageLive`/`Dead`.
-#[derive(Copy, Clone)]
-struct MaybeStorageLive;
-
-impl<'tcx> AnalysisDomain<'tcx> for MaybeStorageLive {
-    type Domain = BitSet<mir::Local>;
-    const NAME: &'static str = "maybe_storage_live";
-
-    fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
-        // bottom = dead
-        BitSet::new_empty(body.local_decls.len())
-    }
-
-    fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut Self::Domain) {
-        for arg in body.args_iter() {
-            state.insert(arg);
-        }
-    }
-}
-
-impl<'tcx> GenKillAnalysis<'tcx> for MaybeStorageLive {
-    type Idx = mir::Local;
-
-    fn statement_effect(&self, trans: &mut impl GenKill<Self::Idx>, stmt: &mir::Statement<'tcx>, _: mir::Location) {
-        match stmt.kind {
-            mir::StatementKind::StorageLive(l) => trans.gen(l),
-            mir::StatementKind::StorageDead(l) => trans.kill(l),
-            _ => (),
-        }
-    }
-
-    fn terminator_effect(
-        &self,
-        _trans: &mut impl GenKill<Self::Idx>,
-        _terminator: &mir::Terminator<'tcx>,
-        _loc: mir::Location,
-    ) {
-    }
-
-    fn call_return_effect(
-        &self,
-        _trans: &mut impl GenKill<Self::Idx>,
-        _block: mir::BasicBlock,
-        _return_places: CallReturnPlaces<'_, 'tcx>,
-    ) {
-        // Nothing to do when a call returns successfully
-    }
-}
-
-/// Collects the possible borrowers of each local.
-/// For example, `b = &a; c = &a;` will make `b` and (transitively) `c`
-/// possible borrowers of `a`.
-struct PossibleBorrowerVisitor<'a, 'tcx> {
-    possible_borrower: TransitiveRelation,
-    body: &'a mir::Body<'tcx>,
-    cx: &'a LateContext<'tcx>,
-    possible_origin: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
-}
-
-impl<'a, 'tcx> PossibleBorrowerVisitor<'a, 'tcx> {
-    fn new(
-        cx: &'a LateContext<'tcx>,
-        body: &'a mir::Body<'tcx>,
-        possible_origin: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
-    ) -> Self {
-        Self {
-            possible_borrower: TransitiveRelation::default(),
-            cx,
-            body,
-            possible_origin,
-        }
-    }
-
-    fn into_map(
-        self,
-        cx: &LateContext<'tcx>,
-        maybe_live: ResultsCursor<'tcx, 'tcx, MaybeStorageLive>,
-    ) -> PossibleBorrowerMap<'a, 'tcx> {
-        let mut map = FxHashMap::default();
-        for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
-            if is_copy(cx, self.body.local_decls[row].ty) {
-                continue;
-            }
-
-            let mut borrowers = self.possible_borrower.reachable_from(row, self.body.local_decls.len());
-            borrowers.remove(mir::Local::from_usize(0));
-            if !borrowers.is_empty() {
-                map.insert(row, borrowers);
-            }
-        }
-
-        let bs = BitSet::new_empty(self.body.local_decls.len());
-        PossibleBorrowerMap {
-            map,
-            maybe_live,
-            bitset: (bs.clone(), bs),
-        }
-    }
-}
-
-impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleBorrowerVisitor<'a, 'tcx> {
-    fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
-        let lhs = place.local;
-        match rvalue {
-            mir::Rvalue::Ref(_, _, borrowed) => {
-                self.possible_borrower.add(borrowed.local, lhs);
-            },
-            other => {
-                if ContainsRegion
-                    .visit_ty(place.ty(&self.body.local_decls, self.cx.tcx).ty)
-                    .is_continue()
-                {
-                    return;
-                }
-                rvalue_locals(other, |rhs| {
-                    if lhs != rhs {
-                        self.possible_borrower.add(rhs, lhs);
-                    }
-                });
-            },
-        }
-    }
-
-    fn visit_terminator(&mut self, terminator: &mir::Terminator<'_>, _loc: mir::Location) {
-        if let mir::TerminatorKind::Call {
-            args,
-            destination: mir::Place { local: dest, .. },
-            ..
-        } = &terminator.kind
-        {
-            // TODO add doc
-            // If the call returns something with lifetimes,
-            // let's conservatively assume the returned value contains lifetime of all the arguments.
-            // For example, given `let y: Foo<'a> = foo(x)`, `y` is considered to be a possible borrower of `x`.
-
-            let mut immutable_borrowers = vec![];
-            let mut mutable_borrowers = vec![];
-
-            for op in args {
-                match op {
-                    mir::Operand::Copy(p) | mir::Operand::Move(p) => {
-                        if let ty::Ref(_, _, Mutability::Mut) = self.body.local_decls[p.local].ty.kind() {
-                            mutable_borrowers.push(p.local);
-                        } else {
-                            immutable_borrowers.push(p.local);
-                        }
-                    },
-                    mir::Operand::Constant(..) => (),
-                }
-            }
-
-            let mut mutable_variables: Vec<mir::Local> = mutable_borrowers
-                .iter()
-                .filter_map(|r| self.possible_origin.get(r))
-                .flat_map(HybridBitSet::iter)
-                .collect();
-
-            if ContainsRegion.visit_ty(self.body.local_decls[*dest].ty).is_break() {
-                mutable_variables.push(*dest);
-            }
-
-            for y in mutable_variables {
-                for x in &immutable_borrowers {
-                    self.possible_borrower.add(*x, y);
-                }
-                for x in &mutable_borrowers {
-                    self.possible_borrower.add(*x, y);
-                }
-            }
-        }
-    }
-}
-
-/// Collect possible borrowed for every `&mut` local.
-/// For example, `_1 = &mut _2` generate _1: {_2,...}
-/// Known Problems: not sure all borrowed are tracked
-struct PossibleOriginVisitor<'a, 'tcx> {
-    possible_origin: TransitiveRelation,
-    body: &'a mir::Body<'tcx>,
-}
-
-impl<'a, 'tcx> PossibleOriginVisitor<'a, 'tcx> {
-    fn new(body: &'a mir::Body<'tcx>) -> Self {
-        Self {
-            possible_origin: TransitiveRelation::default(),
-            body,
-        }
-    }
-
-    fn into_map(self, cx: &LateContext<'tcx>) -> FxHashMap<mir::Local, HybridBitSet<mir::Local>> {
-        let mut map = FxHashMap::default();
-        for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
-            if is_copy(cx, self.body.local_decls[row].ty) {
-                continue;
-            }
-
-            let mut borrowers = self.possible_origin.reachable_from(row, self.body.local_decls.len());
-            borrowers.remove(mir::Local::from_usize(0));
-            if !borrowers.is_empty() {
-                map.insert(row, borrowers);
-            }
-        }
-        map
-    }
-}
-
-impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleOriginVisitor<'a, 'tcx> {
-    fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
-        let lhs = place.local;
-        match rvalue {
-            // Only consider `&mut`, which can modify origin place
-            mir::Rvalue::Ref(_, rustc_middle::mir::BorrowKind::Mut { .. }, borrowed) |
-            // _2: &mut _;
-            // _3 = move _2
-            mir::Rvalue::Use(mir::Operand::Move(borrowed))  |
-            // _3 = move _2 as &mut _;
-            mir::Rvalue::Cast(_, mir::Operand::Move(borrowed), _)
-                => {
-                self.possible_origin.add(lhs, borrowed.local);
-            },
-            _ => {},
-        }
-    }
-}
-
-struct ContainsRegion;
-
-impl TypeVisitor<'_> for ContainsRegion {
-    type BreakTy = ();
-
-    fn visit_region(&mut self, _: ty::Region<'_>) -> ControlFlow<Self::BreakTy> {
-        ControlFlow::BREAK
-    }
-}
-
-fn rvalue_locals(rvalue: &mir::Rvalue<'_>, mut visit: impl FnMut(mir::Local)) {
-    use rustc_middle::mir::Rvalue::{Aggregate, BinaryOp, Cast, CheckedBinaryOp, Repeat, UnaryOp, Use};
-
-    let mut visit_op = |op: &mir::Operand<'_>| match op {
-        mir::Operand::Copy(p) | mir::Operand::Move(p) => visit(p.local),
-        mir::Operand::Constant(..) => (),
-    };
 
