diff --git a/tooling/lints/README.md b/tooling/lints/README.md
index 6804a2e909..6e7f69da49 100644
--- a/tooling/lints/README.md
+++ b/tooling/lints/README.md
@@ -25,6 +25,7 @@ picks them up automatically when run from that directory.
 - `shared_string_from_str_literal` — `SharedString::new/from` etc where `SharedString::from_static` should be used instead.
 - `async_block_without_await` — `async { … }` blocks whose body contains no `.await` expression.
 - `entity_update_in_render` — `Entity::update`/`WeakEntity::update` mutating an entity inside `Render::render`.
+- `map_lookup_then_insert` — map value lookups that branch into an insertion for the same key, on any map with an `entry` method (`HashMap`, `BTreeMap`, `FxHashMap`, `hashbrown`, `indexmap`, ...), and the same pattern on `HashSet`/`BTreeSet` (where a bare `insert` already searches only once). Keys are matched up to borrows/derefs, value-preserving conversions (`clone`, `to_owned`, `to_string` on string types, `to_vec` on slices, `String::from` on `&str` — each resolved by definition, not name), and immutable local aliases, so looking up `&key` and inserting `key.clone()` is caught.
 - `notify_in_render` — `Context::notify()` called inside `Render::render`.
 - `owned_string_into_shared` — `String::from(<lit>).into()` / `<lit>.to_string().into()` / `<lit>.to_owned().into()` whose target is `SharedString`, `Arc<str>`, `Rc<str>`, or `Cow<'_, str>`.
 - `blocking_io_on_foreground` - Catch blocking IO calls that are called on the main thread (but not on closures or background threads)
diff --git a/tooling/lints/src/lib.rs b/tooling/lints/src/lib.rs
index bf4289673d..851ffb739b 100644
--- a/tooling/lints/src/lib.rs
+++ b/tooling/lints/src/lib.rs
@@ -28,12 +28,14 @@ use rustc_span::Span;
 
 mod blocking_io_on_foreground;
 mod entity_update_in_render;
+mod map_lookup_then_insert;
 mod notify_in_render;
 mod owned_string_into_shared;
 mod render_helpers;
 
 use blocking_io_on_foreground::BLOCKING_IO_ON_FOREGROUND;
 use entity_update_in_render::ENTITY_UPDATE_IN_RENDER;
+use map_lookup_then_insert::MAP_LOOKUP_THEN_INSERT;
 use notify_in_render::NOTIFY_IN_RENDER;
 use owned_string_into_shared::OWNED_STRING_INTO_SHARED;
 
@@ -54,6 +56,7 @@ pub fn register_lints(sess: &rustc_session::Session, lint_store: &mut rustc_lint
         ASYNC_BLOCK_WITHOUT_AWAIT,
         BLOCKING_IO_ON_FOREGROUND,
         ENTITY_UPDATE_IN_RENDER,
+        MAP_LOOKUP_THEN_INSERT,
         NOTIFY_IN_RENDER,
         OWNED_STRING_INTO_SHARED,
     ]);
@@ -61,6 +64,7 @@ pub fn register_lints(sess: &rustc_session::Session, lint_store: &mut rustc_lint
     lint_store.register_late_pass(|_| Box::new(AsyncBlockWithoutAwait));
     lint_store.register_late_pass(|_| Box::new(blocking_io_on_foreground::BlockingIoOnForeground));
     lint_store.register_late_pass(|_| Box::new(entity_update_in_render::EntityUpdateInRender));
+    lint_store.register_late_pass(|_| Box::new(map_lookup_then_insert::MapLookupThenInsert));
     lint_store.register_late_pass(|_| Box::new(notify_in_render::NotifyInRender));
     lint_store.register_late_pass(|_| Box::new(owned_string_into_shared::OwnedStringIntoShared));
 }
diff --git a/tooling/lints/src/map_lookup_then_insert.rs b/tooling/lints/src/map_lookup_then_insert.rs
new file mode 100644
index 0000000000..7bf2a193b4
--- /dev/null
+++ b/tooling/lints/src/map_lookup_then_insert.rs
@@ -0,0 +1,450 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::visitors::{Visitable, for_each_expr_without_closures};
+use clippy_utils::{SpanlessEq, expr_or_init, sym};
+use rustc_hir::def_id::DefId;
+use rustc_hir::{Expr, ExprKind, LangItem, LetStmt, MatchSource, UnOp};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::Ty;
+use rustc_span::Symbol;
+use std::ops::ControlFlow;
+
+rustc_session::declare_lint! {
+    /// ### What it does
+    ///
+    /// Flags control flow that branches on map value lookup methods and then
+    /// inserts into the same map with the same key.
+    ///
+    /// The lint applies to any map type with an inherent `entry` method: std's
+    /// `HashMap` and `BTreeMap`, hasher aliases such as `FxHashMap`, and
+    /// third-party maps such as `hashbrown::HashMap` or `indexmap::IndexMap`.
+    ///
+    /// The lint covers direct `match`, `if let`, `is_some`/`is_none`, and
+    /// `let ... else` forms using `get`, `get_mut`, or `get_key_value`.
+    ///
+    /// Keys are compared after stripping borrows and derefs, peeling
+    /// conversions that yield an owned key equal to the original (`clone`,
+    /// `to_owned`, `to_string` on string types, `to_vec` on slices, and
+    /// `String::from` on `&str`), and resolving immutable local aliases to
+    /// their initializers. So looking up `&key` and inserting `key.clone()`
+    /// is recognized as the same key. Each peeled conversion is resolved by
+    /// definition, not by name, so an unrelated user method that happens to
+    /// be called `clone` does not count.
+    ///
+    /// When such a conversion is peeled, the diagnostic carries a note: the
+    /// entry API takes the key by value up front, so a conversion the
+    /// original code performs only on the miss path would run on every call.
