Rollup merge of #123188 - klensy:clippy-me2, r=Nilstrieb

compiler: fix few unused_peekable and needless_pass_by_ref_mut clippy lints

This fixes few instances of `unused_peekable` and `needless_pass_by_ref_mut`. While i expected to fix more warnings, `needless_pass_by_ref_mut` produced too much for one PR, so i stopped here.

Better reviewed commit by commit, as fixes splitted by chunks.

40 files changed, 111 insertions, 120 deletions

diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 3edb832b9a0..80c62d3fecf 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -1552,7 +1552,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 /// When encountering an equality constraint in a `where` clause, emit an error. If the code seems
 /// like it's setting an associated type, provide an appropriate suggestion.
 fn deny_equality_constraints(
-    this: &mut AstValidator<'_>,
+    this: &AstValidator<'_>,
     predicate: &WhereEqPredicate,
     generics: &Generics,
 ) {
diff --git a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
index 578369de4d6..62e16d445c6 100644
--- a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
@@ -337,7 +337,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     fn suggest_ref_or_clone(
-        &mut self,
+        &self,
         mpi: MovePathIndex,
         move_span: Span,
         err: &mut Diag<'tcx>,
@@ -1125,7 +1125,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     pub(crate) fn report_use_while_mutably_borrowed(
-        &mut self,
+        &self,
         location: Location,
         (place, _span): (Place<'tcx>, Span),
         borrow: &BorrowData<'tcx>,
@@ -1174,7 +1174,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     pub(crate) fn report_conflicting_borrow(
-        &mut self,
+        &self,
         location: Location,
         (place, span): (Place<'tcx>, Span),
         gen_borrow_kind: BorrowKind,
@@ -2463,7 +2463,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     fn report_local_value_does_not_live_long_enough(
-        &mut self,
+        &self,
         location: Location,
         name: &str,
         borrow: &BorrowData<'tcx>,
@@ -2642,7 +2642,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     fn report_thread_local_value_does_not_live_long_enough(
-        &mut self,
+        &self,
         drop_span: Span,
         borrow_span: Span,
     ) -> Diag<'tcx> {
@@ -2663,7 +2663,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     #[instrument(level = "debug", skip(self))]
     fn report_temporary_value_does_not_live_long_enough(
-        &mut self,
+        &self,
         location: Location,
         borrow: &BorrowData<'tcx>,
         drop_span: Span,
@@ -2921,7 +2921,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     #[instrument(level = "debug", skip(self))]
     fn report_escaping_closure_capture(
-        &mut self,
+        &self,
         use_span: UseSpans<'tcx>,
         var_span: Span,
         fr_name: &RegionName,
@@ -3031,7 +3031,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     fn report_escaping_data(
-        &mut self,
+        &self,
         borrow_span: Span,
         name: &Option<String>,
         upvar_span: Span,
@@ -3065,7 +3065,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     }
 
     fn get_moved_indexes(
-        &mut self,
+        &self,
         location: Location,
         mpi: MovePathIndex,
     ) -> (Vec<MoveSite>, Vec<Location>) {
@@ -3854,7 +3854,7 @@ enum AnnotatedBorrowFnSignature<'tcx> {
 impl<'tcx> AnnotatedBorrowFnSignature<'tcx> {
     /// Annotate the provided diagnostic with information about borrow from the fn signature that
     /// helps explain.
-    pub(crate) fn emit(&self, cx: &mut MirBorrowckCtxt<'_, 'tcx>, diag: &mut Diag<'_>) -> String {
+    pub(crate) fn emit(&self, cx: &MirBorrowckCtxt<'_, 'tcx>, diag: &mut Diag<'_>) -> String {
         match self {
             &AnnotatedBorrowFnSignature::Closure { argument_ty, argument_span } => {
                 diag.span_label(
diff --git a/compiler/rustc_borrowck/src/type_check/liveness/polonius.rs b/compiler/rustc_borrowck/src/type_check/liveness/polonius.rs
index 45f7b07fd5f..7f530227043 100644
--- a/compiler/rustc_borrowck/src/type_check/liveness/polonius.rs
+++ b/compiler/rustc_borrowck/src/type_check/liveness/polonius.rs
@@ -87,6 +87,8 @@ pub(super) fn populate_access_facts<'a, 'tcx>(
     body: &Body<'tcx>,
     location_table: &LocationTable,
     move_data: &MoveData<'tcx>,
+    //FIXME: this is not mutated, but expected to be modified as
+    // out param, bug?
     dropped_at: &mut Vec<(Local, Location)>,
 ) {
     debug!("populate_access_facts()");
diff --git a/compiler/rustc_borrowck/src/type_check/liveness/trace.rs b/compiler/rustc_borrowck/src/type_check/liveness/trace.rs
index 18975a4e3b2..8bdefdfc0ac 100644
--- a/compiler/rustc_borrowck/src/type_check/liveness/trace.rs
+++ b/compiler/rustc_borrowck/src/type_check/liveness/trace.rs
@@ -200,7 +200,7 @@ impl<'me, 'typeck, 'flow, 'tcx> LivenessResults<'me, 'typeck, 'flow, 'tcx> {
         for local in boring_locals {
             let local_ty = self.cx.body.local_decls[local].ty;
             let drop_data = self.cx.drop_data.entry(local_ty).or_insert_with({
-                let typeck = &mut self.cx.typeck;
+                let typeck = &self.cx.typeck;
                 move || LivenessContext::compute_drop_data(typeck, local_ty)
             });
 
@@ -542,7 +542,7 @@ impl<'tcx> LivenessContext<'_, '_, '_, 'tcx> {
         );
 
         let drop_data = self.drop_data.entry(dropped_ty).or_insert_with({
-            let typeck = &mut self.typeck;
+            let typeck = &self.typeck;
             move || Self::compute_drop_data(typeck, dropped_ty)
         });
 
