diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 9d4c2900eaf..4abb1d90f44 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -259,8 +259,8 @@ fn lower_item_kind(
                         body.as_deref(),
                     );
 
-                    let mut itctx = ImplTraitContext::Universal;
-                    let (generics, decl) = this.lower_generics(generics, id, &mut itctx, |this| {
+                    let itctx = ImplTraitContext::Universal;
+                    let (generics, decl) = this.lower_generics(generics, id, &itctx, |this| {
                         let ret_id = asyncness.opt_return_id();
                         this.lower_fn_decl(&decl, id, *fn_sig_span, FnDeclKind::Fn, ret_id)
                     });
@@ -369,9 +369,9 @@ fn lower_item_kind(
                 // method, it will not be considered an in-band
                 // lifetime to be added, but rather a reference to a
                 // parent lifetime.
-                let mut itctx = ImplTraitContext::Universal;
+                let itctx = ImplTraitContext::Universal;
                 let (generics, (trait_ref, lowered_ty)) =
-                    self.lower_generics(ast_generics, id, &mut itctx, |this| {
+                    self.lower_generics(ast_generics, id, &itctx, |this| {
                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
                             this.lower_trait_ref(
                                 trait_ref,
@@ -590,9 +590,9 @@ fn lower_foreign_item(&mut self, i: &ForeignItem) -> &'hir hir::ForeignItem<'hir
             kind: match &i.kind {
                 ForeignItemKind::Fn(box Fn { sig, generics, .. }) => {
                     let fdec = &sig.decl;
-                    let mut itctx = ImplTraitContext::Universal;
+                    let itctx = ImplTraitContext::Universal;
                     let (generics, (fn_dec, fn_args)) =
-                        self.lower_generics(generics, i.id, &mut itctx, |this| {
+                        self.lower_generics(generics, i.id, &itctx, |this| {
                             (
                                 // Disallow `impl Trait` in foreign items.
                                 this.lower_fn_decl(
@@ -1184,8 +1184,8 @@ fn lower_method_sig(
         is_async: Option<(NodeId, Span)>,
     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
         let header = self.lower_fn_header(sig.header);
-        let mut itctx = ImplTraitContext::Universal;
-        let (generics, decl) = self.lower_generics(generics, id, &mut itctx, |this| {
+        let itctx = ImplTraitContext::Universal;
+        let (generics, decl) = self.lower_generics(generics, id, &itctx, |this| {
             this.lower_fn_decl(&sig.decl, id, sig.span, kind, is_async)
         });
         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 3c16780aa90..a089510493e 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -2029,7 +2029,7 @@ fn lower_async_fn_output_type_to_future_bound(
         &mut self,
         output: &FnRetTy,
         span: Span,
-        mut nested_impl_trait_context: ImplTraitContext,
+        nested_impl_trait_context: ImplTraitContext,
     ) -> hir::GenericBound<'hir> {
         // Compute the `T` in `Future<Output = T>` from the return type.
         let output_ty = match output {
@@ -2037,7 +2037,7 @@ fn lower_async_fn_output_type_to_future_bound(
                 // Not `OpaqueTyOrigin::AsyncFn`: that's only used for the
                 // `impl Future` opaque type that `async fn` implicitly
                 // generates.
-                self.lower_ty(ty, &mut nested_impl_trait_context)
+                self.lower_ty(ty, &nested_impl_trait_context)
             }
             FnRetTy::Default(ret_ty_span) => self.arena.alloc(self.ty_tup(*ret_ty_span, &[])),
         };
diff --git a/compiler/rustc_ast_lowering/src/pat.rs b/compiler/rustc_ast_lowering/src/pat.rs
index 16b012630da..06d885a45fb 100644
--- a/compiler/rustc_ast_lowering/src/pat.rs
+++ b/compiler/rustc_ast_lowering/src/pat.rs
@@ -37,7 +37,7 @@ pub(crate) fn lower_pat_mut(&mut self, mut pattern: &Pat) -> hir::Pat<'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
-                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         );
                         let (pats, ddpos) = self.lower_pat_tuple(pats, "tuple struct");
                         break hir::PatKind::TupleStruct(qpath, pats, ddpos);
@@ -53,7 +53,7 @@ pub(crate) fn lower_pat_mut(&mut self, mut pattern: &Pat) -> hir::Pat<'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
-                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         );
                         break hir::PatKind::Path(qpath);
                     }
@@ -63,7 +63,7 @@ pub(crate) fn lower_pat_mut(&mut self, mut pattern: &Pat) -> hir::Pat<'hir> {
                             qself,
                             path,
                             ParamMode::Optional,
-                            &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
                         );
 