+    ///
+    /// The same pattern on `HashSet` and `BTreeSet` is also flagged. Sets have
+    /// no stable entry API, and need none: `insert` alone searches once, skips
+    /// duplicates, and returns `false` when the value was already present.
+    ///
+    /// Forms that Clippy already rewrites are excluded. `contains_key` or
+    /// `contains` followed by `insert` is `map_entry` or
+    /// `set_contains_or_insert`, and `get(..).is_some()`/`is_none()` on the
+    /// std collections is `unnecessary_get_then_check`, whose `contains_key`/
+    /// `contains` suggestion feeds the former two.
+    ///
+    /// ### Why is this bad?
+    ///
+    /// The lookup searches the map once and `insert` searches it again. The
+    /// map's entry API can reuse the result of one search.
+    pub MAP_LOOKUP_THEN_INSERT,
+    Warn,
+    "a map value lookup is followed by insertion for the same key"
+}
+
+pub(crate) struct MapLookupThenInsert;
+
+rustc_session::impl_lint_pass!(MapLookupThenInsert => [MAP_LOOKUP_THEN_INSERT]);
+
+impl<'tcx> LateLintPass<'tcx> for MapLookupThenInsert {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        if expr.span.from_expansion() {
+            return;
+        }
+
+        let repeated_lookup = match &expr.kind {
+            ExprKind::If(condition, then_expression, else_expression) => {
+                lookup_from_condition(cx, condition).and_then(|lookup| {
+                    find_matching_insert(cx, lookup, *then_expression)
+                        .or_else(|| {
+                            else_expression
+                                .and_then(|expression| find_matching_insert(cx, lookup, expression))
+                        })
+                        .map(|key_converted| (lookup, key_converted))
+                })
+            }
+            ExprKind::Match(scrutinee, arms, MatchSource::Normal) => parse_lookup(cx, scrutinee)
+                .and_then(|lookup| {
+                    arms.iter()
+                        .find_map(|arm| find_matching_insert(cx, lookup, arm.body))
+                        .map(|key_converted| (lookup, key_converted))
+                }),
+            _ => None,
+        };
+
+        if let Some((lookup, key_converted)) = repeated_lookup {
+            emit_lint(cx, expr.span, lookup, key_converted);
+        }
+    }
+
+    fn check_local(&mut self, cx: &LateContext<'tcx>, local: &'tcx LetStmt<'tcx>) {
+        if local.span.from_expansion() {
+            return;
+        }
+
+        let Some(init) = local.init else {
+            return;
+        };
+        let Some(else_block) = local.els else {
+            return;
+        };
+        let Some(lookup) = parse_lookup(cx, init) else {
+            return;
+        };
+
+        if let Some(key_converted) = find_matching_insert(cx, lookup, else_block) {
+            emit_lint(cx, local.span, lookup, key_converted);
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+struct Lookup<'tcx> {
+    map: &'tcx Expr<'tcx>,
+    key: &'tcx Expr<'tcx>,
+}
+
+fn lookup_from_condition<'tcx>(
+    cx: &LateContext<'tcx>,
+    condition: &'tcx Expr<'tcx>,
+) -> Option<Lookup<'tcx>> {
+    match &condition.kind {
+        ExprKind::Let(let_expression) => parse_lookup(cx, let_expression.init),
+        ExprKind::MethodCall(segment, receiver, [], _)
+            if matches!(segment.ident.name.as_str(), "is_some" | "is_none")
+                && !clippy_covers_get_then_check(cx, receiver) =>
+        {
+            parse_lookup(cx, receiver)
+        }
+        _ => None,
+    }
+}
+
+fn parse_lookup<'tcx>(
+    cx: &LateContext<'tcx>,
+    expression: &'tcx Expr<'tcx>,
+) -> Option<Lookup<'tcx>> {
+    let expression = match &expression.kind {
+        ExprKind::MethodCall(segment, receiver, [], _)
+            if matches!(segment.ident.name.as_str(), "copied" | "cloned") =>
+        {
+            receiver
+        }
+        _ => expression,
+    };
+
+    let ExprKind::MethodCall(segment, map, [key], _) = &expression.kind else {
+        return None;
+    };
+    if !matches!(
+        segment.ident.name.as_str(),
+        "get" | "get_mut" | "get_key_value"
+    ) {
+        return None;
+    }
+
+    let map_type = cx.typeck_results().expr_ty_adjusted(map).peel_refs();
+    let map_definition = map_type.ty_adt_def()?;
+    if !is_std_set(cx, map_definition.did()) && !has_inherent_entry_method(cx, map_definition.did())
+    {
+        return None;
+    }
+
+    // The purity check runs on the peeled key so that borrows and
+    // value-preserving conversions do not count as side effects.
+    let mut key_converted = false;
+    let peeled_key = peel_key_expr(cx, key, &mut key_converted);
+    if map.can_have_side_effects() || peeled_key.can_have_side_effects() {
+        return None;
+    }
+
+    Some(Lookup { map, key })
+}
+
+/// Searches `node` for an insertion into `lookup`'s map under `lookup`'s key.
+/// Returns `Some(key_converted)` when one is found, where `key_converted`
+/// records whether the match required peeling an owned-key conversion.
+fn find_matching_insert<'tcx>(
+    cx: &LateContext<'tcx>,
+    lookup: Lookup<'tcx>,
+    node: impl Visitable<'tcx>,
+) -> Option<bool> {
+    for_each_expr_without_closures(node, |expression| {
+        if let Some(insert) = parse_insert(expression)
+            && SpanlessEq::new(cx).eq_expr(lookup.map, insert.map)
+            && let Some(key_converted) = keys_match(cx, lookup.key, insert.key)
+        {
+            ControlFlow::Break(key_converted)
+        } else {
+            ControlFlow::Continue(())
+        }
+    })
+}
+
+fn parse_insert<'tcx>(expression: &'tcx Expr<'tcx>) -> Option<Lookup<'tcx>> {
+    // Maps insert `(key, value)`; sets insert only the value.