@@ -597,10 +597,7 @@ impl<'tcx> LivenessContext<'_, '_, '_, 'tcx> {
         });
     }
 
-    fn compute_drop_data(
-        typeck: &mut TypeChecker<'_, 'tcx>,
-        dropped_ty: Ty<'tcx>,
-    ) -> DropData<'tcx> {
+    fn compute_drop_data(typeck: &TypeChecker<'_, 'tcx>, dropped_ty: Ty<'tcx>) -> DropData<'tcx> {
         debug!("compute_drop_data(dropped_ty={:?})", dropped_ty,);
 
         match typeck
diff --git a/compiler/rustc_builtin_macros/src/asm.rs b/compiler/rustc_builtin_macros/src/asm.rs
index 76805617c93..137ac441579 100644
--- a/compiler/rustc_builtin_macros/src/asm.rs
+++ b/compiler/rustc_builtin_macros/src/asm.rs
@@ -29,7 +29,7 @@ pub struct AsmArgs {
 }
 
 fn parse_args<'a>(
-    ecx: &mut ExtCtxt<'a>,
+    ecx: &ExtCtxt<'a>,
     sp: Span,
     tts: TokenStream,
     is_global_asm: bool,
@@ -303,7 +303,7 @@ pub fn parse_asm_args<'a>(
 ///
 /// This function must be called immediately after the option token is parsed.
 /// Otherwise, the suggestion will be incorrect.
-fn err_duplicate_option(p: &mut Parser<'_>, symbol: Symbol, span: Span) {
+fn err_duplicate_option(p: &Parser<'_>, symbol: Symbol, span: Span) {
     // Tool-only output
     let full_span = if p.token.kind == token::Comma { span.to(p.token.span) } else { span };
     p.psess.dcx.emit_err(errors::AsmOptAlreadyprovided { span, symbol, full_span });
@@ -315,7 +315,7 @@ fn err_duplicate_option(p: &mut Parser<'_>, symbol: Symbol, span: Span) {
 /// This function must be called immediately after the option token is parsed.
 /// Otherwise, the error will not point to the correct spot.
 fn try_set_option<'a>(
-    p: &mut Parser<'a>,
+    p: &Parser<'a>,
     args: &mut AsmArgs,
     symbol: Symbol,
     option: ast::InlineAsmOptions,
diff --git a/compiler/rustc_builtin_macros/src/assert.rs b/compiler/rustc_builtin_macros/src/assert.rs
index 5905bdd7108..d200179f3a0 100644
--- a/compiler/rustc_builtin_macros/src/assert.rs
+++ b/compiler/rustc_builtin_macros/src/assert.rs
@@ -111,7 +111,7 @@ fn expr_if_not(
     cx.expr_if(span, cx.expr(span, ExprKind::Unary(UnOp::Not, cond)), then, els)
 }
 
-fn parse_assert<'a>(cx: &mut ExtCtxt<'a>, sp: Span, stream: TokenStream) -> PResult<'a, Assert> {
+fn parse_assert<'a>(cx: &ExtCtxt<'a>, sp: Span, stream: TokenStream) -> PResult<'a, Assert> {
     let mut parser = cx.new_parser_from_tts(stream);
 
     if parser.token == token::Eof {
diff --git a/compiler/rustc_builtin_macros/src/cfg.rs b/compiler/rustc_builtin_macros/src/cfg.rs
index 9197b9ebdf9..5dc9bbacd06 100644
--- a/compiler/rustc_builtin_macros/src/cfg.rs
+++ b/compiler/rustc_builtin_macros/src/cfg.rs
@@ -35,7 +35,7 @@ pub fn expand_cfg(
     })
 }
 
-fn parse_cfg<'a>(cx: &mut ExtCtxt<'a>, span: Span, tts: TokenStream) -> PResult<'a, ast::MetaItem> {
+fn parse_cfg<'a>(cx: &ExtCtxt<'a>, span: Span, tts: TokenStream) -> PResult<'a, ast::MetaItem> {
     let mut p = cx.new_parser_from_tts(tts);
 
     if p.token == token::Eof {
diff --git a/compiler/rustc_builtin_macros/src/cfg_accessible.rs b/compiler/rustc_builtin_macros/src/cfg_accessible.rs
index 1933b2e1fb7..98c0ca3a526 100644
--- a/compiler/rustc_builtin_macros/src/cfg_accessible.rs
+++ b/compiler/rustc_builtin_macros/src/cfg_accessible.rs
@@ -10,7 +10,7 @@ use rustc_span::Span;
 
 pub(crate) struct Expander;
 
-fn validate_input<'a>(ecx: &mut ExtCtxt<'_>, mi: &'a ast::MetaItem) -> Option<&'a ast::Path> {
+fn validate_input<'a>(ecx: &ExtCtxt<'_>, mi: &'a ast::MetaItem) -> Option<&'a ast::Path> {
     use errors::CfgAccessibleInvalid::*;
     match mi.meta_item_list() {
         None => {}
diff --git a/compiler/rustc_builtin_macros/src/concat_bytes.rs b/compiler/rustc_builtin_macros/src/concat_bytes.rs
index a2f827c5567..45fec294578 100644
--- a/compiler/rustc_builtin_macros/src/concat_bytes.rs
+++ b/compiler/rustc_builtin_macros/src/concat_bytes.rs
@@ -8,7 +8,7 @@ use crate::errors;
 
