diff --git a/compiler/rustc_const_eval/src/interpret/eval_context.rs b/compiler/rustc_const_eval/src/interpret/eval_context.rs
index 99218a30e16b1..a74b323c395e6 100644
--- a/compiler/rustc_const_eval/src/interpret/eval_context.rs
+++ b/compiler/rustc_const_eval/src/interpret/eval_context.rs
@@ -285,12 +285,12 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
 
     #[inline(always)]
     pub fn frame(&self) -> &Frame<'tcx, M::Provenance, M::FrameExtra> {
-        self.stack().last().expect("no call frames exist")
+        self.stack().last().expect("a call frame should exist")
     }
 
     #[inline(always)]
     pub fn frame_mut(&mut self) -> &mut Frame<'tcx, M::Provenance, M::FrameExtra> {
-        self.stack_mut().last_mut().expect("no call frames exist")
+        self.stack_mut().last_mut().expect("a call frame should exist")
     }
 
     #[inline(always)]
diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
index 2651ce55281fe..e2dff59a92990 100644
--- a/compiler/rustc_lint/src/unused.rs
+++ b/compiler/rustc_lint/src/unused.rs
@@ -783,6 +783,14 @@ impl UnusedParens {
         }
     }
 
+    /// A pattern that a `macro_rules!` metavariable brings in from the call site keeps the call
+    /// site's syntax context, while the pattern's surroundings keep the macro's. Parentheses in
+    /// such a pattern can be required by the matcher even when they are redundant in the
+    /// expansion, so removing them would break the macro call. (Issue #86959)
+    fn pat_is_macro_arg(pat_span: Span, parent_span: Span) -> bool {
+        !pat_span.eq_ctxt(parent_span)
+    }
+
     fn cast_followed_by_lt(&self, expr: &ast::Expr) -> Option<ast::NodeId> {
         if let ExprKind::Binary(op, lhs, _rhs) = &expr.kind
             && (op.node == ast::BinOpKind::Lt || op.node == ast::BinOpKind::Shl)
@@ -811,7 +819,8 @@ impl EarlyLintPass for UnusedParens {
 
         match e.kind {
             ExprKind::Let(ref pat, _, _, _) | ExprKind::ForLoop(ForLoop { ref pat, .. }) => {
-                self.check_unused_parens_pat(cx, pat, false, false, (true, true));
+                let avoid_or = Self::pat_is_macro_arg(pat.span, e.span);
+                self.check_unused_parens_pat(cx, pat, avoid_or, false, (true, true));
             }
             // We ignore parens in cases like `if (((let Some(0) = Some(1))))` because we already
             // handle a hard error for them during AST lowering in `lower_expr_mut`, but we still
@@ -890,13 +899,15 @@ impl EarlyLintPass for UnusedParens {
             | Err(_) => {}
             // These are list-like patterns; parens can always be removed.
             TupleStruct(_, _, ps) | Tuple(ps) | Slice(ps) | Or(ps) => {
-                for p in ps {
-                    self.check_unused_parens_pat(cx, p, false, false, keep_space);
+                for sub in ps {
+                    let avoid_or = Self::pat_is_macro_arg(sub.span, p.span);
+                    self.check_unused_parens_pat(cx, sub, avoid_or, false, keep_space);
                 }
             }
             Struct(_, _, fps, _) => {
                 for f in fps {
-                    self.check_unused_parens_pat(cx, &f.pat, false, false, keep_space);
+                    let avoid_or = Self::pat_is_macro_arg(f.pat.span, p.span);
+                    self.check_unused_parens_pat(cx, &f.pat, avoid_or, false, keep_space);
                 }
             }
             // Avoid linting on `i @ (p0 | .. | pn)`, #64106.
@@ -938,7 +949,8 @@ impl EarlyLintPass for UnusedParens {
     }
 
     fn check_arm(&mut self, cx: &EarlyContext<'_>, arm: &ast::Arm) {
-        self.check_unused_parens_pat(cx, &arm.pat, false, false, (false, false));
+        let avoid_or = Self::pat_is_macro_arg(arm.pat.span, arm.span);
+        self.check_unused_parens_pat(cx, &arm.pat, avoid_or, false, (false, false));
     }
 