+    let ExprKind::MethodCall(segment, map, [key] | [key, _], _) = &expression.kind else {
+        return None;
+    };
+    if segment.ident.name.as_str() != "insert" {
+        return None;
+    }
+
+    Some(Lookup { map, key })
+}
+
+/// Checks whether the two key expressions denote the same key value. Returns
+/// `Some(key_converted)` on a match, where `key_converted` records whether an
+/// owned-key conversion had to be peeled to see it.
+///
+/// Two forms are recognized:
+/// * the keys are equal after peeling borrows, derefs, and value-preserving
+///   conversions, e.g. looking up `&entry.key` and inserting
+///   `entry.key.clone()`;
+/// * after additionally resolving immutable local aliases to their
+///   initializers, e.g. `let owned = key.clone();` before a lookup of `&key`
+///   and an insertion of `owned`. Both resolved forms must be free of side
+///   effects: two structurally equal side-effecting initializers (say, two
+///   separate `next_key()` calls) are two different key values.
+fn keys_match<'tcx>(
+    cx: &LateContext<'tcx>,
+    lookup_key: &'tcx Expr<'tcx>,
+    insert_key: &'tcx Expr<'tcx>,
+) -> Option<bool> {
+    let mut key_converted = false;
+    let lookup_key = peel_key_expr(cx, lookup_key, &mut key_converted);
+    let insert_key = peel_key_expr(cx, insert_key, &mut key_converted);
+    if SpanlessEq::new(cx).eq_expr(lookup_key, insert_key) {
+        return Some(key_converted);
+    }
+
+    let lookup_key = resolve_local_aliases(cx, lookup_key, &mut key_converted);
+    let insert_key = resolve_local_aliases(cx, insert_key, &mut key_converted);
+    if !lookup_key.can_have_side_effects()
+        && !insert_key.can_have_side_effects()
+        && SpanlessEq::new(cx).eq_expr(lookup_key, insert_key)
+    {
+        return Some(key_converted);
+    }
+
+    None
+}
+
+/// Follows immutable local bindings back to their initializers, peeling each
+/// initializer down to its underlying key expression. `expr_or_init` only
+/// resolves bindings that cannot be reassigned, so the binding's value is the
+/// initializer's value wherever the binding is referenced.
+fn resolve_local_aliases<'tcx>(
+    cx: &LateContext<'tcx>,
+    mut expression: &'tcx Expr<'tcx>,
+    key_converted: &mut bool,
+) -> &'tcx Expr<'tcx> {
+    loop {
+        let resolved = expr_or_init(cx, expression);
+        if resolved.hir_id == expression.hir_id {
+            return expression;
+        }
+        expression = peel_key_expr(cx, resolved, key_converted);
+    }
+}
+
+/// Strips layers that do not change which slot of the map the key selects:
+/// borrows, derefs, and owned-key conversions of the underlying expression.
+fn peel_key_expr<'tcx>(
+    cx: &LateContext<'tcx>,
+    mut expression: &'tcx Expr<'tcx>,
+    key_converted: &mut bool,
+) -> &'tcx Expr<'tcx> {
+    loop {
+        match &expression.kind {
+            ExprKind::AddrOf(_, _, inner) | ExprKind::Unary(UnOp::Deref, inner) => {
+                expression = inner;
+            }
+            _ => match peel_value_preserving_conversion(cx, expression) {
+                Some(inner) => {
+                    *key_converted = true;
+                    expression = inner;
+                }
+                None => return expression,
+            },
+        }
+    }
+}
+
+/// If `expression` is a conversion that yields an owned key equal to its
+/// input — under the same `Eq`/`Hash`/`Ord` contract the map lookup itself
+/// relies on — returns the input.
+///
+/// Each conversion is resolved by definition, not by name, so a user-defined
+/// method that merely happens to be called `clone` or `to_owned` does not
+/// count. For the `ToOwned` family, the `Borrow` contract (the owned and
+/// borrowed forms hash and compare identically) is the same contract that
+/// makes the map lookup itself well-defined. For `Clone`, the original code
+/// already inserts the clone under the probed key, so the lint assumes
+/// nothing the code did not.
+fn peel_value_preserving_conversion<'tcx>(
+    cx: &LateContext<'tcx>,
+    expression: &'tcx Expr<'tcx>,
+) -> Option<&'tcx Expr<'tcx>> {
+    match &expression.kind {
+        ExprKind::MethodCall(segment, receiver, [], _) => {
+            let method = cx
+                .typeck_results()
+                .type_dependent_def_id(expression.hir_id)?;
+            let preserves_value = match segment.ident.name.as_str() {
+                "clone" => is_assoc_of_diag_trait(cx, method, sym::Clone),
+                "to_owned" => is_assoc_of_diag_trait(cx, method, sym::ToOwned),
+                // `ToString` runs an arbitrary `Display` impl; only the
+                // string types are guaranteed to format as themselves.
+                "to_string" => {
+                    is_assoc_of_diag_trait(cx, method, sym::ToString) && {
+                        let receiver_type =
+                            cx.typeck_results().expr_ty_adjusted(receiver).peel_refs();
+                        receiver_type.is_str() || is_string(cx, receiver_type)
+                    }
+                }
+                "to_vec" => cx.tcx.impl_of_assoc(method).is_some_and(|impl_id| {
+                    cx.tcx.type_of(impl_id).instantiate_identity().is_slice()
+                }),
+                _ => false,
+            };
+            preserves_value.then_some(receiver)
+        }
+        // UFCS forms: `Clone::clone(&key)`, `ToOwned::to_owned(&key)`, and
+        // `String::from(key)` where `key` is a `&str`.