 /// Emits errors for literal expressions that are invalid inside and outside of an array.
 fn invalid_type_err(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     token_lit: token::Lit,
     span: Span,
     is_nested: bool,
@@ -65,7 +65,7 @@ fn invalid_type_err(
 /// Otherwise, returns `None`, and either pushes the `expr`'s span to `missing_literals` or
 /// updates `guar` accordingly.
 fn handle_array_element(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     guar: &mut Option<ErrorGuaranteed>,
     missing_literals: &mut Vec<rustc_span::Span>,
     expr: &P<rustc_ast::Expr>,
diff --git a/compiler/rustc_builtin_macros/src/deriving/bounds.rs b/compiler/rustc_builtin_macros/src/deriving/bounds.rs
index 8027ca2e7bb..26ef3da3a91 100644
--- a/compiler/rustc_builtin_macros/src/deriving/bounds.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/bounds.rs
@@ -6,7 +6,7 @@ use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::Span;
 
 pub fn expand_deriving_copy(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
@@ -29,7 +29,7 @@ pub fn expand_deriving_copy(
 }
 
 pub fn expand_deriving_const_param_ty(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
diff --git a/compiler/rustc_builtin_macros/src/deriving/clone.rs b/compiler/rustc_builtin_macros/src/deriving/clone.rs
index 267405ac32e..0a44bd42b91 100644
--- a/compiler/rustc_builtin_macros/src/deriving/clone.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/clone.rs
@@ -9,7 +9,7 @@ use rustc_span::Span;
 use thin_vec::{thin_vec, ThinVec};
 
 pub fn expand_deriving_clone(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
@@ -94,7 +94,7 @@ pub fn expand_deriving_clone(
 
 fn cs_clone_simple(
     name: &str,
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
     is_union: bool,
@@ -157,14 +157,14 @@ fn cs_clone_simple(
 
 fn cs_clone(
     name: &str,
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
 ) -> BlockOrExpr {
     let ctor_path;
     let all_fields;
     let fn_path = cx.std_path(&[sym::clone, sym::Clone, sym::clone]);
-    let subcall = |cx: &mut ExtCtxt<'_>, field: &FieldInfo| {
+    let subcall = |cx: &ExtCtxt<'_>, field: &FieldInfo| {
         let args = thin_vec![field.self_expr.clone()];
         cx.expr_call_global(field.span, fn_path.clone(), args)
     };
diff --git a/compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs
index ce3fa1ab32c..45c4467a109 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs
@@ -10,7 +10,7 @@ use rustc_span::Span;
 use thin_vec::{thin_vec, ThinVec};
 
 pub fn expand_deriving_eq(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
@@ -49,7 +49,7 @@ pub fn expand_deriving_eq(
 }
 
 fn cs_total_eq_assert(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
 ) -> BlockOrExpr {
diff --git a/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
index 0923acfeedd..1d7a69540ab 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs
@@ -8,7 +8,7 @@ use rustc_span::Span;
 use thin_vec::thin_vec;
 
 pub fn expand_deriving_ord(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
@@ -39,7 +39,7 @@ pub fn expand_deriving_ord(
     trait_def.expand(cx, mitem, item, push)
 }
 
-pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
+pub fn cs_cmp(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
     let test_id = Ident::new(sym::cmp, span);
     let equal_path = cx.path_global(span, cx.std_path(&[sym::cmp, sym::Ordering, sym::Equal]));
     let cmp_path = cx.std_path(&[sym::cmp, sym::Ord, sym::cmp]);
diff --git a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs
index 006110cd4b1..234918ae429 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs
@@ -9,14 +9,14 @@ use rustc_span::Span;
 use thin_vec::thin_vec;
 
 pub fn expand_deriving_partial_eq(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
     push: &mut dyn FnMut(Annotatable),
     is_const: bool,
 ) {
-    fn cs_eq(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
+    fn cs_eq(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
         let base = true;
         let expr = cs_fold(
             true, // use foldl
diff --git a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
index 60dbdf8b544..49fe89b18b0 100644
--- a/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs
@@ -8,7 +8,7 @@ use rustc_span::Span;
 use thin_vec::thin_vec;
 
 pub fn expand_deriving_partial_ord(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
@@ -69,7 +69,7 @@ pub fn expand_deriving_partial_ord(
 }
 
 fn cs_partial_cmp(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     substr: &Substructure<'_>,
     tag_then_data: bool,
diff --git a/compiler/rustc_builtin_macros/src/deriving/debug.rs b/compiler/rustc_builtin_macros/src/deriving/debug.rs
index 03acd7f489f..e442b3520b2 100644
--- a/compiler/rustc_builtin_macros/src/deriving/debug.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/debug.rs
@@ -9,7 +9,7 @@ use rustc_span::Span;
 use thin_vec::{thin_vec, ThinVec};
 
 pub fn expand_deriving_debug(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
@@ -45,7 +45,7 @@ pub fn expand_deriving_debug(
     trait_def.expand(cx, mitem, item, push)
 }
 
-fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
+fn show_substructure(cx: &ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
     // We want to make sure we have the ctxt set so that we can use unstable methods
     let span = cx.with_def_site_ctxt(span);
 