     fn check_ty(&mut self, cx: &EarlyContext<'_>, ty: &ast::Ty) {
diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index 6b74024613dee..109beed7de947 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -205,8 +205,17 @@ pub struct BTreeMap<
 #[stable(feature = "btree_drop", since = "1.7.0")]
 unsafe impl<#[may_dangle] K, #[may_dangle] V, A: AllocatorClone> Drop for BTreeMap<K, V, A> {
     fn drop(&mut self) {
-        // ignore-tidy-undocumented-unsafe
-        drop(unsafe { ptr::read(self) }.into_iter())
+        // Skip `into_iter` for an empty map: `dying_next` is too costly to inline, so the
+        // empty drop isn't optimised away (see #161375).
+        if self.root.is_some() {
+            // SAFETY: `self` is not used after this and none of its fields are dropped again:
+            // `alloc` is `ManuallyDrop` and `root` has no drop glue.
+            drop(unsafe { ptr::read(self) }.into_iter())
+        } else {
+            // SAFETY: With no root there are no nodes to free, so only the allocator needs
+            // dropping. `self` is not used after this, and `alloc` is dropped only here.
+            unsafe { ManuallyDrop::drop(&mut self.alloc) }
+        }
     }
 }
 
diff --git a/tests/codegen-llvm/btree-empty-drop.rs b/tests/codegen-llvm/btree-empty-drop.rs
new file mode 100644
index 0000000000000..f1aaf52e95729
--- /dev/null
+++ b/tests/codegen-llvm/btree-empty-drop.rs
@@ -0,0 +1,15 @@
+// Regression test for https://github.com/rust-lang/rust/issues/161375
+// Checking that an empty BTree's drop is optimized away.
+//@ compile-flags: -Copt-level=3
+
+#![crate_type = "lib"]
+
+use std::collections::BTreeMap;
+
+// CHECK-LABEL: @drop_btree(
+// CHECK-NEXT: {{.*}}:
+// CHECK-NEXT: ret void
+#[no_mangle]
+pub fn drop_btree() {
+    let _ = BTreeMap::<(), ()>::new();
+}
diff --git a/tests/ui/lint/unused-parens-macro-pat-issue-86959.e2018.stderr b/tests/ui/lint/unused-parens-macro-pat-issue-86959.e2018.stderr
new file mode 100644
index 0000000000000..4a04b5b3d1fe2
--- /dev/null
+++ b/tests/ui/lint/unused-parens-macro-pat-issue-86959.e2018.stderr
@@ -0,0 +1,42 @@
+error: unnecessary parentheses around pattern
+  --> $DIR/unused-parens-macro-pat-issue-86959.rs:76:16
+   |
+LL |     match_pat!(('a'));
+   |                ^   ^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-parens-macro-pat-issue-86959.rs:9:9
+   |
+LL | #![deny(unused_parens)]
+   |         ^^^^^^^^^^^^^
+help: remove these parentheses
+   |
+LL -     match_pat!(('a'));
+LL +     match_pat!('a');
+   |
+
+error: unnecessary parentheses around pattern
+  --> $DIR/unused-parens-macro-pat-issue-86959.rs:54:13
+   |
+LL | macro_rules! body_or_pat {
+...
+LL |             ('a' | 'A') => {}
+   |             ^^^^^^^^^^^
+...
+LL |     body_or_pat!();
+   |     -------------- in this macro invocation
+
+error: unnecessary parentheses around pattern
+  --> $DIR/unused-parens-macro-pat-issue-86959.rs:80:9
+   |
+LL |     let (_x) = 1;
+   |         ^  ^
+   |
+help: remove these parentheses
+   |
+LL -     let (_x) = 1;
+LL +     let _x = 1;
+   |
+
+error: aborting due to 3 previous errors
+
diff --git a/tests/ui/lint/unused-parens-macro-pat-issue-86959.e2021.stderr b/tests/ui/lint/unused-parens-macro-pat-issue-86959.e2021.stderr
new file mode 100644
index 0000000000000..4a04b5b3d1fe2
--- /dev/null
+++ b/tests/ui/lint/unused-parens-macro-pat-issue-86959.e2021.stderr
@@ -0,0 +1,42 @@
+error: unnecessary parentheses around pattern
+  --> $DIR/unused-parens-macro-pat-issue-86959.rs:76:16
+   |
+LL |     match_pat!(('a'));
+   |                ^   ^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-parens-macro-pat-issue-86959.rs:9:9
+   |
+LL | #![deny(unused_parens)]
+   |         ^^^^^^^^^^^^^
+help: remove these parentheses
+   |