+        ExprKind::Call(callee, [argument]) => {
+            let ExprKind::Path(qpath) = &callee.kind else {
+                return None;
+            };
+            let function = cx.qpath_res(qpath, callee.hir_id).opt_def_id()?;
+            let preserves_value = is_assoc_of_diag_trait(cx, function, sym::Clone)
+                || is_assoc_of_diag_trait(cx, function, sym::ToOwned)
+                || (is_assoc_of_diag_trait(cx, function, sym::From)
+                    && is_string(cx, cx.typeck_results().expr_ty(expression))
+                    && cx.typeck_results().expr_ty(argument).peel_refs().is_str());
+            preserves_value.then_some(argument)
+        }
+        _ => None,
+    }
+}
+
+/// Whether `item` is an associated item of the trait named by the diagnostic
+/// item `trait_name`. Qualified paths can resolve to a trait impl's item
+/// rather than the trait's, so impl items are first mapped back to the trait
+/// item they implement.
+fn is_assoc_of_diag_trait(cx: &LateContext<'_>, item: DefId, trait_name: Symbol) -> bool {
+    let item = cx
+        .tcx
+        .opt_associated_item(item)
+        .and_then(|associated| associated.trait_item_def_id())
+        .unwrap_or(item);
+    cx.tcx
+        .trait_of_assoc(item)
+        .is_some_and(|trait_id| cx.tcx.is_diagnostic_item(trait_name, trait_id))
+}
+
+fn is_string(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
+    ty.ty_adt_def()
+        .is_some_and(|definition| cx.tcx.is_lang_item(definition.did(), LangItem::String))
+}
+
+/// Clippy already rewrites `get(..).is_some()`/`is_none()` on the std maps
+/// and sets: `unnecessary_get_then_check` turns the condition into
+/// `contains_key`/`contains`, which `map_entry`/`set_contains_or_insert` then
+/// turn into the final form. Defer to that chain instead of duplicating it.
+/// Conditions using `get_mut` or `get_key_value`, and any `get` on a non-std
+/// map, are not covered by Clippy and stay ours.
+fn clippy_covers_get_then_check(cx: &LateContext<'_>, receiver: &Expr<'_>) -> bool {
+    let ExprKind::MethodCall(segment, map, [_], _) = &receiver.kind else {
+        return false;
+    };
+    if segment.ident.name.as_str() != "get" {
+        return false;
+    }
+    cx.typeck_results()
+        .expr_ty_adjusted(map)
+        .peel_refs()
+        .ty_adt_def()
+        .is_some_and(|definition| {
+            matches!(
+                cx.tcx.get_diagnostic_name(definition.did()),
+                Some(sym::HashMap | sym::BTreeMap | sym::HashSet | sym::BTreeSet)
+            )
+        })
+}
+
+/// Sets are gated by diagnostic name rather than by an `entry` method: their
+/// entry API is unstable (`hash_set_entry`), and the suggested rewrite (plain
+/// `insert`) relies on std's no-op-when-present insertion semantics.
+fn is_std_set(cx: &LateContext<'_>, definition: DefId) -> bool {
+    matches!(
+        cx.tcx.get_diagnostic_name(definition),
+        Some(sym::HashSet | sym::BTreeSet)
+    )
+}
+
+/// A lookup-then-insert pattern is only worth reporting when the map offers an
+/// entry API to rewrite it with. Rather than hard-coding `HashMap` and
+/// `BTreeMap`, accept any type whose inherent impls provide an `entry` method;
+/// this also covers maps like `hashbrown::HashMap` and `indexmap::IndexMap`.
+/// Hasher aliases such as `FxHashMap` need no special case because they
+/// resolve to the underlying map type.
+fn has_inherent_entry_method(cx: &LateContext<'_>, map_definition: DefId) -> bool {
+    let entry = Symbol::intern("entry");
+    cx.tcx.inherent_impls(map_definition).iter().any(|&imp| {
+        cx.tcx
+            .associated_items(imp)
+            .filter_by_name_unhygienic(entry)
+            .next()
+            .is_some()
+    })
+}
+
+fn emit_lint(
+    cx: &LateContext<'_>,
+    span: rustc_span::Span,
+    lookup: Lookup<'_>,
+    key_converted: bool,
+) {
+    let is_set = cx
+        .typeck_results()
+        .expr_ty_adjusted(lookup.map)
+        .peel_refs()
+        .ty_adt_def()
+        .is_some_and(|definition| is_std_set(cx, definition.did()));
+
+    let (message, help) = if is_set {
+        (
+            "this set lookup and insertion search for the same value twice",
+            "`insert` alone searches once, skips duplicates, and returns `false` \
+             when the value was already present",
+        )
+    } else {
+        (
+            "this map lookup and insertion search for the same key twice",
+            "to search the map only once, use the entry API: \
+             https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html",
+        )
+    };
+    span_lint_and_then(cx, MAP_LOOKUP_THEN_INSERT, span, message, |diagnostic| {
+        diagnostic.help(help);
+        if key_converted {
+            diagnostic.note(
+                "the inserted key is an owned copy of the looked-up key; a rewrite that \
+                 passes the key by value up front would make that copy on hits as well \
+                 as misses",
+            );
+        }
+    });
+}
diff --git a/tooling/lints/ui/map_lookup_then_insert.rs b/tooling/lints/ui/map_lookup_then_insert.rs
new file mode 100644
index 0000000000..aa80986923
--- /dev/null
+++ b/tooling/lints/ui/map_lookup_then_insert.rs
@@ -0,0 +1,573 @@
+// Tests for the `map_lookup_then_insert` lint.
+
+#![allow(unused)]
+
+use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
+
+fn main() {}
+
+// ------------ SHOULD FIRE -----------
+
+fn hash_map_if_let(map: &mut HashMap<String, usize>, key: String) -> usize {
+    if let Some(value) = map.get(&key) {
+        *value
+    } else {
+        map.insert(key, 1);
+        1
+    }
+}
+
+fn hash_map_match(map: &mut HashMap<String, usize>, key: String) -> usize {
+    match map.get(&key).copied() {
+        Some(value) => value,
+        None => {
+            map.insert(key, 1);
+            1
+        }
+    }
+}
+
+// Fix: `map.entry(key).and_modify(|value| *value += 1).or_insert(1)`.