@@ -209,7 +209,7 @@ fn show_substructure(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>
 /// }
 /// ```
 fn show_fieldless_enum(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     def: &EnumDef,
     substr: &Substructure<'_>,
diff --git a/compiler/rustc_builtin_macros/src/deriving/decodable.rs b/compiler/rustc_builtin_macros/src/deriving/decodable.rs
index bf4693cd541..34798ab0a17 100644
--- a/compiler/rustc_builtin_macros/src/deriving/decodable.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/decodable.rs
@@ -11,7 +11,7 @@ use rustc_span::Span;
 use thin_vec::{thin_vec, ThinVec};
 
 pub fn expand_deriving_rustc_decodable(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
@@ -63,7 +63,7 @@ pub fn expand_deriving_rustc_decodable(
 }
 
 fn decodable_substructure(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
     krate: Symbol,
@@ -186,14 +186,14 @@ fn decodable_substructure(
 /// - `outer_pat_path` is the path to this enum variant/struct
 /// - `getarg` should retrieve the `usize`-th field with name `@str`.
 fn decode_static_fields<F>(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     outer_pat_path: ast::Path,
     fields: &StaticFields,
     mut getarg: F,
 ) -> P<Expr>
 where
-    F: FnMut(&mut ExtCtxt<'_>, Span, Symbol, usize) -> P<Expr>,
+    F: FnMut(&ExtCtxt<'_>, Span, Symbol, usize) -> P<Expr>,
 {
     match fields {
         Unnamed(fields, is_tuple) => {
diff --git a/compiler/rustc_builtin_macros/src/deriving/default.rs b/compiler/rustc_builtin_macros/src/deriving/default.rs
index 292a916e2a7..328770ce10d 100644
--- a/compiler/rustc_builtin_macros/src/deriving/default.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/default.rs
@@ -13,7 +13,7 @@ use smallvec::SmallVec;
 use thin_vec::{thin_vec, ThinVec};
 
 pub fn expand_deriving_default(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &ast::MetaItem,
     item: &Annotatable,
@@ -54,7 +54,7 @@ pub fn expand_deriving_default(
 }
 
 fn default_struct_substructure(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
     summary: &StaticFields,
@@ -81,7 +81,7 @@ fn default_struct_substructure(
 }
 
 fn default_enum_substructure(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     enum_def: &EnumDef,
 ) -> BlockOrExpr {
@@ -103,7 +103,7 @@ fn default_enum_substructure(
 }
 
 fn extract_default_variant<'a>(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     enum_def: &'a EnumDef,
     trait_span: Span,
 ) -> Result<&'a rustc_ast::Variant, ErrorGuaranteed> {
@@ -173,7 +173,7 @@ fn extract_default_variant<'a>(
 }
 
 fn validate_default_attribute(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     default_variant: &rustc_ast::Variant,
 ) -> Result<(), ErrorGuaranteed> {
     let attrs: SmallVec<[_; 1]> =
diff --git a/compiler/rustc_builtin_macros/src/deriving/encodable.rs b/compiler/rustc_builtin_macros/src/deriving/encodable.rs
index d939f8c7aeb..2e5f1173825 100644
--- a/compiler/rustc_builtin_macros/src/deriving/encodable.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/encodable.rs
@@ -95,7 +95,7 @@ use rustc_span::Span;
 use thin_vec::{thin_vec, ThinVec};
 
 pub fn expand_deriving_rustc_encodable(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
@@ -147,7 +147,7 @@ pub fn expand_deriving_rustc_encodable(
 }
 
 fn encodable_substructure(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
     krate: Symbol,
diff --git a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
index afa73b672da..e16d74eed4e 100644
--- a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
@@ -330,7 +330,7 @@ pub enum SubstructureFields<'a> {
 /// Combine the values of all the fields together. The last argument is
 /// all the fields of all the structures.
 pub type CombineSubstructureFunc<'a> =
-    Box<dyn FnMut(&mut ExtCtxt<'_>, Span, &Substructure<'_>) -> BlockOrExpr + 'a>;
+    Box<dyn FnMut(&ExtCtxt<'_>, Span, &Substructure<'_>) -> BlockOrExpr + 'a>;
 
 pub fn combine_substructure(
     f: CombineSubstructureFunc<'_>,
@@ -454,7 +454,7 @@ fn find_type_parameters(
 impl<'a> TraitDef<'a> {
     pub fn expand(
         self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         mitem: &ast::MetaItem,
         item: &'a Annotatable,
         push: &mut dyn FnMut(Annotatable),
@@ -464,7 +464,7 @@ impl<'a> TraitDef<'a> {
 
     pub fn expand_ext(
         self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         mitem: &ast::MetaItem,
         item: &'a Annotatable,
         push: &mut dyn FnMut(Annotatable),
@@ -577,7 +577,7 @@ impl<'a> TraitDef<'a> {
     /// therefore does not get bound by the derived trait.
     fn create_derived_impl(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         type_ident: Ident,
         generics: &Generics,
         field_tys: Vec<P<ast::Ty>>,
@@ -802,7 +802,7 @@ impl<'a> TraitDef<'a> {
 
     fn expand_struct_def(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         struct_def: &'a VariantData,
         type_ident: Ident,
         generics: &Generics,
@@ -856,7 +856,7 @@ impl<'a> TraitDef<'a> {
 
     fn expand_enum_def(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         enum_def: &'a EnumDef,
         type_ident: Ident,
         generics: &Generics,
@@ -914,7 +914,7 @@ impl<'a> TraitDef<'a> {
 impl<'a> MethodDef<'a> {
     fn call_substructure_method(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         type_ident: Ident,
         nonselflike_args: &[P<Expr>],
@@ -929,7 +929,7 @@ impl<'a> MethodDef<'a> {
 
     fn get_ret_ty(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         generics: &Generics,
         type_ident: Ident,
@@ -950,7 +950,7 @@ impl<'a> MethodDef<'a> {
     //   `&self`.
     fn extract_arg_details(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         type_ident: Ident,
         generics: &Generics,
@@ -986,7 +986,7 @@ impl<'a> MethodDef<'a> {
 
     fn create_method(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         type_ident: Ident,
         generics: &Generics,
@@ -1077,7 +1077,7 @@ impl<'a> MethodDef<'a> {
     /// ```
     fn expand_struct_method_body<'b>(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'b>,
         struct_def: &'b VariantData,
         type_ident: Ident,
@@ -1100,7 +1100,7 @@ impl<'a> MethodDef<'a> {
 
     fn expand_static_struct_method_body(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         struct_def: &VariantData,
         type_ident: Ident,
@@ -1154,7 +1154,7 @@ impl<'a> MethodDef<'a> {
     /// `Unify`), and possibly a default arm.
     fn expand_enum_method_body<'b>(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'b>,
         enum_def: &'b EnumDef,
         type_ident: Ident,
@@ -1403,7 +1403,7 @@ impl<'a> MethodDef<'a> {
 
     fn expand_static_enum_method_body(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         trait_: &TraitDef<'_>,
         enum_def: &EnumDef,
         type_ident: Ident,
@@ -1430,7 +1430,7 @@ impl<'a> MethodDef<'a> {
 