-    match rvalue {
-        Use(op) | Repeat(op, _) | Cast(_, op, _) | UnaryOp(_, op) => visit_op(op),
-        Aggregate(_, ops) => ops.iter().for_each(visit_op),
-        BinaryOp(_, box (lhs, rhs)) | CheckedBinaryOp(_, box (lhs, rhs)) => {
-            visit_op(lhs);
-            visit_op(rhs);
+fn visit_clone_usage(cloned: mir::Local, clone: mir::Local, mir: &mir::Body<'_>, bb: mir::BasicBlock) -> CloneUsage {
+    if let Some((
+        LocalUsage {
+            local_use_locs: cloned_use_locs,
+            local_consume_or_mutate_locs: cloned_consume_or_mutate_locs,
         },
-        _ => (),
-    }
-}
-
-/// Result of `PossibleBorrowerVisitor`.
-struct PossibleBorrowerMap<'a, 'tcx> {
-    /// Mapping `Local -> its possible borrowers`
-    map: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
-    maybe_live: ResultsCursor<'a, 'tcx, MaybeStorageLive>,
-    // Caches to avoid allocation of `BitSet` on every query
-    bitset: (BitSet<mir::Local>, BitSet<mir::Local>),
-}
-
-impl PossibleBorrowerMap<'_, '_> {
-    /// Returns true if the set of borrowers of `borrowed` living at `at` matches with `borrowers`.
-    fn only_borrowers(&mut self, borrowers: &[mir::Local], borrowed: mir::Local, at: mir::Location) -> bool {
-        self.maybe_live.seek_after_primary_effect(at);
-
-        self.bitset.0.clear();
-        let maybe_live = &mut self.maybe_live;
-        if let Some(bitset) = self.map.get(&borrowed) {
-            for b in bitset.iter().filter(move |b| maybe_live.contains(*b)) {
-                self.bitset.0.insert(b);
-            }
-        } else {
-            return false;
-        }
-
-        self.bitset.1.clear();
-        for b in borrowers {
-            self.bitset.1.insert(*b);
+        LocalUsage {
+            local_use_locs: _,
+            local_consume_or_mutate_locs: clone_consume_or_mutate_locs,
+        },
+    )) = visit_local_usage(
+        &[cloned, clone],
+        mir,
+        mir::Location {
+            block: bb,
+            statement_index: mir.basic_blocks[bb].statements.len(),
+        },
+    )
+    .map(|mut vec| (vec.remove(0), vec.remove(0)))
+    {
+        CloneUsage {
+            cloned_used: !cloned_use_locs.is_empty(),
+            cloned_consume_or_mutate_loc: cloned_consume_or_mutate_locs.first().copied(),
+            // Consider non-temporary clones consumed.
+            // TODO: Actually check for mutation of non-temporaries.
+            clone_consumed_or_mutated: mir.local_kind(clone) != mir::LocalKind::Temp
+                || !clone_consume_or_mutate_locs.is_empty(),
         }
-
-        self.bitset.0 == self.bitset.1
-    }
-
-    fn local_is_alive_at(&mut self, local: mir::Local, at: mir::Location) -> bool {
-        self.maybe_live.seek_after_primary_effect(at);
-        self.maybe_live.contains(local)
-    }
-}
-
-#[derive(Default)]
-struct TransitiveRelation {
-    relations: FxHashMap<mir::Local, Vec<mir::Local>>,
-}
-impl TransitiveRelation {
-    fn add(&mut self, a: mir::Local, b: mir::Local) {
-        self.relations.entry(a).or_default().push(b);
-    }
-
-    fn reachable_from(&self, a: mir::Local, domain_size: usize) -> HybridBitSet<mir::Local> {
-        let mut seen = HybridBitSet::new_empty(domain_size);
-        let mut stack = vec![a];
-        while let Some(u) = stack.pop() {
-            if let Some(edges) = self.relations.get(&u) {
-                for &v in edges {
-                    if seen.insert(v) {
-                        stack.push(v);
-                    }
-                }
-            }
+    } else {
+        CloneUsage {
+            cloned_used: true,
+            cloned_consume_or_mutate_loc: None,
+            clone_consumed_or_mutated: true,
         }
-        seen
     }
 }
diff --git a/clippy_utils/src/attrs.rs b/clippy_utils/src/attrs.rs
index d9b22664fd2..cd8575c90e8 100644
--- a/clippy_utils/src/attrs.rs
+++ b/clippy_utils/src/attrs.rs
@@ -136,7 +136,7 @@ pub fn get_unique_inner_attr(sess: &Session, attrs: &[ast::Attribute], name: &'s
                     .emit();
             },
             ast::AttrStyle::Outer => {
-                sess.span_err(attr.span, &format!("`{name}` cannot be an outer attribute"));
+                sess.span_err(attr.span, format!("`{name}` cannot be an outer attribute"));
             },
         }
     }
diff --git a/clippy_utils/src/lib.rs b/clippy_utils/src/lib.rs
index e6492d76260..dbe75b43cb2 100644
--- a/clippy_utils/src/lib.rs
+++ b/clippy_utils/src/lib.rs
@@ -25,10 +25,12 @@ extern crate rustc_data_structures;
 extern crate rustc_errors;
 extern crate rustc_hir;
 extern crate rustc_hir_analysis;
+extern crate rustc_index;
 extern crate rustc_infer;
 extern crate rustc_lexer;
 extern crate rustc_lint;
 extern crate rustc_middle;
+extern crate rustc_mir_dataflow;
 extern crate rustc_parse_format;
 extern crate rustc_session;
 extern crate rustc_span;
@@ -48,6 +50,7 @@ pub mod eager_or_lazy;
 pub mod higher;
 mod hir_utils;
 pub mod macros;
+pub mod mir;
 pub mod msrvs;
 pub mod numeric_literal;
 pub mod paths;
@@ -122,7 +125,7 @@ pub fn parse_msrv(msrv: &str, sess: Option<&Session>, span: Option<Span>) -> Opt
         return Some(version);
     } else if let Some(sess) = sess {
         if let Some(span) = span {
-            sess.span_err(span, &format!("`{msrv}` is not a valid Rust version"));
+            sess.span_err(span, format!("`{msrv}` is not a valid Rust version"));
         }
     }
     None
diff --git a/clippy_utils/src/mir/maybe_storage_live.rs b/clippy_utils/src/mir/maybe_storage_live.rs
new file mode 100644
index 00000000000..d262b335d99
--- /dev/null
+++ b/clippy_utils/src/mir/maybe_storage_live.rs
@@ -0,0 +1,52 @@
+use rustc_index::bit_set::BitSet;
+use rustc_middle::mir;
+use rustc_mir_dataflow::{AnalysisDomain, CallReturnPlaces, GenKill, GenKillAnalysis};
+
+/// Determines liveness of each local purely based on `StorageLive`/`Dead`.
+#[derive(Copy, Clone)]
+pub(super) struct MaybeStorageLive;
+
+impl<'tcx> AnalysisDomain<'tcx> for MaybeStorageLive {
+    type Domain = BitSet<mir::Local>;
+    const NAME: &'static str = "maybe_storage_live";
+
+    fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
+        // bottom = dead
+        BitSet::new_empty(body.local_decls.len())
+    }
+
+    fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut Self::Domain) {
+        for arg in body.args_iter() {
+            state.insert(arg);
+        }
+    }
+}
+
+impl<'tcx> GenKillAnalysis<'tcx> for MaybeStorageLive {
+    type Idx = mir::Local;
+
+    fn statement_effect(&self, trans: &mut impl GenKill<Self::Idx>, stmt: &mir::Statement<'tcx>, _: mir::Location) {
+        match stmt.kind {
+            mir::StatementKind::StorageLive(l) => trans.gen(l),
+            mir::StatementKind::StorageDead(l) => trans.kill(l),
+            _ => (),
+        }
+    }
+
+    fn terminator_effect(
+        &self,
+        _trans: &mut impl GenKill<Self::Idx>,
+        _terminator: &mir::Terminator<'tcx>,
+        _loc: mir::Location,
+    ) {
+    }
+
+    fn call_return_effect(
+        &self,
+        _trans: &mut impl GenKill<Self::Idx>,
+        _block: mir::BasicBlock,
+        _return_places: CallReturnPlaces<'_, 'tcx>,
+    ) {
+        // Nothing to do when a call returns successfully
+    }
+}
diff --git a/clippy_utils/src/mir/mod.rs b/clippy_utils/src/mir/mod.rs
new file mode 100644
index 00000000000..c8aa6f3e595
--- /dev/null
+++ b/clippy_utils/src/mir/mod.rs
@@ -0,0 +1,165 @@
+use rustc_hir::{Expr, HirId};
+use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
+use rustc_middle::mir::{
+    traversal, Body, InlineAsmOperand, Local, Location, Place, StatementKind, TerminatorKind, START_BLOCK,
+};
+use rustc_middle::ty::TyCtxt;
+
+mod maybe_storage_live;
+
+mod possible_borrower;
+pub use possible_borrower::PossibleBorrowerMap;
+
+mod possible_origin;
+
+mod transitive_relation;
+
+#[derive(Clone, Debug, Default)]
+pub struct LocalUsage {
+    /// The locations where the local is used, if any.
+    pub local_use_locs: Vec<Location>,
+    /// The locations where the local is consumed or mutated, if any.
+    pub local_consume_or_mutate_locs: Vec<Location>,
+}
+
+pub fn visit_local_usage(locals: &[Local], mir: &Body<'_>, location: Location) -> Option<Vec<LocalUsage>> {
+    let init = vec![
+        LocalUsage {
+            local_use_locs: Vec::new(),
+            local_consume_or_mutate_locs: Vec::new(),
+        };
+        locals.len()
+    ];
+
+    traversal::ReversePostorder::new(mir, location.block).try_fold(init, |usage, (tbb, tdata)| {
+        // Give up on loops
+        if tdata.terminator().successors().any(|s| s == location.block) {
+            return None;
+        }
+
+        let mut v = V {
+            locals,
+            location,
+            results: usage,
+        };
+        v.visit_basic_block_data(tbb, tdata);
+        Some(v.results)
+    })
+}
+
+struct V<'a> {
+    locals: &'a [Local],
+    location: Location,
+    results: Vec<LocalUsage>,
+}
+
+impl<'a, 'tcx> Visitor<'tcx> for V<'a> {
+    fn visit_place(&mut self, place: &Place<'tcx>, ctx: PlaceContext, loc: Location) {
+        if loc.block == self.location.block && loc.statement_index <= self.location.statement_index {