                         let fs = self.arena.alloc_from_iter(fields.iter().map(|f| {
diff --git a/compiler/rustc_borrowck/src/type_check/input_output.rs b/compiler/rustc_borrowck/src/type_check/input_output.rs
index 62c6f958137..8fa43f8528c 100644
--- a/compiler/rustc_borrowck/src/type_check/input_output.rs
+++ b/compiler/rustc_borrowck/src/type_check/input_output.rs
@@ -37,31 +37,30 @@ pub(super) fn equate_inputs_and_outputs(
         // those.
         //
         // e.g., `|x: FxHashMap<_, &'static u32>| ...`
-        let user_provided_sig;
-        if !self.tcx().is_closure(mir_def_id.to_def_id()) {
-            user_provided_sig = None;
+        let user_provided_sig = if !self.tcx().is_closure(mir_def_id.to_def_id()) {
+            None
         } else {
             let typeck_results = self.tcx().typeck(mir_def_id);
-            user_provided_sig =
-                typeck_results.user_provided_sigs.get(&mir_def_id).map(|user_provided_poly_sig| {
-                    // Instantiate the canonicalized variables from
-                    // user-provided signature (e.g., the `_` in the code
-                    // above) with fresh variables.
-                    let poly_sig = self.instantiate_canonical_with_fresh_inference_vars(
-                        body.span,
-                        &user_provided_poly_sig,
-                    );
 
-                    // Replace the bound items in the fn sig with fresh
-                    // variables, so that they represent the view from
-                    // "inside" the closure.
-                    self.infcx.replace_bound_vars_with_fresh_vars(
-                        body.span,
-                        LateBoundRegionConversionTime::FnCall,
-                        poly_sig,
-                    )
-                });
-        }
+            typeck_results.user_provided_sigs.get(&mir_def_id).map(|user_provided_poly_sig| {
+                // Instantiate the canonicalized variables from
+                // user-provided signature (e.g., the `_` in the code
+                // above) with fresh variables.
+                let poly_sig = self.instantiate_canonical_with_fresh_inference_vars(
+                    body.span,
+                    &user_provided_poly_sig,
+                );
+
+                // Replace the bound items in the fn sig with fresh
+                // variables, so that they represent the view from
+                // "inside" the closure.
+                self.infcx.replace_bound_vars_with_fresh_vars(
+                    body.span,
+                    LateBoundRegionConversionTime::FnCall,
+                    poly_sig,
+                )
+            })
+        };
 
         debug!(?normalized_input_tys, ?body.local_decls);
 
diff --git a/compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs b/compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs
index 819c2678d6c..3ae6c531b44 100644
--- a/compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs
+++ b/compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs
@@ -509,7 +509,7 @@ pub fn compute_debuginfo_vtable_name<'tcx>(
         visited.clear();
         push_generic_params_internal(tcx, trait_ref.substs, &mut vtable_name, &mut visited);
     } else {
-        vtable_name.push_str("_");
+        vtable_name.push('_');
     }
 
     push_close_angle_bracket(cpp_like_debuginfo, &mut vtable_name);
diff --git a/compiler/rustc_driver/src/lib.rs b/compiler/rustc_driver/src/lib.rs
index 236f66eae72..31e08c44d29 100644
--- a/compiler/rustc_driver/src/lib.rs
+++ b/compiler/rustc_driver/src/lib.rs
@@ -653,7 +653,7 @@ fn print_crate_info(
     for req in &sess.opts.prints {
         match *req {
             TargetList => {
-                let mut targets = rustc_target::spec::TARGETS.iter().copied().collect::<Vec<_>>();
+                let mut targets = rustc_target::spec::TARGETS.to_vec();
                 targets.sort_unstable();
                 println!("{}", targets.join("\n"));
             }
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index 518b5ec10f8..b03352d5fec 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -1212,8 +1212,8 @@ fn must_teach(&mut self, code: &DiagnosticId) -> bool {
         self.taught_diagnostics.insert(code.clone())
     }
 
-    fn force_print_diagnostic(&mut self, mut db: Diagnostic) {
-        self.emitter.emit_diagnostic(&mut db);
+    fn force_print_diagnostic(&mut self, db: Diagnostic) {
+        self.emitter.emit_diagnostic(&db);
     }
 