 // general helper methods.
 impl<'a> TraitDef<'a> {
-    fn summarise_struct(&self, cx: &mut ExtCtxt<'_>, struct_def: &VariantData) -> StaticFields {
+    fn summarise_struct(&self, cx: &ExtCtxt<'_>, struct_def: &VariantData) -> StaticFields {
         let mut named_idents = Vec::new();
         let mut just_spans = Vec::new();
         for field in struct_def.fields() {
@@ -1460,7 +1460,7 @@ impl<'a> TraitDef<'a> {
 
     fn create_struct_patterns(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         struct_path: ast::Path,
         struct_def: &'a VariantData,
         prefixes: &[String],
@@ -1553,7 +1553,7 @@ impl<'a> TraitDef<'a> {
 
     fn create_struct_pattern_fields(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         struct_def: &'a VariantData,
         prefixes: &[String],
     ) -> Vec<FieldInfo> {
@@ -1570,7 +1570,7 @@ impl<'a> TraitDef<'a> {
 
     fn create_struct_field_access_fields(
         &self,
-        cx: &mut ExtCtxt<'_>,
+        cx: &ExtCtxt<'_>,
         selflike_args: &[P<Expr>],
         struct_def: &'a VariantData,
         is_packed: bool,
@@ -1668,13 +1668,13 @@ pub enum CsFold<'a> {
 /// Statics may not be folded over.
 pub fn cs_fold<F>(
     use_foldl: bool,
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     trait_span: Span,
     substructure: &Substructure<'_>,
     mut f: F,
 ) -> P<Expr>
 where
-    F: FnMut(&mut ExtCtxt<'_>, CsFold<'_>) -> P<Expr>,
+    F: FnMut(&ExtCtxt<'_>, CsFold<'_>) -> P<Expr>,
 {
     match substructure.fields {
         EnumMatching(.., all_fields) | Struct(_, all_fields) => {
diff --git a/compiler/rustc_builtin_macros/src/deriving/hash.rs b/compiler/rustc_builtin_macros/src/deriving/hash.rs
index dd6149cf614..6bb61311bd2 100644
--- a/compiler/rustc_builtin_macros/src/deriving/hash.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/hash.rs
@@ -8,7 +8,7 @@ use rustc_span::Span;
 use thin_vec::thin_vec;
 
 pub fn expand_deriving_hash(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &MetaItem,
     item: &Annotatable,
@@ -46,11 +46,7 @@ pub fn expand_deriving_hash(
     hash_trait_def.expand(cx, mitem, item, push);
 }
 
-fn hash_substructure(
-    cx: &mut ExtCtxt<'_>,
-    trait_span: Span,
-    substr: &Substructure<'_>,
-) -> BlockOrExpr {
+fn hash_substructure(cx: &ExtCtxt<'_>, trait_span: Span, substr: &Substructure<'_>) -> BlockOrExpr {
     let [state_expr] = substr.nonselflike_args else {
         cx.dcx().span_bug(trait_span, "incorrect number of arguments in `derive(Hash)`");
     };
diff --git a/compiler/rustc_builtin_macros/src/deriving/mod.rs b/compiler/rustc_builtin_macros/src/deriving/mod.rs
index 8a3375cba9d..9f786d22c93 100644
--- a/compiler/rustc_builtin_macros/src/deriving/mod.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/mod.rs
@@ -40,7 +40,7 @@ pub mod partial_ord;
 pub mod generic;
 
 pub(crate) type BuiltinDeriveFn =
-    fn(&mut ExtCtxt<'_>, Span, &MetaItem, &Annotatable, &mut dyn FnMut(Annotatable), bool);
+    fn(&ExtCtxt<'_>, Span, &MetaItem, &Annotatable, &mut dyn FnMut(Annotatable), bool);
 
 pub(crate) struct BuiltinDerive(pub(crate) BuiltinDeriveFn);
 
@@ -117,7 +117,7 @@ fn call_unreachable(cx: &ExtCtxt<'_>, span: Span) -> P<ast::Expr> {
 }
 
 fn assert_ty_bounds(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     stmts: &mut ThinVec<ast::Stmt>,
     ty: P<ast::Ty>,
     span: Span,
diff --git a/compiler/rustc_builtin_macros/src/edition_panic.rs b/compiler/rustc_builtin_macros/src/edition_panic.rs
index fa22e911642..bb3c83e8c0e 100644
--- a/compiler/rustc_builtin_macros/src/edition_panic.rs
+++ b/compiler/rustc_builtin_macros/src/edition_panic.rs
@@ -40,7 +40,7 @@ pub fn expand_unreachable<'cx>(
 