+            return;
+        }
+
+        let local = place.local;
+
+        for (i, self_local) in self.locals.iter().enumerate() {
+            if local == *self_local {
+                if !matches!(
+                    ctx,
+                    PlaceContext::MutatingUse(MutatingUseContext::Drop) | PlaceContext::NonUse(_)
+                ) {
+                    self.results[i].local_use_locs.push(loc);
+                }
+                if matches!(
+                    ctx,
+                    PlaceContext::NonMutatingUse(NonMutatingUseContext::Move)
+                        | PlaceContext::MutatingUse(MutatingUseContext::Borrow)
+                ) {
+                    self.results[i].local_consume_or_mutate_locs.push(loc);
+                }
+            }
+        }
+    }
+}
+
+/// Convenience wrapper around `visit_local_usage`.
+pub fn used_exactly_once(mir: &rustc_middle::mir::Body<'_>, local: rustc_middle::mir::Local) -> Option<bool> {
+    visit_local_usage(
+        &[local],
+        mir,
+        Location {
+            block: START_BLOCK,
+            statement_index: 0,
+        },
+    )
+    .map(|mut vec| {
+        let LocalUsage { local_use_locs, .. } = vec.remove(0);
+        local_use_locs
+            .into_iter()
+            .filter(|location| !is_local_assignment(mir, local, *location))
+            .count()
+            == 1
+    })
+}
+
+/// Returns the `mir::Body` containing the node associated with `hir_id`.
+#[allow(clippy::module_name_repetitions)]
+pub fn enclosing_mir(tcx: TyCtxt<'_>, hir_id: HirId) -> &Body<'_> {
+    let body_owner_local_def_id = tcx.hir().enclosing_body_owner(hir_id);
+    tcx.optimized_mir(body_owner_local_def_id.to_def_id())
+}
+
+/// Tries to determine the `Local` corresponding to `expr`, if any.
+/// This function is expensive and should be used sparingly.
+pub fn expr_local(tcx: TyCtxt<'_>, expr: &Expr<'_>) -> Option<Local> {
+    let mir = enclosing_mir(tcx, expr.hir_id);
+    mir.local_decls.iter_enumerated().find_map(|(local, local_decl)| {
+        if local_decl.source_info.span == expr.span {
+            Some(local)
+        } else {
+            None
+        }
+    })
+}
+
+/// Returns a vector of `mir::Location` where `local` is assigned. Each statement referred to has
+/// kind `StatementKind::Assign`.
+pub fn local_assignments(mir: &Body<'_>, local: Local) -> Vec<Location> {
+    let mut locations = Vec::new();
+    for (block, data) in mir.basic_blocks.iter_enumerated() {
+        for statement_index in 0..=data.statements.len() {
+            let location = Location { block, statement_index };
+            if is_local_assignment(mir, local, location) {
+                locations.push(location);
+            }
+        }
+    }
+    locations
+}
+
+// `is_local_assignment` is based on `is_place_assignment`:
+// https://github.com/rust-lang/rust/blob/b7413511dc85ec01ef4b91785f86614589ac6103/compiler/rustc_middle/src/mir/visit.rs#L1350
+fn is_local_assignment(mir: &Body<'_>, local: Local, location: Location) -> bool {
+    let Location { block, statement_index } = location;
+    let basic_block = &mir.basic_blocks[block];
+    if statement_index < basic_block.statements.len() {
+        let statement = &basic_block.statements[statement_index];
+        if let StatementKind::Assign(box (place, _)) = statement.kind {
+            place.as_local() == Some(local)
+        } else {
+            false
+        }
+    } else {
+        let terminator = basic_block.terminator();
+        match &terminator.kind {
+            TerminatorKind::Call { destination, .. } => destination.as_local() == Some(local),
+            TerminatorKind::InlineAsm { operands, .. } => operands.iter().any(|operand| {
+                if let InlineAsmOperand::Out { place: Some(place), .. } = operand {
+                    place.as_local() == Some(local)
+                } else {
+                    false
+                }
+            }),
+            _ => false,
+        }
+    }
+}
diff --git a/clippy_utils/src/mir/possible_borrower.rs b/clippy_utils/src/mir/possible_borrower.rs
new file mode 100644
index 00000000000..25717bf3d2f
--- /dev/null
+++ b/clippy_utils/src/mir/possible_borrower.rs
@@ -0,0 +1,241 @@
+use super::{
+    maybe_storage_live::MaybeStorageLive, possible_origin::PossibleOriginVisitor,
+    transitive_relation::TransitiveRelation,
+};
+use crate::ty::is_copy;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_index::bit_set::{BitSet, HybridBitSet};
+use rustc_lint::LateContext;
+use rustc_middle::mir::{self, visit::Visitor as _, Mutability};
+use rustc_middle::ty::{self, visit::TypeVisitor};
+use rustc_mir_dataflow::{Analysis, ResultsCursor};
+use std::ops::ControlFlow;
+
+/// Collects the possible borrowers of each local.
+/// For example, `b = &a; c = &a;` will make `b` and (transitively) `c`
+/// possible borrowers of `a`.
+#[allow(clippy::module_name_repetitions)]
+struct PossibleBorrowerVisitor<'a, 'b, 'tcx> {
+    possible_borrower: TransitiveRelation,
+    body: &'b mir::Body<'tcx>,
+    cx: &'a LateContext<'tcx>,
+    possible_origin: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
+}
+
+impl<'a, 'b, 'tcx> PossibleBorrowerVisitor<'a, 'b, 'tcx> {
+    fn new(
+        cx: &'a LateContext<'tcx>,
+        body: &'b mir::Body<'tcx>,
+        possible_origin: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
+    ) -> Self {
+        Self {
+            possible_borrower: TransitiveRelation::default(),
+            cx,
+            body,
+            possible_origin,
+        }
+    }
+
+    fn into_map(
+        self,
+        cx: &'a LateContext<'tcx>,
+        maybe_live: ResultsCursor<'b, 'tcx, MaybeStorageLive>,
+    ) -> PossibleBorrowerMap<'b, 'tcx> {
+        let mut map = FxHashMap::default();
+        for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
+            if is_copy(cx, self.body.local_decls[row].ty) {
+                continue;
+            }
+
+            let mut borrowers = self.possible_borrower.reachable_from(row, self.body.local_decls.len());
+            borrowers.remove(mir::Local::from_usize(0));
+            if !borrowers.is_empty() {
+                map.insert(row, borrowers);
+            }
+        }
+
+        let bs = BitSet::new_empty(self.body.local_decls.len());
+        PossibleBorrowerMap {
+            map,
+            maybe_live,
+            bitset: (bs.clone(), bs),
+        }
+    }
+}
+
+impl<'a, 'b, 'tcx> mir::visit::Visitor<'tcx> for PossibleBorrowerVisitor<'a, 'b, 'tcx> {
+    fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
+        let lhs = place.local;
+        match rvalue {
+            mir::Rvalue::Ref(_, _, borrowed) => {
+                self.possible_borrower.add(borrowed.local, lhs);
+            },
+            other => {
+                if ContainsRegion
+                    .visit_ty(place.ty(&self.body.local_decls, self.cx.tcx).ty)
+                    .is_continue()
+                {
+                    return;
+                }
+                rvalue_locals(other, |rhs| {
+                    if lhs != rhs {
+                        self.possible_borrower.add(rhs, lhs);
+                    }
+                });
+            },
+        }
+    }
+
+    fn visit_terminator(&mut self, terminator: &mir::Terminator<'_>, _loc: mir::Location) {
+        if let mir::TerminatorKind::Call {
+            args,
+            destination: mir::Place { local: dest, .. },
+            ..
+        } = &terminator.kind
+        {
+            // TODO add doc
+            // If the call returns something with lifetimes,
+            // let's conservatively assume the returned value contains lifetime of all the arguments.
+            // For example, given `let y: Foo<'a> = foo(x)`, `y` is considered to be a possible borrower of `x`.
+
+            let mut immutable_borrowers = vec![];
+            let mut mutable_borrowers = vec![];
+
+            for op in args {
+                match op {
+                    mir::Operand::Copy(p) | mir::Operand::Move(p) => {
+                        if let ty::Ref(_, _, Mutability::Mut) = self.body.local_decls[p.local].ty.kind() {
+                            mutable_borrowers.push(p.local);
+                        } else {
+                            immutable_borrowers.push(p.local);
+                        }
+                    },
+                    mir::Operand::Constant(..) => (),
+                }
+            }
+
+            let mut mutable_variables: Vec<mir::Local> = mutable_borrowers
+                .iter()
+                .filter_map(|r| self.possible_origin.get(r))
+                .flat_map(HybridBitSet::iter)
+                .collect();
+
+            if ContainsRegion.visit_ty(self.body.local_decls[*dest].ty).is_break() {
+                mutable_variables.push(*dest);
+            }
+
+            for y in mutable_variables {
+                for x in &immutable_borrowers {
+                    self.possible_borrower.add(*x, y);
+                }
+                for x in &mutable_borrowers {
+                    self.possible_borrower.add(*x, y);
+                }
+            }