     /// Emit all stashed diagnostics.
diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
index 320c533a66e..3e6bc81f672 100644
--- a/compiler/rustc_expand/src/mbe/macro_rules.rs
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs
@@ -1166,11 +1166,7 @@ fn check_matcher_core<'tt>(
                                     err.note(&format!(
                                         "{}{} or {}",
                                         msg,
-                                        ts[..ts.len() - 1]
-                                            .iter()
-                                            .copied()
-                                            .collect::<Vec<_>>()
-                                            .join(", "),
+                                        ts[..ts.len() - 1].to_vec().join(", "),
                                         ts[ts.len() - 1],
                                     ));
                                 }
diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
index aedc736b023..4f27068429c 100644
--- a/compiler/rustc_hir_analysis/src/check/wfcheck.rs
+++ b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -410,10 +410,7 @@ fn check_gat_where_clauses(tcx: TyCtxt<'_>, associated_items: &[hir::TraitItemRe
                             tcx,
                             param_env,
                             item_hir_id,
-                            tcx.explicit_item_bounds(item_def_id)
-                                .iter()
-                                .copied()
-                                .collect::<Vec<_>>(),
+                            tcx.explicit_item_bounds(item_def_id).to_vec(),
                             &FxIndexSet::default(),
                             gat_def_id.def_id,
                             gat_generics,
diff --git a/compiler/rustc_lint/src/context.rs b/compiler/rustc_lint/src/context.rs
index 3c9ad410663..a16bb7f1a5f 100644
--- a/compiler/rustc_lint/src/context.rs
+++ b/compiler/rustc_lint/src/context.rs
@@ -355,14 +355,12 @@ pub fn check_lint_name_cmdline(
                     sub: RequestedLevel { level, lint_name },
                 });
             }
-            CheckLintNameResult::Tool(result) => {
-                if let Err((Some(_), new_name)) = result {
-                    sess.emit_warning(CheckNameDeprecated {
-                        lint_name: lint_name.clone(),
-                        new_name,
-                        sub: RequestedLevel { level, lint_name },
-                    });
-                }
+            CheckLintNameResult::Tool(Err((Some(_), new_name))) => {
+                sess.emit_warning(CheckNameDeprecated {
+                    lint_name: lint_name.clone(),
+                    new_name,
+                    sub: RequestedLevel { level, lint_name },
+                });
             }
             CheckLintNameResult::NoTool => {
                 sess.emit_err(CheckNameUnknownTool {
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index a2212a33ade..65beb744cf8 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -1564,7 +1564,7 @@ fn encode_info_for_item(&mut self, def_id: DefId, item: &'tcx hir::Item<'tcx>) {
                 let trait_ref = self.tcx.impl_trait_ref(def_id);
                 if let Some(trait_ref) = trait_ref {
                     let trait_def = self.tcx.trait_def(trait_ref.def_id);
-                    if let Some(mut an) = trait_def.ancestors(self.tcx, def_id).ok() {
+                    if let Ok(mut an) = trait_def.ancestors(self.tcx, def_id) {
                         if let Some(specialization_graph::Node::Impl(parent)) = an.nth(1) {
                             self.tables.impl_parent.set(def_id.index, parent.into());
                         }
diff --git a/compiler/rustc_middle/src/hir/map/mod.rs b/compiler/rustc_middle/src/hir/map/mod.rs
index fe640f185b6..99e692ccb58 100644
--- a/compiler/rustc_middle/src/hir/map/mod.rs
+++ b/compiler/rustc_middle/src/hir/map/mod.rs
@@ -704,12 +704,10 @@ pub fn is_inside_const_context(self, hir_id: HirId) -> bool {
     pub fn get_return_block(self, id: HirId) -> Option<HirId> {
         let mut iter = self.parent_iter(id).peekable();
         let mut ignore_tail = false;
-        if let Some(node) = self.find(id) {
-            if let Node::Expr(Expr { kind: ExprKind::Ret(_), .. }) = node {
-                // When dealing with `return` statements, we don't care about climbing only tail
-                // expressions.
-                ignore_tail = true;
-            }
+        if let Some(Node::Expr(Expr { kind: ExprKind::Ret(_), .. })) = self.find(id) {
+            // When dealing with `return` statements, we don't care about climbing only tail
+            // expressions.
+            ignore_tail = true;
         }
         while let Some((hir_id, node)) = iter.next() {
             if let (Some((_, next_node)), false) = (iter.peek(), ignore_tail) {
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 50e41e3e006..a89e6566d56 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -2894,7 +2894,7 @@ fn pretty_print_const_value<'tcx>(
                 if let Some(contents) = tcx.try_destructure_mir_constant(
                     ty::ParamEnv::reveal_all().and(ConstantKind::Val(ct, ty)),
                 ) {
-                    let fields = contents.fields.iter().copied().collect::<Vec<_>>();
+                    let fields = contents.fields.to_vec();
                     match *ty.kind() {
                         ty::Array(..) => {
                             fmt.write_str("[")?;
diff --git a/compiler/rustc_monomorphize/src/partitioning/mod.rs b/compiler/rustc_monomorphize/src/partitioning/mod.rs
index a55a3fe5cca..4f25fc71314 100644
--- a/compiler/rustc_monomorphize/src/partitioning/mod.rs
+++ b/compiler/rustc_monomorphize/src/partitioning/mod.rs
@@ -305,7 +305,7 @@ fn debug_dump<'a, 'tcx, I>(tcx: TyCtxt<'tcx>, label: &str, cgus: I)
                 );
             }
 