+fn hash_map_get_mut(map: &mut HashMap<String, usize>, key: String) {
+    match map.get_mut(&key) {
+        Some(value) => *value += 1,
+        None => {
+            map.insert(key, 1);
+        }
+    }
+}
+
+fn hash_map_get_key_value(map: &mut HashMap<String, usize>, key: String) -> usize {
+    match map.get_key_value(&key) {
+        Some((_, value)) => *value,
+        None => {
+            map.insert(key, 1);
+            1
+        }
+    }
+}
+
+fn btree_map_match(map: &mut BTreeMap<String, usize>, key: String) -> usize {
+    match map.get(&key) {
+        Some(value) => *value,
+        None => {
+            map.insert(key, 1);
+            1
+        }
+    }
+}
+
+fn btree_map_if_let(map: &mut BTreeMap<String, usize>, key: String) -> usize {
+    if let Some(value) = map.get(&key) {
+        *value
+    } else {
+        map.insert(key, 1);
+        1
+    }
+}
+
+// Fix: `map.entry(key).and_modify(|value| *value += 1).or_insert(1)`.
+fn btree_map_get_mut(map: &mut BTreeMap<String, usize>, key: String) {
+    match map.get_mut(&key) {
+        Some(value) => *value += 1,
+        None => {
+            map.insert(key, 1);
+        }
+    }
+}
+
+fn btree_map_get_key_value(map: &mut BTreeMap<String, usize>, key: String) -> usize {
+    match map.get_key_value(&key) {
+        Some((_, value)) => *value,
+        None => {
+            map.insert(key, 1);
+            1
+        }
+    }
+}
+
+fn btree_map_let_else(map: &mut BTreeMap<String, usize>, key: String) -> usize {
+    let Some(value) = map.get(&key) else {
+        map.insert(key, 1);
+        return 1;
+    };
+    *value
+}
+
+// `btree_map::Entry` has no `insert_entry`; the fix here is
+// `map.entry(key).and_modify(|value| *value = 2)`.
+fn btree_map_overwrite_when_present(map: &mut BTreeMap<String, usize>, key: String) {
+    if let Some(_current) = map.get(&key) {
+        map.insert(key, 2);
+    }
+}
+
+// Sets repeat the search too. They need no entry API: the fix is plain
+// `set.insert(value)`, which returns `false` when the value was present.
+fn hash_set_if_let(set: &mut HashSet<String>, value: String) -> bool {
+    if let Some(existing) = set.get(&value) {
+        existing.is_empty()
+    } else {
+        set.insert(value)
+    }
+}
+
+fn btree_set_match(set: &mut BTreeSet<String>, value: String) -> bool {
+    match set.get(&value) {
+        Some(_) => false,
+        None => set.insert(value),
+    }
+}
+
+fn hash_map_let_else(map: &mut HashMap<String, usize>, key: String) -> usize {
+    let Some(value) = map.get(&key) else {
+        map.insert(key, 1);
+        return 1;
+    };
+    *value
+}
+
+fn expensive() -> usize {
+    42
+}
+
+// Fix: `map.entry(key).or_insert_with(expensive)`.
+fn fix_with_or_insert_with(map: &mut HashMap<String, usize>, key: String) {
+    match map.get(&key) {
+        Some(_) => {}
+        None => {
+            map.insert(key, expensive());
+        }
+    }
+}
+
+// Fix: `map.entry(key).or_insert_with_key(|key| key.len())`.
+fn fix_with_or_insert_with_key(map: &mut HashMap<String, usize>, key: String) {
+    match map.get(&key) {
+        Some(_) => {}
+        None => {
+            let value = key.len();
+            map.insert(key, value);
+        }
+    }
+}
+
+// Fix: `map.entry(key).or_default()`.
+fn fix_with_or_default(map: &mut HashMap<String, usize>, key: String) {
+    match map.get(&key) {
+        Some(_) => {}
+        None => {
+            map.insert(key, usize::default());
+        }
+    }
+}
+
+// Fix: `map.entry(key).insert_entry(2)`, keeping the occupancy check.
+fn fix_with_insert_entry(map: &mut HashMap<String, usize>, key: String) {
+    if let Some(_current) = map.get(&key) {
+        map.insert(key, 2);
+    }
+}
+
+// Hasher aliases like `FxHashMap` resolve to `HashMap`, so they are covered.
+type FxLikeMap = HashMap<
+    String,
+    usize,
+    std::hash::BuildHasherDefault<std::collections::hash_map::DefaultHasher>,
+>;
+
+fn custom_hasher_alias(map: &mut FxLikeMap, key: String) {
+    match map.get(&key) {
+        Some(_) => {}
+        None => {
+            map.insert(key, 1);
+        }
+    }
+}
+
+// A third-party map (`hashbrown`, `indexmap`, ...) that offers an inherent
+// `entry` method is covered.
+struct EntryMap;
+
+impl EntryMap {
+    fn get(&self, _key: &str) -> Option<&usize> {
+        None
+    }
+
+    fn insert(&mut self, _key: String, _value: usize) {}
+
+    fn entry(&mut self, _key: String) -> std::collections::hash_map::Entry<'_, String, usize> {
+        unimplemented!()
+    }
+}
+
+// Clippy's `unnecessary_get_then_check` only covers the std collections, so
+// the `get(..).is_none()` form on a third-party map is still ours to catch.
+fn entry_compatible_custom_map(map: &mut EntryMap, key: String) {
+    if map.get(&key).is_none() {
+        map.insert(key, 1);
+    }
+}
+
+// --- Keys that differ only by a value-preserving conversion ---
+
+// The shape that motivated conversion peeling: code in the wild looks up
+// `&key` and inserts `key.clone()`, which a textual comparison misses.