+        }
+    }
+}
+
+struct ContainsRegion;
+
+impl TypeVisitor<'_> for ContainsRegion {
+    type BreakTy = ();
+
+    fn visit_region(&mut self, _: ty::Region<'_>) -> ControlFlow<Self::BreakTy> {
+        ControlFlow::BREAK
+    }
+}
+
+fn rvalue_locals(rvalue: &mir::Rvalue<'_>, mut visit: impl FnMut(mir::Local)) {
+    use rustc_middle::mir::Rvalue::{Aggregate, BinaryOp, Cast, CheckedBinaryOp, Repeat, UnaryOp, Use};
+
+    let mut visit_op = |op: &mir::Operand<'_>| match op {
+        mir::Operand::Copy(p) | mir::Operand::Move(p) => visit(p.local),
+        mir::Operand::Constant(..) => (),
+    };
+
+    match rvalue {
+        Use(op) | Repeat(op, _) | Cast(_, op, _) | UnaryOp(_, op) => visit_op(op),
+        Aggregate(_, ops) => ops.iter().for_each(visit_op),
+        BinaryOp(_, box (lhs, rhs)) | CheckedBinaryOp(_, box (lhs, rhs)) => {
+            visit_op(lhs);
+            visit_op(rhs);
+        },
+        _ => (),
+    }
+}
+
+/// Result of `PossibleBorrowerVisitor`.
+#[allow(clippy::module_name_repetitions)]
+pub struct PossibleBorrowerMap<'b, 'tcx> {
+    /// Mapping `Local -> its possible borrowers`
+    pub map: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
+    maybe_live: ResultsCursor<'b, 'tcx, MaybeStorageLive>,
+    // Caches to avoid allocation of `BitSet` on every query
+    pub bitset: (BitSet<mir::Local>, BitSet<mir::Local>),
+}
+
+impl<'a, 'b, 'tcx> PossibleBorrowerMap<'b, 'tcx> {
+    pub fn new(cx: &'a LateContext<'tcx>, mir: &'b mir::Body<'tcx>) -> Self {
+        let possible_origin = {
+            let mut vis = PossibleOriginVisitor::new(mir);
+            vis.visit_body(mir);
+            vis.into_map(cx)
+        };
+        let maybe_storage_live_result = MaybeStorageLive
+            .into_engine(cx.tcx, mir)
+            .pass_name("redundant_clone")
+            .iterate_to_fixpoint()
+            .into_results_cursor(mir);
+        let mut vis = PossibleBorrowerVisitor::new(cx, mir, possible_origin);
+        vis.visit_body(mir);
+        vis.into_map(cx, maybe_storage_live_result)
+    }
+
+    /// Returns true if the set of borrowers of `borrowed` living at `at` matches with `borrowers`.
+    pub fn only_borrowers(&mut self, borrowers: &[mir::Local], borrowed: mir::Local, at: mir::Location) -> bool {
+        self.bounded_borrowers(borrowers, borrowers, borrowed, at)
+    }
+
+    /// Returns true if the set of borrowers of `borrowed` living at `at` includes at least `below`
+    /// but no more than `above`.
+    pub fn bounded_borrowers(
+        &mut self,
+        below: &[mir::Local],
+        above: &[mir::Local],
+        borrowed: mir::Local,
+        at: mir::Location,
+    ) -> bool {
+        self.maybe_live.seek_after_primary_effect(at);
+
+        self.bitset.0.clear();
+        let maybe_live = &mut self.maybe_live;
+        if let Some(bitset) = self.map.get(&borrowed) {
+            for b in bitset.iter().filter(move |b| maybe_live.contains(*b)) {
+                self.bitset.0.insert(b);
+            }
+        } else {
+            return false;
+        }
+
+        self.bitset.1.clear();
+        for b in below {
+            self.bitset.1.insert(*b);
+        }
+
+        if !self.bitset.0.superset(&self.bitset.1) {
+            return false;
+        }
+
+        for b in above {
+            self.bitset.0.remove(*b);
+        }
+
+        self.bitset.0.is_empty()
+    }
+
+    pub fn local_is_alive_at(&mut self, local: mir::Local, at: mir::Location) -> bool {
+        self.maybe_live.seek_after_primary_effect(at);
+        self.maybe_live.contains(local)
+    }
+}
diff --git a/clippy_utils/src/mir/possible_origin.rs b/clippy_utils/src/mir/possible_origin.rs
new file mode 100644
index 00000000000..8e7513d740a
--- /dev/null
+++ b/clippy_utils/src/mir/possible_origin.rs
@@ -0,0 +1,59 @@
+use super::transitive_relation::TransitiveRelation;
+use crate::ty::is_copy;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_index::bit_set::HybridBitSet;
+use rustc_lint::LateContext;
+use rustc_middle::mir;
+
+/// Collect possible borrowed for every `&mut` local.
+/// For example, `_1 = &mut _2` generate _1: {_2,...}
+/// Known Problems: not sure all borrowed are tracked
+#[allow(clippy::module_name_repetitions)]
+pub(super) struct PossibleOriginVisitor<'a, 'tcx> {
+    possible_origin: TransitiveRelation,
+    body: &'a mir::Body<'tcx>,
+}
+
+impl<'a, 'tcx> PossibleOriginVisitor<'a, 'tcx> {
+    pub fn new(body: &'a mir::Body<'tcx>) -> Self {
+        Self {
+            possible_origin: TransitiveRelation::default(),
+            body,
+        }
+    }
+
+    pub fn into_map(self, cx: &LateContext<'tcx>) -> FxHashMap<mir::Local, HybridBitSet<mir::Local>> {
+        let mut map = FxHashMap::default();
+        for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
+            if is_copy(cx, self.body.local_decls[row].ty) {
+                continue;
+            }
+
+            let mut borrowers = self.possible_origin.reachable_from(row, self.body.local_decls.len());
+            borrowers.remove(mir::Local::from_usize(0));
+            if !borrowers.is_empty() {
+                map.insert(row, borrowers);
+            }
+        }
+        map
+    }
+}
+
+impl<'a, 'tcx> mir::visit::Visitor<'tcx> for PossibleOriginVisitor<'a, 'tcx> {
+    fn visit_assign(&mut self, place: &mir::Place<'tcx>, rvalue: &mir::Rvalue<'_>, _location: mir::Location) {
+        let lhs = place.local;
+        match rvalue {
+            // Only consider `&mut`, which can modify origin place
+            mir::Rvalue::Ref(_, rustc_middle::mir::BorrowKind::Mut { .. }, borrowed) |
+            // _2: &mut _;
+            // _3 = move _2
+            mir::Rvalue::Use(mir::Operand::Move(borrowed))  |
+            // _3 = move _2 as &mut _;
+            mir::Rvalue::Cast(_, mir::Operand::Move(borrowed), _)
+                => {
+                self.possible_origin.add(lhs, borrowed.local);
+            },
+            _ => {},
+        }
+    }
+}
diff --git a/clippy_utils/src/mir/transitive_relation.rs b/clippy_utils/src/mir/transitive_relation.rs
new file mode 100644
index 00000000000..7fe2960739f
--- /dev/null
+++ b/clippy_utils/src/mir/transitive_relation.rs
@@ -0,0 +1,29 @@
+use rustc_data_structures::fx::FxHashMap;
+use rustc_index::bit_set::HybridBitSet;
+use rustc_middle::mir;
+
+#[derive(Default)]
+pub(super) struct TransitiveRelation {
+    relations: FxHashMap<mir::Local, Vec<mir::Local>>,
+}
+
+impl TransitiveRelation {
+    pub fn add(&mut self, a: mir::Local, b: mir::Local) {
+        self.relations.entry(a).or_default().push(b);
+    }
+
+    pub fn reachable_from(&self, a: mir::Local, domain_size: usize) -> HybridBitSet<mir::Local> {
+        let mut seen = HybridBitSet::new_empty(domain_size);
+        let mut stack = vec![a];
+        while let Some(u) = stack.pop() {
+            if let Some(edges) = self.relations.get(&u) {
+                for &v in edges {
+                    if seen.insert(v) {
+                        stack.push(v);
+                    }
+                }
+            }
+        }
+        seen
+    }
+}
diff --git a/clippy_utils/src/paths.rs b/clippy_utils/src/paths.rs
index 13938645fc3..78adc453654 100644
--- a/clippy_utils/src/paths.rs
+++ b/clippy_utils/src/paths.rs
@@ -119,17 +119,11 @@ pub const RANGE_ARGUMENT_TRAIT: [&str; 3] = ["core", "ops", "RangeBounds"];
 pub const RC_PTR_EQ: [&str; 4] = ["alloc", "rc", "Rc", "ptr_eq"];
 pub const REFCELL_REF: [&str; 3] = ["core", "cell", "Ref"];
 pub const REFCELL_REFMUT: [&str; 3] = ["core", "cell", "RefMut"];
-#[expect(clippy::invalid_paths)] // internal lints do not know about all external crates
 pub const REGEX_BUILDER_NEW: [&str; 5] = ["regex", "re_builder", "unicode", "RegexBuilder", "new"];
-#[expect(clippy::invalid_paths)] // internal lints do not know about all external crates
 pub const REGEX_BYTES_BUILDER_NEW: [&str; 5] = ["regex", "re_builder", "bytes", "RegexBuilder", "new"];
-#[expect(clippy::invalid_paths)] // internal lints do not know about all external crates
 pub const REGEX_BYTES_NEW: [&str; 4] = ["regex", "re_bytes", "Regex", "new"];
-#[expect(clippy::invalid_paths)] // internal lints do not know about all external crates
 pub const REGEX_BYTES_SET_NEW: [&str; 5] = ["regex", "re_set", "bytes", "RegexSet", "new"];
-#[expect(clippy::invalid_paths)] // internal lints do not know about all external crates
 pub const REGEX_NEW: [&str; 4] = ["regex", "re_unicode", "Regex", "new"];
-#[expect(clippy::invalid_paths)] // internal lints do not know about all external crates
 pub const REGEX_SET_NEW: [&str; 5] = ["regex", "re_set", "unicode", "RegexSet", "new"];
 /// Preferably use the diagnostic item `sym::Result` where possible
 pub const RESULT: [&str; 3] = ["core", "result", "Result"];
diff --git a/lintcheck/src/main.rs b/lintcheck/src/main.rs
index cc2b3e1acec..95b20d7f024 100644
--- a/lintcheck/src/main.rs
+++ b/lintcheck/src/main.rs
@@ -345,7 +345,7 @@ impl Crate {
                 clippy_args.push(opt);
             }
         } else {
-            clippy_args.extend(&["-Wclippy::pedantic", "-Wclippy::cargo"])
+            clippy_args.extend(["-Wclippy::pedantic", "-Wclippy::cargo"])
         }
 