-            let _ = writeln!(s, "");
+            let _ = writeln!(s);
         }
 
         std::mem::take(s)
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index 2e2874dbccb..c59c06ac31e 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -1090,9 +1090,7 @@ fn check_doc_attrs(
                                     errors::DocTestUnknownInclude {
                                         path,
                                         value: value.to_string(),
-                                        inner: (attr.style == AttrStyle::Inner)
-                                            .then_some("!")
-                                            .unwrap_or(""),
+                                        inner: if attr.style == AttrStyle::Inner { "!" } else { "" },
                                         sugg: (attr.meta().unwrap().span, applicability),
                                     }
                                 );
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 9656bfbf4ec..472086eca8f 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -1291,29 +1291,25 @@ fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut
 
         if let ObligationCauseCode::AwaitableExpr(hir_id) = obligation.cause.code().peel_derives() {
             let hir = self.tcx.hir();
-            if let Some(node) = hir_id.and_then(|hir_id| hir.find(hir_id)) {
-                if let hir::Node::Expr(expr) = node {
-                    // FIXME: use `obligation.predicate.kind()...trait_ref.self_ty()` to see if we have `()`
-                    // and if not maybe suggest doing something else? If we kept the expression around we
-                    // could also check if it is an fn call (very likely) and suggest changing *that*, if
-                    // it is from the local crate.
-                    err.span_suggestion(
-                        span,
-                        "remove the `.await`",
-                        "",
-                        Applicability::MachineApplicable,
-                    );
-                    // FIXME: account for associated `async fn`s.
-                    if let hir::Expr { span, kind: hir::ExprKind::Call(base, _), .. } = expr {
-                        if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) =
-                            obligation.predicate.kind().skip_binder()
-                        {
-                            err.span_label(
-                                *span,
-                                &format!("this call returns `{}`", pred.self_ty()),
-                            );
-                        }
-                        if let Some(typeck_results) = &self.typeck_results
+            if let Some(hir::Node::Expr(expr)) = hir_id.and_then(|hir_id| hir.find(hir_id)) {
+                // FIXME: use `obligation.predicate.kind()...trait_ref.self_ty()` to see if we have `()`
+                // and if not maybe suggest doing something else? If we kept the expression around we
+                // could also check if it is an fn call (very likely) and suggest changing *that*, if
+                // it is from the local crate.
+                err.span_suggestion(
+                    span,
+                    "remove the `.await`",
+                    "",
+                    Applicability::MachineApplicable,
+                );
+                // FIXME: account for associated `async fn`s.
+                if let hir::Expr { span, kind: hir::ExprKind::Call(base, _), .. } = expr {
+                    if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) =
+                        obligation.predicate.kind().skip_binder()
+                    {
+                        err.span_label(*span, &format!("this call returns `{}`", pred.self_ty()));
+                    }
+                    if let Some(typeck_results) = &self.typeck_results
                             && let ty = typeck_results.expr_ty_adjusted(base)
                             && let ty::FnDef(def_id, _substs) = ty.kind()
                             && let Some(hir::Node::Item(hir::Item { ident, span, vis_span, .. })) =
@@ -1339,7 +1335,6 @@ fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut
                                 );
                             }
                         }
-                    }
                 }
             }
         }