+LL -     match_pat!(('a'));
+LL +     match_pat!('a');
+   |
+
+error: unnecessary parentheses around pattern
+  --> $DIR/unused-parens-macro-pat-issue-86959.rs:54:13
+   |
+LL | macro_rules! body_or_pat {
+...
+LL |             ('a' | 'A') => {}
+   |             ^^^^^^^^^^^
+...
+LL |     body_or_pat!();
+   |     -------------- in this macro invocation
+
+error: unnecessary parentheses around pattern
+  --> $DIR/unused-parens-macro-pat-issue-86959.rs:80:9
+   |
+LL |     let (_x) = 1;
+   |         ^  ^
+   |
+help: remove these parentheses
+   |
+LL -     let (_x) = 1;
+LL +     let _x = 1;
+   |
+
+error: aborting due to 3 previous errors
+
diff --git a/tests/ui/lint/unused-parens-macro-pat-issue-86959.rs b/tests/ui/lint/unused-parens-macro-pat-issue-86959.rs
new file mode 100644
index 0000000000000..9db969f4635cd
--- /dev/null
+++ b/tests/ui/lint/unused-parens-macro-pat-issue-86959.rs
@@ -0,0 +1,81 @@
+//@ revisions: e2018 e2021
+//@[e2018] edition: 2018
+//@[e2021] edition: 2021
+
+// Parentheses around a pattern handed to a `macro_rules!` metavariable can be required by the
+// matcher even though they are redundant in the expanded pattern, so removing them breaks the
+// macro call. rust-lang/rust#86959
+
+#![deny(unused_parens)]
+
+macro_rules! match_pat {
+    ($p:pat) => {
+        match 'a' {
+            $p => {}
+            _ => {}
+        }
+    };
+}
+
+macro_rules! match_pat_param {
+    ($p:pat_param) => {
+        match 'a' {
+            $p => {}
+            _ => {}
+        }
+    };
+}
+
+macro_rules! if_let_pat {
+    ($p:pat) => {
+        if let $p = 'a' {}
+    };
+}
+
+macro_rules! nested_pat {
+    ($p:pat) => {
+        match Some(1) {
+            Some($p) => {}
+            _ => {}
+        }
+    };
+}
+
+// `body_or_pat!()` breaks the context leg. its a `PatKind::Or`, but the parens and the arm
+// both come from the macro body so they share one `SyntaxContext`, `eq_ctxt` is true and
+// `avoid_or` is false. these are the macro authors own parens, no metavariable involved, so
+// removing them is safe. this is the case that matters most: `from_expansion()` is true here
+// and `!eq_ctxt` is false, so if the gate were ever keyed on "are we inside a macro" instead
+// of "did this cross a context boundary" it would be wrongly silenced. nothing else in the
+// file detects that.
+macro_rules! body_or_pat {
+    () => {
+        match 'a' {
+            ('a' | 'A') => {} //~ ERROR unnecessary parentheses around pattern
+            _ => {}
+        }
+    };
+}
+
+fn main() {
+    // `$p:pat` only accepts a top-level `|` from edition 2021 onwards.
+    match_pat!(('a' | 'A'));
+    // `$p:pat_param` never accepts a top-level `|`, in any edition.
+    match_pat_param!(('a' | 'A'));
+    if_let_pat!(('a' | 'A'));
+    nested_pat!((1 | 2));
+    // A leading `..=` is rejected by both fragment specifiers. rust-lang/rust#120737
+    match_pat!((..='a' | 'z'));
+
+    // `match_pat!(('a'))` breaks the or leg. `avoid_or` is true, since the pattern arrives
+    // through `$p:pat` and its `SyntaxContext` differs from the arms. but the inner pattern is
+    // `PatKind::Expr`, not `PatKind::Or`, so `unused.rs:676` never matches and the lint fires.
+    // the suggestion rewrites the call site to `match_pat!('a')`, which is right — `$p:pat`
+    // takes a bare literal. this guards against widening the gate to all patterns, or moving
+    // the `avoid_or` check above the `match inner.kind`.
+    match_pat!(('a')); //~ ERROR unnecessary parentheses around pattern
+    body_or_pat!();
+    // `let (_x) = 1;` is the no-macro baseline for the `check_expr` call site, one of the four
+    // that went from a hardcoded `false` to a computed value.
+    let (_x) = 1; //~ ERROR unnecessary parentheses around pattern
+}