         if lint_filter.is_empty() {
@@ -457,15 +457,11 @@ fn build_clippy() {
 /// Read a `lintcheck_crates.toml` file
 fn read_crates(toml_path: &Path) -> (Vec<CrateSource>, RecursiveOptions) {
     let toml_content: String =
-        std::fs::read_to_string(&toml_path).unwrap_or_else(|_| panic!("Failed to read {}", toml_path.display()));
+        std::fs::read_to_string(toml_path).unwrap_or_else(|_| panic!("Failed to read {}", toml_path.display()));
     let crate_list: SourceList =
         toml::from_str(&toml_content).unwrap_or_else(|e| panic!("Failed to parse {}: \n{}", toml_path.display(), e));
     // parse the hashmap of the toml file into a list of crates
-    let tomlcrates: Vec<TomlCrate> = crate_list
-        .crates
-        .into_iter()
-        .map(|(_cratename, tomlcrate)| tomlcrate)
-        .collect();
+    let tomlcrates: Vec<TomlCrate> = crate_list.crates.into_values().collect();
 
     // flatten TomlCrates into CrateSources (one TomlCrates may represent several versions of a crate =>
     // multiple Cratesources)
@@ -602,10 +598,10 @@ fn main() {
     ) {
         let shared_target_dir = "target/lintcheck/shared_target_dir";
         // if we get an Err here, the shared target dir probably does simply not exist
-        if let Ok(metadata) = std::fs::metadata(&shared_target_dir) {
+        if let Ok(metadata) = std::fs::metadata(shared_target_dir) {
             if metadata.is_dir() {
                 println!("Clippy is newer than lint check logs, clearing lintcheck shared target dir...");
-                std::fs::remove_dir_all(&shared_target_dir)
+                std::fs::remove_dir_all(shared_target_dir)
                     .expect("failed to remove target/lintcheck/shared_target_dir");
             }
         }
@@ -779,7 +775,7 @@ fn read_stats_from_file(file_path: &Path) -> HashMap<String, usize> {
 fn print_stats(old_stats: HashMap<String, usize>, new_stats: HashMap<&String, usize>, lint_filter: &Vec<String>) {
     let same_in_both_hashmaps = old_stats
         .iter()
-        .filter(|(old_key, old_val)| new_stats.get::<&String>(&old_key) == Some(old_val))
+        .filter(|(old_key, old_val)| new_stats.get::<&String>(old_key) == Some(old_val))
         .map(|(k, v)| (k.to_string(), *v))
         .collect::<Vec<(String, usize)>>();
 