 fn expand<'cx>(
     mac: rustc_span::Symbol,
-    cx: &'cx mut ExtCtxt<'_>,
+    cx: &'cx ExtCtxt<'_>,
     sp: Span,
     tts: TokenStream,
 ) -> MacroExpanderResult<'cx> {
diff --git a/compiler/rustc_builtin_macros/src/format.rs b/compiler/rustc_builtin_macros/src/format.rs
index 6f031f270ca..51d6058a744 100644
--- a/compiler/rustc_builtin_macros/src/format.rs
+++ b/compiler/rustc_builtin_macros/src/format.rs
@@ -65,7 +65,7 @@ struct MacroInput {
 /// ```text
 /// Ok((fmtstr, parsed arguments))
 /// ```
-fn parse_args<'a>(ecx: &mut ExtCtxt<'a>, sp: Span, tts: TokenStream) -> PResult<'a, MacroInput> {
+fn parse_args<'a>(ecx: &ExtCtxt<'a>, sp: Span, tts: TokenStream) -> PResult<'a, MacroInput> {
     let mut args = FormatArguments::new();
 
     let mut p = ecx.new_parser_from_tts(tts);
@@ -604,7 +604,7 @@ fn invalid_placeholder_type_error(
 }
 
 fn report_missing_placeholders(
-    ecx: &mut ExtCtxt<'_>,
+    ecx: &ExtCtxt<'_>,
     unused: Vec<(Span, bool)>,
     used: &[bool],
     args: &FormatArguments,
@@ -734,7 +734,7 @@ fn report_missing_placeholders(
 /// This function detects and reports unused format!() arguments that are
 /// redundant due to implicit captures (e.g. `format!("{x}", x)`).
 fn report_redundant_format_arguments<'a>(
-    ecx: &mut ExtCtxt<'a>,
+    ecx: &ExtCtxt<'a>,
     args: &FormatArguments,
     used: &[bool],
     placeholders: Vec<(Span, &str)>,
@@ -806,7 +806,7 @@ fn report_redundant_format_arguments<'a>(
 /// there are named arguments or numbered positional arguments in the
 /// format string.
 fn report_invalid_references(
-    ecx: &mut ExtCtxt<'_>,
+    ecx: &ExtCtxt<'_>,
     invalid_refs: &[(usize, Option<Span>, PositionUsedAs, FormatArgPositionKind)],
     template: &[FormatArgsPiece],
     fmt_span: Span,
diff --git a/compiler/rustc_builtin_macros/src/source_util.rs b/compiler/rustc_builtin_macros/src/source_util.rs
index abcdfabcaed..61a6361ae8d 100644
--- a/compiler/rustc_builtin_macros/src/source_util.rs
+++ b/compiler/rustc_builtin_macros/src/source_util.rs
@@ -234,7 +234,7 @@ pub fn expand_include_bytes(
 }
 
 fn load_binary_file(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     original_path: &Path,
     macro_span: Span,
     path_span: Span,
diff --git a/compiler/rustc_builtin_macros/src/test.rs b/compiler/rustc_builtin_macros/src/test.rs
index 81ac78dd58f..c7568f1461c 100644
--- a/compiler/rustc_builtin_macros/src/test.rs
+++ b/compiler/rustc_builtin_macros/src/test.rs
@@ -96,7 +96,7 @@ pub fn expand_bench(
 }
 
 pub fn expand_test_or_bench(
-    cx: &mut ExtCtxt<'_>,
+    cx: &ExtCtxt<'_>,
     attr_sp: Span,
     item: Annotatable,
     is_bench: bool,
diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index c8b8594c0dd..7c7f702b2c3 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -2089,14 +2089,14 @@ fn add_rpath_args(
                     .map(|(path, _)| &**path)
             })
             .collect::<Vec<_>>();
-        let mut rpath_config = RPathConfig {
+        let rpath_config = RPathConfig {
             libs: &*libs,
             out_filename: out_filename.to_path_buf(),
             has_rpath: sess.target.has_rpath,
             is_like_osx: sess.target.is_like_osx,
             linker_is_gnu: sess.target.linker_flavor.is_gnu(),
         };
-        cmd.args(&rpath::get_rpath_flags(&mut rpath_config));
+        cmd.args(&rpath::get_rpath_flags(&rpath_config));
     }
 }
 
diff --git a/compiler/rustc_codegen_ssa/src/back/rpath.rs b/compiler/rustc_codegen_ssa/src/back/rpath.rs
index 60346228625..ebbf49af184 100644
--- a/compiler/rustc_codegen_ssa/src/back/rpath.rs
+++ b/compiler/rustc_codegen_ssa/src/back/rpath.rs
@@ -12,7 +12,7 @@ pub struct RPathConfig<'a> {
     pub linker_is_gnu: bool,
 }
 
-pub fn get_rpath_flags(config: &mut RPathConfig<'_>) -> Vec<OsString> {
+pub fn get_rpath_flags(config: &RPathConfig<'_>) -> Vec<OsString> {
     // No rpath on windows
     if !config.has_rpath {
         return Vec::new();
@@ -52,7 +52,7 @@ fn rpaths_to_flags(rpaths: Vec<OsString>) -> Vec<OsString> {
     ret
 }
 
-fn get_rpaths(config: &mut RPathConfig<'_>) -> Vec<OsString> {
+fn get_rpaths(config: &RPathConfig<'_>) -> Vec<OsString> {
     debug!("output: {:?}", config.out_filename.display());
     debug!("libs:");
     for libpath in config.libs {
@@ -73,11 +73,11 @@ fn get_rpaths(config: &mut RPathConfig<'_>) -> Vec<OsString> {
     minimize_rpaths(&rpaths)
 }
 
-fn get_rpaths_relative_to_output(config: &mut RPathConfig<'_>) -> Vec<OsString> {
+fn get_rpaths_relative_to_output(config: &RPathConfig<'_>) -> Vec<OsString> {
     config.libs.iter().map(|a| get_rpath_relative_to_output(config, a)).collect()
 }
 