+fn insert_clones_the_key(map: &mut HashMap<String, Vec<usize>>, key: String, value: usize) {
+    if let Some(values) = map.get_mut(&key) {
+        values.push(value);
+    } else {
+        map.insert(key.clone(), vec![value]);
+    }
+}
+
+fn insert_clones_the_key_ufcs(map: &mut HashMap<String, usize>, key: String) {
+    match map.get(&key) {
+        Some(_) => {}
+        None => {
+            map.insert(Clone::clone(&key), 1);
+        }
+    }
+}
+
+fn insert_key_to_owned(map: &mut HashMap<String, usize>, key: &str) {
+    match map.get(key) {
+        Some(_) => {}
+        None => {
+            map.insert(key.to_owned(), 1);
+        }
+    }
+}
+
+fn insert_key_to_string(map: &mut HashMap<String, usize>, key: &str) {
+    match map.get(key) {
+        Some(_) => {}
+        None => {
+            map.insert(key.to_string(), 1);
+        }
+    }
+}
+
+fn insert_key_string_from(map: &mut HashMap<String, usize>, key: &str) {
+    match map.get(key) {
+        Some(_) => {}
+        None => {
+            map.insert(String::from(key), 1);
+        }
+    }
+}
+
+fn insert_key_to_vec(map: &mut HashMap<Vec<u8>, usize>, key: &[u8]) {
+    match map.get(key) {
+        Some(_) => {}
+        None => {
+            map.insert(key.to_vec(), 1);
+        }
+    }
+}
+
+// The lookup and the insertion may each add their own borrow/deref layers.
+fn insert_clone_of_deref(map: &mut HashMap<String, usize>, key: &String) {
+    match map.get(key) {
+        Some(_) => {}
+        None => {
+            map.insert((*key).clone(), 1);
+        }
+    }
+}
+
+// An immutable local alias of the cloned key is still the same key.
+fn insert_through_alias(map: &mut HashMap<String, usize>, key: String) {
+    let owned = key.clone();
+    match map.get(&key) {
+        Some(_) => {}
+        None => {
+            map.insert(owned, 1);
+        }
+    }
+}
+
+fn set_insert_clone(set: &mut HashSet<String>, value: String) -> bool {
+    if let Some(existing) = set.get(&value) {
+        existing.is_empty()
+    } else {
+        set.insert(value.clone())
+    }
+}
+
+// ------------- SHOULD NOT FIRE -------------
+
+// `contains_key` followed by `insert` is Clippy's `map_entry` lint.
+fn clippy_handles_contains_key(map: &mut HashMap<String, usize>, key: String) {
+    if !map.contains_key(&key) {
+        map.insert(key, 1);
+    }
+}
+
+// `get(..).is_none()`/`is_some()` on std collections is Clippy's chain:
+// `unnecessary_get_then_check` rewrites the condition to
+// `contains_key`/`contains`, which `map_entry`/`set_contains_or_insert`
+// rewrite again.
+fn clippy_handles_hash_map_get_is_none(map: &mut HashMap<String, usize>, key: String) {
+    if map.get(&key).is_none() {
+        map.insert(key, 1);
+    }
+}
+
+fn clippy_handles_hash_map_get_is_some(map: &mut HashMap<String, usize>, key: String) {
+    if map.get(&key).is_some() {
+        map.insert(key, 2);
+    }
+}
+
+fn clippy_handles_btree_map_get_is_none(map: &mut BTreeMap<String, usize>, key: String) {
+    if map.get(&key).is_none() {
+        map.insert(key, 1);
+    }
+}
+
+fn clippy_handles_hash_set_get_is_none(set: &mut HashSet<String>, value: String) {
+    if set.get(&value).is_none() {
+        set.insert(value);
+    }
+}
+
+fn clippy_handles_btree_set_get_is_none(set: &mut BTreeSet<String>, value: String) {
+    if set.get(&value).is_none() {
+        set.insert(value);
+    }
+}
+
+fn already_uses_entry_or_insert(map: &mut HashMap<String, usize>, key: String) {
+    map.entry(key).or_insert(1);
+}
+
+fn already_uses_entry_or_insert_with(map: &mut HashMap<String, usize>, key: String) {
+    map.entry(key).or_insert_with(expensive);
+}
+
+fn already_uses_entry_or_insert_with_key(map: &mut HashMap<String, usize>, key: String) {
+    map.entry(key).or_insert_with_key(|key| key.len());
+}
+
+fn already_uses_entry_or_default(map: &mut HashMap<String, usize>, key: String) {
+    map.entry(key).or_default();
+}
+
+fn already_uses_entry_and_modify(map: &mut HashMap<String, usize>, key: String) {
+    map.entry(key).and_modify(|value| *value += 1).or_insert(1);
+}
+
+fn already_uses_entry_insert_entry(map: &mut HashMap<String, usize>, key: String) {
+    map.entry(key).insert_entry(2);
+}
+
+fn already_uses_entry_key(map: &mut HashMap<String, usize>, key: String) -> usize {
+    map.entry(key).key().len()
+}
+
+// `btree_map::Entry` offers every method that `hash_map::Entry` does except
+// `insert_entry`, so there is no BTreeMap mirror of that case.
+fn btree_already_uses_entry_or_insert(map: &mut BTreeMap<String, usize>, key: String) {
+    map.entry(key).or_insert(1);
+}
+
+fn btree_already_uses_entry_or_insert_with(map: &mut BTreeMap<String, usize>, key: String) {
+    map.entry(key).or_insert_with(expensive);
+}
+
+fn btree_already_uses_entry_or_insert_with_key(map: &mut BTreeMap<String, usize>, key: String) {
+    map.entry(key).or_insert_with_key(|key| key.len());
+}
+
+fn btree_already_uses_entry_or_default(map: &mut BTreeMap<String, usize>, key: String) {
+    map.entry(key).or_default();
+}
+
+fn btree_already_uses_entry_and_modify(map: &mut BTreeMap<String, usize>, key: String) {
+    map.entry(key).and_modify(|value| *value += 1).or_insert(1);
+}
+
+fn btree_already_uses_entry_key(map: &mut BTreeMap<String, usize>, key: String) -> usize {
+    map.entry(key).key().len()
+}
+
+// `contains` followed by `insert` is Clippy's `set_contains_or_insert` lint.