@@ -797,7 +793,7 @@ fn print_stats(old_stats: HashMap<String, usize>, new_stats: HashMap<&String, us
     // list all new counts  (key is in new stats but not in old stats)
     new_stats_deduped
         .iter()
-        .filter(|(new_key, _)| old_stats_deduped.get::<str>(&new_key).is_none())
+        .filter(|(new_key, _)| old_stats_deduped.get::<str>(new_key).is_none())
         .for_each(|(new_key, new_value)| {
             println!("{} 0 => {}", new_key, new_value);
         });
@@ -805,16 +801,16 @@ fn print_stats(old_stats: HashMap<String, usize>, new_stats: HashMap<&String, us
     // list all changed counts (key is in both maps but value differs)
     new_stats_deduped
         .iter()
-        .filter(|(new_key, _new_val)| old_stats_deduped.get::<str>(&new_key).is_some())
+        .filter(|(new_key, _new_val)| old_stats_deduped.get::<str>(new_key).is_some())
         .for_each(|(new_key, new_val)| {
-            let old_val = old_stats_deduped.get::<str>(&new_key).unwrap();
+            let old_val = old_stats_deduped.get::<str>(new_key).unwrap();
             println!("{} {} => {}", new_key, old_val, new_val);
         });
 
     // list all gone counts (key is in old status but not in new stats)
     old_stats_deduped
         .iter()
-        .filter(|(old_key, _)| new_stats_deduped.get::<&String>(&old_key).is_none())
+        .filter(|(old_key, _)| new_stats_deduped.get::<&String>(old_key).is_none())
         .filter(|(old_key, _)| lint_filter.is_empty() || lint_filter.contains(old_key))
         .for_each(|(old_key, old_value)| {
             println!("{} {} => 0", old_key, old_value);
@@ -832,12 +828,12 @@ fn create_dirs(krate_download_dir: &Path, extract_dir: &Path) {
             panic!("cannot create lintcheck target dir");
         }
     });
-    std::fs::create_dir(&krate_download_dir).unwrap_or_else(|err| {
+    std::fs::create_dir(krate_download_dir).unwrap_or_else(|err| {
         if err.kind() != ErrorKind::AlreadyExists {
             panic!("cannot create crate download dir");
         }
     });
-    std::fs::create_dir(&extract_dir).unwrap_or_else(|err| {
+    std::fs::create_dir(extract_dir).unwrap_or_else(|err| {
         if err.kind() != ErrorKind::AlreadyExists {
             panic!("cannot create crate extraction dir");
         }
@@ -863,7 +859,7 @@ fn lintcheck_test() {
         "lintcheck/test_sources.toml",
     ];
     let status = std::process::Command::new("cargo")
-        .args(&args)
+        .args(args)
         .current_dir("..") // repo root
         .status();
     //.output();
diff --git a/tests/compile-test.rs b/tests/compile-test.rs
index fa769222d1a..c10ee969c01 100644
--- a/tests/compile-test.rs
+++ b/tests/compile-test.rs
@@ -283,7 +283,7 @@ fn run_ui_cargo() {
                 env::set_current_dir(&src_path)?;
 
                 let cargo_toml_path = case.path().join("Cargo.toml");
-                let cargo_content = fs::read(&cargo_toml_path)?;
+                let cargo_content = fs::read(cargo_toml_path)?;
                 let cargo_parsed: toml::Value = toml::from_str(
                     std::str::from_utf8(&cargo_content).expect("`Cargo.toml` is not a valid utf-8 file!"),
                 )
diff --git a/tests/ui/needless_borrow.fixed b/tests/ui/needless_borrow.fixed
index aa2687159ef..340e89d2db1 100644
--- a/tests/ui/needless_borrow.fixed
+++ b/tests/ui/needless_borrow.fixed
@@ -3,7 +3,11 @@
 