-fn get_rpath_relative_to_output(config: &mut RPathConfig<'_>, lib: &Path) -> OsString {
+fn get_rpath_relative_to_output(config: &RPathConfig<'_>, lib: &Path) -> OsString {
     // Mac doesn't appear to support $ORIGIN
     let prefix = if config.is_like_osx { "@loader_path" } else { "$ORIGIN" };
 
diff --git a/compiler/rustc_expand/src/expand.rs b/compiler/rustc_expand/src/expand.rs
index cac1e8f80e3..6029caa965c 100644
--- a/compiler/rustc_expand/src/expand.rs
+++ b/compiler/rustc_expand/src/expand.rs
@@ -871,7 +871,7 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         let mut parser = self.cx.new_parser_from_tts(toks);
         match parse_ast_fragment(&mut parser, kind) {
             Ok(fragment) => {
-                ensure_complete_parse(&mut parser, path, kind.name(), span);
+                ensure_complete_parse(&parser, path, kind.name(), span);
                 fragment
             }
             Err(mut err) => {
@@ -958,7 +958,7 @@ pub fn parse_ast_fragment<'a>(
 }
 
 pub fn ensure_complete_parse<'a>(
-    parser: &mut Parser<'a>,
+    parser: &Parser<'a>,
     macro_path: &ast::Path,
     kind_name: &str,
     span: Span,
diff --git a/compiler/rustc_infer/src/infer/snapshot/fudge.rs b/compiler/rustc_infer/src/infer/snapshot/fudge.rs
index 14de461cd17..f8f1c1b4c45 100644
--- a/compiler/rustc_infer/src/infer/snapshot/fudge.rs
+++ b/compiler/rustc_infer/src/infer/snapshot/fudge.rs
@@ -13,7 +13,7 @@ use ut::UnifyKey;
 use std::ops::Range;
 
 fn vars_since_snapshot<'tcx, T>(
-    table: &mut UnificationTable<'_, 'tcx, T>,
+    table: &UnificationTable<'_, 'tcx, T>,
     snapshot_var_len: usize,
 ) -> Range<T>
 where
@@ -124,11 +124,11 @@ impl<'tcx> InferCtxt<'tcx> {
                     let type_vars =
                         inner.type_variables().vars_since_snapshot(variable_lengths.type_var_len);
                     let int_vars = vars_since_snapshot(
-                        &mut inner.int_unification_table(),
+                        &inner.int_unification_table(),
                         variable_lengths.int_var_len,
                     );
                     let float_vars = vars_since_snapshot(
-                        &mut inner.float_unification_table(),
+                        &inner.float_unification_table(),
                         variable_lengths.float_var_len,
                     );
                     let region_vars = inner
diff --git a/compiler/rustc_mir_build/src/build/matches/mod.rs b/compiler/rustc_mir_build/src/build/matches/mod.rs
index b4c98834d0a..f8aaf629131 100644
--- a/compiler/rustc_mir_build/src/build/matches/mod.rs
+++ b/compiler/rustc_mir_build/src/build/matches/mod.rs
@@ -1593,10 +1593,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     /// [`Switch`]: TestKind::Switch
     /// [`SwitchInt`]: TestKind::SwitchInt
     /// [`Range`]: TestKind::Range
-    fn pick_test(
-        &mut self,
-        candidates: &mut [&mut Candidate<'_, 'tcx>],
-    ) -> (Place<'tcx>, Test<'tcx>) {
+    fn pick_test(&mut self, candidates: &[&mut Candidate<'_, 'tcx>]) -> (Place<'tcx>, Test<'tcx>) {
         // Extract the match-pair from the highest priority candidate
         let match_pair = &candidates.first().unwrap().match_pairs[0];
         let test = self.test(match_pair);
diff --git a/compiler/rustc_mir_transform/src/coroutine.rs b/compiler/rustc_mir_transform/src/coroutine.rs
index f0a13f66555..c3f175f150d 100644
--- a/compiler/rustc_mir_transform/src/coroutine.rs
+++ b/compiler/rustc_mir_transform/src/coroutine.rs
@@ -1226,7 +1226,7 @@ fn create_coroutine_drop_shim<'tcx>(
     tcx: TyCtxt<'tcx>,
     transform: &TransformVisitor<'tcx>,
     coroutine_ty: Ty<'tcx>,
-    body: &mut Body<'tcx>,
+    body: &Body<'tcx>,
     drop_clean: BasicBlock,
 ) -> Body<'tcx> {
     let mut body = body.clone();
diff --git a/compiler/rustc_mir_transform/src/nrvo.rs b/compiler/rustc_mir_transform/src/nrvo.rs
index c3a92911bbf..232c290e0fb 100644
--- a/compiler/rustc_mir_transform/src/nrvo.rs
+++ b/compiler/rustc_mir_transform/src/nrvo.rs
@@ -84,7 +84,7 @@ impl<'tcx> MirPass<'tcx> for RenameReturnPlace {
 ///
 /// If the MIR fulfills both these conditions, this function returns the `Local` that is assigned
 /// to the return place along all possible paths through the control-flow graph.
-fn local_eligible_for_nrvo(body: &mut mir::Body<'_>) -> Option<Local> {
+fn local_eligible_for_nrvo(body: &mir::Body<'_>) -> Option<Local> {
     if IsReturnPlaceRead::run(body) {
         return None;
     }
@@ -118,10 +118,7 @@ fn local_eligible_for_nrvo(body: &mut mir::Body<'_>) -> Option<Local> {
     copied_to_return_place
 }
 
-fn find_local_assigned_to_return_place(
-    start: BasicBlock,
-    body: &mut mir::Body<'_>,
-) -> Option<Local> {
+fn find_local_assigned_to_return_place(start: BasicBlock, body: &mir::Body<'_>) -> Option<Local> {
     let mut block = start;
     let mut seen = BitSet::new_empty(body.basic_blocks.len());
 