+fn clippy_handles_set_contains(set: &mut HashSet<String>, value: String) {
+    if !set.contains(&value) {
+        set.insert(value);
+    }
+}
+
+// A bare `insert` already searches only once.
+fn set_insert_alone(set: &mut HashSet<String>, value: String) -> bool {
+    set.insert(value)
+}
+
+fn btree_set_insert_alone(set: &mut BTreeSet<String>, value: String) -> bool {
+    set.insert(value)
+}
+
+fn different_key(map: &mut HashMap<String, usize>, first: String, second: String) {
+    match map.get(&first) {
+        Some(_) => {}
+        None => {
+            map.insert(second, 1);
+        }
+    }
+}
+
+fn different_map(
+    first: &HashMap<String, usize>,
+    second: &mut HashMap<String, usize>,
+    key: String,
+) {
+    match first.get(&key) {
+        Some(_) => {}
+        None => {
+            second.insert(key, 1);
+        }
+    }
+}
+
+fn lookup_without_insert(map: &HashMap<String, usize>, key: &str) -> Option<usize> {
+    map.get(key).copied()
+}
+
+fn insertion_in_deferred_closure(map: &mut HashMap<String, usize>, key: String) {
+    match map.get(&key).copied() {
+        Some(_) => {}
+        None => {
+            let _insert_later = || {
+                map.insert(key, 1);
+            };
+        }
+    }
+}
+
+// `CustomMap` has no `entry` method, so there is no entry API to suggest.
+struct CustomMap;
+
+impl CustomMap {
+    fn get(&self, _key: &str) -> Option<&usize> {
+        None
+    }
+
+    fn insert(&mut self, _key: String, _value: usize) {}
+}
+
+fn non_standard_map(map: &mut CustomMap, key: String) {
+    if map.get(&key).is_none() {
+        map.insert(key, 1);
+    }
+}
+
+fn next_key() -> String {
+    String::new()
+}
+
+fn side_effecting_key(map: &mut HashMap<String, usize>) {
+    match map.get(&next_key()) {
+        Some(_) => {}
+        None => {
+            map.insert(next_key(), 1);
+        }
+    }
+}
+
+fn global_map() -> &'static mut HashMap<String, usize> {
+    unimplemented!()
+}
+
+fn side_effecting_map(key: String) {
+    match global_map().get(&key) {
+        Some(_) => {}
+        None => {
+            global_map().insert(key, 1);
+        }
+    }
+}
+
+// Conversions are peeled by definition, not by name: `DisguisedKey`'s
+// inherent `clone` and `to_owned` are not `Clone::clone`/`ToOwned::to_owned`
+// and may return a different key.
+#[derive(Clone, PartialEq, Eq, Hash)]
+struct DisguisedKey(usize);
+
+impl DisguisedKey {
+    fn clone(&self) -> DisguisedKey {
+        DisguisedKey(self.0 + 1)
+    }
+
+    fn to_owned(&self) -> DisguisedKey {
+        DisguisedKey(self.0 * 2)
+    }
+}
+
+fn inherent_clone_is_a_different_key(map: &mut HashMap<DisguisedKey, usize>, key: DisguisedKey) {
+    match map.get(&key) {
+        Some(_) => {}
+        None => {
+            map.insert(key.clone(), 1);
+        }
+    }
+}
+
+fn inherent_to_owned_is_a_different_key(
+    map: &mut HashMap<DisguisedKey, usize>,
+    key: DisguisedKey,
+) {
+    match map.get(&key) {
+        Some(_) => {}
+        None => {
+            map.insert(key.to_owned(), 1);
+        }
+    }
+}
+
+// A mutable alias may no longer hold the looked-up key by insertion time.
+fn mutated_alias(map: &mut HashMap<String, usize>, key: String) {
+    let mut owned = key.clone();
+    owned.push('!');
+    match map.get(&key) {
+        Some(_) => {}
+        None => {
+            map.insert(owned, 1);
+        }
+    }
+}
+
+// A clone of a different key is a different key.
+fn clone_of_different_key(map: &mut HashMap<String, usize>, first: String, second: String) {
+    match map.get(&first) {
+        Some(_) => {}
+        None => {
+            map.insert(second.clone(), 1);
+        }
+    }
+}
+
+// `to_string` runs an arbitrary `Display` impl; only the string types are
+// guaranteed to format as themselves, so a non-string receiver stays a
+// side-effecting key and is not matched.
+fn to_string_of_non_string_key(map: &mut HashMap<String, usize>, id: u32) {
+    match map.get(&id.to_string()) {
+        Some(_) => {}
+        None => {
+            map.insert(id.to_string(), 1);
+        }
+    }
+}
+
+// Alias resolution must not equate structurally equal but side-effecting
+// initializers: the two `next_key()` calls produce two different keys.