 #[warn(clippy::all, clippy::needless_borrow)]
 #[allow(unused_variables)]
-#[allow(clippy::uninlined_format_args, clippy::unnecessary_mut_passed)]
+#[allow(
+    clippy::uninlined_format_args,
+    clippy::unnecessary_mut_passed,
+    clippy::unnecessary_to_owned
+)]
 fn main() {
     let a = 5;
     let ref_a = &a;
@@ -134,6 +138,7 @@ fn main() {
     multiple_constraints([[""]]);
     multiple_constraints_normalizes_to_same(X, X);
     let _ = Some("").unwrap_or("");
+    let _ = std::fs::write("x", "".to_string());
 
     only_sized(&""); // Don't lint. `Sized` is only bound
     let _ = std::any::Any::type_id(&""); // Don't lint. `Any` is only bound
@@ -276,8 +281,9 @@ mod copyable_iterator {
     fn dont_warn(mut x: Iter) {
         takes_iter(&mut x);
     }
+    #[allow(unused_mut)]
     fn warn(mut x: &mut Iter) {
-        takes_iter(&mut x)
+        takes_iter(x)
     }
 }
 
@@ -327,3 +333,55 @@ fn issue9383() {
         ManuallyDrop::drop(&mut ocean.coral);
     }
 }
+
+#[allow(dead_code)]
+fn closure_test() {
+    let env = "env".to_owned();
+    let arg = "arg".to_owned();
+    let f = |arg| {
+        let loc = "loc".to_owned();
+        let _ = std::fs::write("x", &env); // Don't lint. In environment
+        let _ = std::fs::write("x", arg);
+        let _ = std::fs::write("x", loc);
+    };
+    let _ = std::fs::write("x", &env); // Don't lint. Borrowed by `f`
+    f(arg);
+}
+
+#[allow(dead_code)]
+mod significant_drop {
+    #[derive(Debug)]
+    struct X;
+
+    #[derive(Debug)]
+    struct Y;
+
+    impl Drop for Y {
+        fn drop(&mut self) {}
+    }
+
+    fn foo(x: X, y: Y) {
+        debug(x);
+        debug(&y); // Don't lint. Has significant drop
+    }
+
+    fn debug(_: impl std::fmt::Debug) {}
+}
+
+#[allow(dead_code)]
+mod used_exactly_once {
+    fn foo(x: String) {
+        use_x(x);
+    }
+    fn use_x(_: impl AsRef<str>) {}
+}
+
+#[allow(dead_code)]
+mod used_more_than_once {
+    fn foo(x: String) {
+        use_x(&x);
+        use_x_again(&x);
+    }
+    fn use_x(_: impl AsRef<str>) {}
+    fn use_x_again(_: impl AsRef<str>) {}
+}
diff --git a/tests/ui/needless_borrow.rs b/tests/ui/needless_borrow.rs
index d41251e8f6a..c93711ac8e2 100644
--- a/tests/ui/needless_borrow.rs
+++ b/tests/ui/needless_borrow.rs
@@ -3,7 +3,11 @@
 
 #[warn(clippy::all, clippy::needless_borrow)]
 #[allow(unused_variables)]
-#[allow(clippy::uninlined_format_args, clippy::unnecessary_mut_passed)]
+#[allow(
+    clippy::uninlined_format_args,
+    clippy::unnecessary_mut_passed,
+    clippy::unnecessary_to_owned
+)]
 fn main() {
     let a = 5;
     let ref_a = &a;
@@ -134,6 +138,7 @@ fn main() {
     multiple_constraints(&[[""]]);
     multiple_constraints_normalizes_to_same(&X, X);
     let _ = Some("").unwrap_or(&"");
+    let _ = std::fs::write("x", &"".to_string());
 
     only_sized(&""); // Don't lint. `Sized` is only bound
     let _ = std::any::Any::type_id(&""); // Don't lint. `Any` is only bound
@@ -276,6 +281,7 @@ mod copyable_iterator {
     fn dont_warn(mut x: Iter) {
         takes_iter(&mut x);
     }
+    #[allow(unused_mut)]
     fn warn(mut x: &mut Iter) {
         takes_iter(&mut x)
     }
@@ -327,3 +333,55 @@ fn issue9383() {
         ManuallyDrop::drop(&mut ocean.coral);
     }
 }
+
+#[allow(dead_code)]
+fn closure_test() {
+    let env = "env".to_owned();
+    let arg = "arg".to_owned();
+    let f = |arg| {
+        let loc = "loc".to_owned();
+        let _ = std::fs::write("x", &env); // Don't lint. In environment
+        let _ = std::fs::write("x", &arg);
+        let _ = std::fs::write("x", &loc);
+    };
+    let _ = std::fs::write("x", &env); // Don't lint. Borrowed by `f`
+    f(arg);
+}
+
+#[allow(dead_code)]
+mod significant_drop {
+    #[derive(Debug)]
+    struct X;
+
+    #[derive(Debug)]
+    struct Y;
+
+    impl Drop for Y {
+        fn drop(&mut self) {}
+    }
+
+    fn foo(x: X, y: Y) {
+        debug(&x);
+        debug(&y); // Don't lint. Has significant drop
+    }
+
+    fn debug(_: impl std::fmt::Debug) {}
+}
+
+#[allow(dead_code)]
+mod used_exactly_once {
+    fn foo(x: String) {
+        use_x(&x);
+    }
+    fn use_x(_: impl AsRef<str>) {}
+}
+
+#[allow(dead_code)]
+mod used_more_than_once {
+    fn foo(x: String) {
+        use_x(&x);
+        use_x_again(&x);
+    }
+    fn use_x(_: impl AsRef<str>) {}
+    fn use_x_again(_: impl AsRef<str>) {}
+}
diff --git a/tests/ui/needless_borrow.stderr b/tests/ui/needless_borrow.stderr
index 5af68706d4b..8b593268bec 100644
--- a/tests/ui/needless_borrow.stderr
+++ b/tests/ui/needless_borrow.stderr
@@ -1,5 +1,5 @@
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:11:15
+  --> $DIR/needless_borrow.rs:15:15
    |
 LL |     let _ = x(&&a); // warn
    |               ^^^ help: change this to: `&a`
@@ -7,172 +7,208 @@ LL |     let _ = x(&&a); // warn
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:15:13
+  --> $DIR/needless_borrow.rs:19:13
    |
 LL |     mut_ref(&mut &mut b); // warn
    |             ^^^^^^^^^^^ help: change this to: `&mut b`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:27:13
+  --> $DIR/needless_borrow.rs:31:13
    |
 LL |             &&a
    |             ^^^ help: change this to: `&a`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:29:15
+  --> $DIR/needless_borrow.rs:33:15
    |
 LL |         46 => &&a,
    |               ^^^ help: change this to: `&a`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:35:27
+  --> $DIR/needless_borrow.rs:39:27
    |
 LL |                     break &ref_a;
    |                           ^^^^^^ help: change this to: `ref_a`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:42:15
+  --> $DIR/needless_borrow.rs:46:15
    |
 LL |     let _ = x(&&&a);
    |               ^^^^ help: change this to: `&a`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:43:15
+  --> $DIR/needless_borrow.rs:47:15
    |
 LL |     let _ = x(&mut &&a);
    |               ^^^^^^^^ help: change this to: `&a`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:44:15
+  --> $DIR/needless_borrow.rs:48:15
    |
 LL |     let _ = x(&&&mut b);
    |               ^^^^^^^^ help: change this to: `&mut b`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:45:15
+  --> $DIR/needless_borrow.rs:49:15
    |
 LL |     let _ = x(&&ref_a);
    |               ^^^^^^^ help: change this to: `ref_a`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:48:11
+  --> $DIR/needless_borrow.rs:52:11
    |
 LL |         x(&b);
    |           ^^ help: change this to: `b`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:55:13
+  --> $DIR/needless_borrow.rs:59:13
    |
 LL |     mut_ref(&mut x);
    |             ^^^^^^ help: change this to: `x`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:56:13
+  --> $DIR/needless_borrow.rs:60:13
    |
 LL |     mut_ref(&mut &mut x);
    |             ^^^^^^^^^^^ help: change this to: `x`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:57:23
+  --> $DIR/needless_borrow.rs:61:23
    |
 LL |     let y: &mut i32 = &mut x;
    |                       ^^^^^^ help: change this to: `x`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:58:23
+  --> $DIR/needless_borrow.rs:62:23
    |
 LL |     let y: &mut i32 = &mut &mut x;
    |                       ^^^^^^^^^^^ help: change this to: `x`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:67:14
+  --> $DIR/needless_borrow.rs:71:14
    |
 LL |         0 => &mut x,
    |              ^^^^^^ help: change this to: `x`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:73:14
+  --> $DIR/needless_borrow.rs:77:14
    |
 LL |         0 => &mut x,
    |              ^^^^^^ help: change this to: `x`
 