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index f612e8b5b1a..4bf0e51905f 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -2010,7 +2010,7 @@ pub fn parse_crate_edition(early_dcx: &EarlyDiagCtxt, matches: &getopts::Matches
 }
 
 fn check_error_format_stability(
-    early_dcx: &mut EarlyDiagCtxt,
+    early_dcx: &EarlyDiagCtxt,
     unstable_opts: &UnstableOptions,
     error_format: ErrorOutputType,
 ) {
@@ -2108,7 +2108,7 @@ fn should_override_cgus_and_disable_thinlto(
 fn collect_print_requests(
     early_dcx: &EarlyDiagCtxt,
     cg: &mut CodegenOptions,
-    unstable_opts: &mut UnstableOptions,
+    unstable_opts: &UnstableOptions,
     matches: &getopts::Matches,
 ) -> Vec<PrintRequest> {
     let mut prints = Vec::<PrintRequest>::new();
@@ -2574,7 +2574,7 @@ fn parse_remap_path_prefix(
 }
 
 fn parse_logical_env(
-    early_dcx: &mut EarlyDiagCtxt,
+    early_dcx: &EarlyDiagCtxt,
     matches: &getopts::Matches,
 ) -> FxIndexMap<String, String> {
     let mut vars = FxIndexMap::default();
@@ -2732,6 +2732,8 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
     }
 
     if let Ok(graphviz_font) = std::env::var("RUSTC_GRAPHVIZ_FONT") {
+        // FIXME: this is only mutation of UnstableOptions here, move into
+        // UnstableOptions::build?
         unstable_opts.graphviz_font = graphviz_font;
     }
 
@@ -2781,7 +2783,7 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
         ));
     }
 
-    let prints = collect_print_requests(early_dcx, &mut cg, &mut unstable_opts, matches);
+    let prints = collect_print_requests(early_dcx, &mut cg, &unstable_opts, matches);
 
     let cg = cg;
 
diff --git a/compiler/rustc_session/src/utils.rs b/compiler/rustc_session/src/utils.rs
index 50ebbdccf67..58de5cb31a5 100644
--- a/compiler/rustc_session/src/utils.rs
+++ b/compiler/rustc_session/src/utils.rs
@@ -127,7 +127,7 @@ pub fn extra_compiler_flags() -> Option<(Vec<String>, bool)> {
 
     const ICE_REPORT_COMPILER_FLAGS_STRIP_VALUE: &[&str] = &["incremental"];
 
-    let mut args = std::env::args_os().map(|arg| arg.to_string_lossy().to_string()).peekable();
+    let mut args = std::env::args_os().map(|arg| arg.to_string_lossy().to_string());
 
     let mut result = Vec::new();
     let mut excluded_cargo_defaults = false;
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 28f4f81e7d8..fe2691e9d4d 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -4931,7 +4931,7 @@ fn point_at_assoc_type_restriction<G: EmissionGuarantee>(
         let hir::WherePredicate::BoundPredicate(pred) = pred else {
             continue;
         };
-        let mut bounds = pred.bounds.iter().peekable();
+        let mut bounds = pred.bounds.iter();
         while let Some(bound) = bounds.next() {
             let Some(trait_ref) = bound.trait_ref() else {
                 continue;
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 12371155303..c33e24b1094 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -508,7 +508,7 @@ pub(super) fn opt_normalize_projection_type<'a, 'b, 'tcx>(
 /// because it contains `[type error]`. Yuck! (See issue #29857 for
 /// one case where this arose.)
 fn normalize_to_error<'a, 'tcx>(
-    selcx: &mut SelectionContext<'a, 'tcx>,
+    selcx: &SelectionContext<'a, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     projection_ty: ty::AliasTy<'tcx>,
     cause: ObligationCause<'tcx>,
diff --git a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
index 27dd8f26489..43c750ebbb5 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
@@ -239,10 +239,10 @@ fn fulfill_implication<'tcx>(
 
     let source_trait = ImplSubject::Trait(source_trait_ref);
 
-    let selcx = &mut SelectionContext::new(infcx);
+    let selcx = SelectionContext::new(infcx);
     let target_args = infcx.fresh_args_for_item(DUMMY_SP, target_impl);
     let (target_trait, obligations) =
-        util::impl_subject_and_oblig(selcx, param_env, target_impl, target_args, error_cause);
+        util::impl_subject_and_oblig(&selcx, param_env, target_impl, target_args, error_cause);
 
     // do the impls unify? If not, no specialization.
     let Ok(InferOk { obligations: more_obligations, .. }) = infcx
diff --git a/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs b/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
index 1aa65b87f0b..dba014d58b0 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
@@ -198,7 +198,7 @@ impl<'tcx> Children {
     }
 }
 
-fn iter_children(children: &mut Children) -> impl Iterator<Item = DefId> + '_ {
+fn iter_children(children: &Children) -> impl Iterator<Item = DefId> + '_ {
     let nonblanket = children.non_blanket_impls.iter().flat_map(|(_, v)| v.iter());
     children.blanket_impls.iter().chain(nonblanket).cloned()
 }
diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
index 29d063321a7..466a53d8ce0 100644
--- a/compiler/rustc_trait_selection/src/traits/util.rs
+++ b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -205,7 +205,7 @@ impl Iterator for SupertraitDefIds<'_> {
 /// returning the resulting subject and all obligations that arise.
 /// The obligations are closed under normalization.
 pub fn impl_subject_and_oblig<'a, 'tcx>(
-    selcx: &mut SelectionContext<'a, 'tcx>,
+    selcx: &SelectionContext<'a, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     impl_def_id: DefId,
     impl_args: GenericArgsRef<'tcx>,