+fn aliases_of_two_generated_keys(map: &mut HashMap<String, usize>) {
+    let first = next_key();
+    let second = next_key();
+    match map.get(&first) {
+        Some(_) => {}
+        None => {
+            map.insert(second, 1);
+        }
+    }
+}
diff --git a/tooling/lints/ui/map_lookup_then_insert.stderr b/tooling/lints/ui/map_lookup_then_insert.stderr
new file mode 100644
index 0000000000..2406471f31
--- /dev/null
+++ b/tooling/lints/ui/map_lookup_then_insert.stderr
@@ -0,0 +1,359 @@
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:12:5
+   |
+LL | /     if let Some(value) = map.get(&key) {
+LL | |         *value
+LL | |     } else {
+LL | |         map.insert(key, 1);
+LL | |         1
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+   = note: `-D map-lookup-then-insert` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(map_lookup_then_insert)]`
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:21:5
+   |
+LL | /     match map.get(&key).copied() {
+LL | |         Some(value) => value,
+LL | |         None => {
+LL | |             map.insert(key, 1);
+...  |
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:32:5
+   |
+LL | /     match map.get_mut(&key) {
+LL | |         Some(value) => *value += 1,
+LL | |         None => {
+LL | |             map.insert(key, 1);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:41:5
+   |
+LL | /     match map.get_key_value(&key) {
+LL | |         Some((_, value)) => *value,
+LL | |         None => {
+LL | |             map.insert(key, 1);
+...  |
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:51:5
+   |
+LL | /     match map.get(&key) {
+LL | |         Some(value) => *value,
+LL | |         None => {
+LL | |             map.insert(key, 1);
+...  |
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:61:5
+   |
+LL | /     if let Some(value) = map.get(&key) {
+LL | |         *value
+LL | |     } else {
+LL | |         map.insert(key, 1);
+LL | |         1
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:71:5
+   |
+LL | /     match map.get_mut(&key) {
+LL | |         Some(value) => *value += 1,
+LL | |         None => {
+LL | |             map.insert(key, 1);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:80:5
+   |
+LL | /     match map.get_key_value(&key) {
+LL | |         Some((_, value)) => *value,
+LL | |         None => {
+LL | |             map.insert(key, 1);
+...  |
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:90:5
+   |
+LL | /     let Some(value) = map.get(&key) else {
+LL | |         map.insert(key, 1);
+LL | |         return 1;
+LL | |     };
+   | |______^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:100:5
+   |
+LL | /     if let Some(_current) = map.get(&key) {
+LL | |         map.insert(key, 2);
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this set lookup and insertion search for the same value twice
+  --> $DIR/map_lookup_then_insert.rs:108:5
+   |
+LL | /     if let Some(existing) = set.get(&value) {
+LL | |         existing.is_empty()
+LL | |     } else {
+LL | |         set.insert(value)
+LL | |     }
+   | |_____^
+   |
+   = help: `insert` alone searches once, skips duplicates, and returns `false` when the value was already present
+
+error: this set lookup and insertion search for the same value twice
+  --> $DIR/map_lookup_then_insert.rs:116:5
+   |
+LL | /     match set.get(&value) {
+LL | |         Some(_) => false,
+LL | |         None => set.insert(value),
+LL | |     }
+   | |_____^
+   |
+   = help: `insert` alone searches once, skips duplicates, and returns `false` when the value was already present
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:123:5
+   |
+LL | /     let Some(value) = map.get(&key) else {
+LL | |         map.insert(key, 1);
+LL | |         return 1;
+LL | |     };
+   | |______^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:136:5
+   |
+LL | /     match map.get(&key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             map.insert(key, expensive());
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:146:5
+   |
+LL | /     match map.get(&key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             let value = key.len();
+...  |
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:157:5
+   |
+LL | /     match map.get(&key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             map.insert(key, usize::default());
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:167:5
+   |
+LL | /     if let Some(_current) = map.get(&key) {
+LL | |         map.insert(key, 2);
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:180:5
+   |
+LL | /     match map.get(&key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             map.insert(key, 1);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:207:5
+   |
+LL | /     if map.get(&key).is_none() {
+LL | |         map.insert(key, 1);
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:217:5
+   |
+LL | /     if let Some(values) = map.get_mut(&key) {
+LL | |         values.push(value);
+LL | |     } else {
+LL | |         map.insert(key.clone(), vec![value]);
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+   = note: the inserted key is an owned copy of the looked-up key; a rewrite that passes the key by value up front would make that copy on hits as well as misses
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:225:5
+   |
+LL | /     match map.get(&key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             map.insert(Clone::clone(&key), 1);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+   = note: the inserted key is an owned copy of the looked-up key; a rewrite that passes the key by value up front would make that copy on hits as well as misses
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:234:5
+   |
+LL | /     match map.get(key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             map.insert(key.to_owned(), 1);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+   = note: the inserted key is an owned copy of the looked-up key; a rewrite that passes the key by value up front would make that copy on hits as well as misses
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:243:5
+   |
+LL | /     match map.get(key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             map.insert(key.to_string(), 1);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+   = note: the inserted key is an owned copy of the looked-up key; a rewrite that passes the key by value up front would make that copy on hits as well as misses
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:252:5
+   |
+LL | /     match map.get(key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             map.insert(String::from(key), 1);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+   = note: the inserted key is an owned copy of the looked-up key; a rewrite that passes the key by value up front would make that copy on hits as well as misses
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:261:5
+   |
+LL | /     match map.get(key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             map.insert(key.to_vec(), 1);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+   = note: the inserted key is an owned copy of the looked-up key; a rewrite that passes the key by value up front would make that copy on hits as well as misses
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:271:5
+   |
+LL | /     match map.get(key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             map.insert((*key).clone(), 1);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+   = note: the inserted key is an owned copy of the looked-up key; a rewrite that passes the key by value up front would make that copy on hits as well as misses
+
+error: this map lookup and insertion search for the same key twice
+  --> $DIR/map_lookup_then_insert.rs:282:5
+   |
+LL | /     match map.get(&key) {
+LL | |         Some(_) => {}
+LL | |         None => {
+LL | |             map.insert(owned, 1);
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+   = help: to search the map only once, use the entry API: https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html
+   = note: the inserted key is an owned copy of the looked-up key; a rewrite that passes the key by value up front would make that copy on hits as well as misses
+
+error: this set lookup and insertion search for the same value twice
+  --> $DIR/map_lookup_then_insert.rs:291:5
+   |
+LL | /     if let Some(existing) = set.get(&value) {
+LL | |         existing.is_empty()
+LL | |     } else {
+LL | |         set.insert(value.clone())
+LL | |     }
+   | |_____^
+   |
+   = help: `insert` alone searches once, skips duplicates, and returns `false` when the value was already present
+   = note: the inserted key is an owned copy of the looked-up key; a rewrite that passes the key by value up front would make that copy on hits as well as misses
+
+error: aborting due to 28 previous errors
+