 error: this expression borrows a value the compiler would automatically borrow
-  --> $DIR/needless_borrow.rs:85:13
+  --> $DIR/needless_borrow.rs:89:13
    |
 LL |     let _ = (&x).0;
    |             ^^^^ help: change this to: `x`
 
 error: this expression borrows a value the compiler would automatically borrow
-  --> $DIR/needless_borrow.rs:87:22
+  --> $DIR/needless_borrow.rs:91:22
    |
 LL |     let _ = unsafe { (&*x).0 };
    |                      ^^^^^ help: change this to: `(*x)`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:97:5
+  --> $DIR/needless_borrow.rs:101:5
    |
 LL |     (&&()).foo();
    |     ^^^^^^ help: change this to: `(&())`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:106:5
+  --> $DIR/needless_borrow.rs:110:5
    |
 LL |     (&&5).foo();
    |     ^^^^^ help: change this to: `(&5)`
 
 error: the borrowed expression implements the required traits
-  --> $DIR/needless_borrow.rs:131:51
+  --> $DIR/needless_borrow.rs:135:51
    |
 LL |     let _ = std::process::Command::new("ls").args(&["-a", "-l"]).status().unwrap();
    |                                                   ^^^^^^^^^^^^^ help: change this to: `["-a", "-l"]`
 
 error: the borrowed expression implements the required traits
-  --> $DIR/needless_borrow.rs:132:44
+  --> $DIR/needless_borrow.rs:136:44
    |
 LL |     let _ = std::path::Path::new(".").join(&&".");
    |                                            ^^^^^ help: change this to: `"."`
 
 error: the borrowed expression implements the required traits
-  --> $DIR/needless_borrow.rs:133:23
+  --> $DIR/needless_borrow.rs:137:23
    |
 LL |     deref_target_is_x(&X);
    |                       ^^ help: change this to: `X`
 
 error: the borrowed expression implements the required traits
-  --> $DIR/needless_borrow.rs:134:26
+  --> $DIR/needless_borrow.rs:138:26
    |
 LL |     multiple_constraints(&[[""]]);
    |                          ^^^^^^^ help: change this to: `[[""]]`
 
 error: the borrowed expression implements the required traits
-  --> $DIR/needless_borrow.rs:135:45
+  --> $DIR/needless_borrow.rs:139:45
    |
 LL |     multiple_constraints_normalizes_to_same(&X, X);
    |                                             ^^ help: change this to: `X`
 
 error: this expression creates a reference which is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:136:32
+  --> $DIR/needless_borrow.rs:140:32
    |
 LL |     let _ = Some("").unwrap_or(&"");
    |                                ^^^ help: change this to: `""`
 
+error: the borrowed expression implements the required traits
+  --> $DIR/needless_borrow.rs:141:33
+   |
+LL |     let _ = std::fs::write("x", &"".to_string());
+   |                                 ^^^^^^^^^^^^^^^ help: change this to: `"".to_string()`
+
 error: this expression borrows a value the compiler would automatically borrow
-  --> $DIR/needless_borrow.rs:187:13
+  --> $DIR/needless_borrow.rs:192:13
    |
 LL |             (&self.f)()
    |             ^^^^^^^^^ help: change this to: `(self.f)`
 
 error: this expression borrows a value the compiler would automatically borrow
-  --> $DIR/needless_borrow.rs:196:13
+  --> $DIR/needless_borrow.rs:201:13
    |
 LL |             (&mut self.f)()
    |             ^^^^^^^^^^^^^ help: change this to: `(self.f)`
 
 error: the borrowed expression implements the required traits
-  --> $DIR/needless_borrow.rs:298:55
+  --> $DIR/needless_borrow.rs:286:20
+   |
+LL |         takes_iter(&mut x)
+   |                    ^^^^^^ help: change this to: `x`
+
+error: the borrowed expression implements the required traits
+  --> $DIR/needless_borrow.rs:304:55
    |
 LL |         let _ = std::process::Command::new("ls").args(&["-a", "-l"]).status().unwrap();
    |                                                       ^^^^^^^^^^^^^ help: change this to: `["-a", "-l"]`
 
-error: aborting due to 29 previous errors
+error: the borrowed expression implements the required traits
+  --> $DIR/needless_borrow.rs:344:37
+   |
+LL |         let _ = std::fs::write("x", &arg);
+   |                                     ^^^^ help: change this to: `arg`
+
+error: the borrowed expression implements the required traits
+  --> $DIR/needless_borrow.rs:345:37
+   |
+LL |         let _ = std::fs::write("x", &loc);
+   |                                     ^^^^ help: change this to: `loc`
+
+error: the borrowed expression implements the required traits
+  --> $DIR/needless_borrow.rs:364:15
+   |
+LL |         debug(&x);
+   |               ^^ help: change this to: `x`
+
+error: the borrowed expression implements the required traits
+  --> $DIR/needless_borrow.rs:374:15
+   |
+LL |         use_x(&x);
+   |               ^^ help: change this to: `x`
+
+error: aborting due to 35 previous errors
 
diff --git a/tests/ui/unnecessary_to_owned.fixed b/tests/ui/unnecessary_to_owned.fixed
index f97583aa22f..fe09aad06bc 100644
--- a/tests/ui/unnecessary_to_owned.fixed
+++ b/tests/ui/unnecessary_to_owned.fixed
@@ -1,6 +1,6 @@
 // run-rustfix
 
-#![allow(clippy::ptr_arg)]
+#![allow(clippy::needless_borrow, clippy::ptr_arg)]
 #![warn(clippy::unnecessary_to_owned)]
 #![feature(custom_inner_attributes)]
 
diff --git a/tests/ui/unnecessary_to_owned.rs b/tests/ui/unnecessary_to_owned.rs
index aa5394a5657..3de6d0903c0 100644
--- a/tests/ui/unnecessary_to_owned.rs
+++ b/tests/ui/unnecessary_to_owned.rs
@@ -1,6 +1,6 @@
 // run-rustfix
 
-#![allow(clippy::ptr_arg)]
+#![allow(clippy::needless_borrow, clippy::ptr_arg)]
 #![warn(clippy::unnecessary_to_owned)]
 #![feature(custom_inner_attributes)]
 
diff --git a/tests/versioncheck.rs b/tests/versioncheck.rs
index 9e07769a8e4..a6d8d0307ce 100644
--- a/tests/versioncheck.rs
+++ b/tests/versioncheck.rs
@@ -48,7 +48,7 @@ fn check_that_clippy_has_the_same_major_version_as_rustc() {
     // `RUSTC_REAL` if Clippy is build in the Rust repo with `./x.py`.
     let rustc = std::env::var("RUSTC_REAL").unwrap_or_else(|_| "rustc".to_string());
     let rustc_version = String::from_utf8(
-        std::process::Command::new(&rustc)
+        std::process::Command::new(rustc)
             .arg("--version")
             .output()
             .expect("failed to run `rustc --version`")