don't use .into() to convert types into identical types.

example:
    let s: String = format!("hello").into();

src/librustc/mir/interpret/allocation.rs

@@ -472,7 +472,7 @@ impl<'tcx, Tag: Copy, Extra: AllocationExtra<Tag>> Allocation<Tag, Extra> {
         val: ScalarMaybeUndef<Tag>,
     ) -> InterpResult<'tcx> {
         let ptr_size = cx.data_layout().pointer_size;
-        self.write_scalar(cx, ptr.into(), val, ptr_size)
+        self.write_scalar(cx, ptr, val, ptr_size)
     }
 }
 

src/librustc/mir/mod.rs

@@ -1519,7 +1519,7 @@ impl<'tcx> TerminatorKind<'tcx> {
                 values
                     .iter()
                     .map(|&u| {
-                        ty::Const::from_scalar(tcx, Scalar::from_uint(u, size).into(), switch_ty)
+                        ty::Const::from_scalar(tcx, Scalar::from_uint(u, size), switch_ty)
                             .to_string()
                             .into()
                     })

src/librustc/mir/tcx.rs

@@ -156,7 +156,7 @@ impl<'tcx> Rvalue<'tcx> {
             }
             Rvalue::AddressOf(mutability, ref place) => {
                 let place_ty = place.ty(local_decls, tcx).ty;
-                tcx.mk_ptr(ty::TypeAndMut { ty: place_ty, mutbl: mutability.into() })
+                tcx.mk_ptr(ty::TypeAndMut { ty: place_ty, mutbl: mutability })
             }
             Rvalue::Len(..) => tcx.types.usize,
             Rvalue::Cast(.., ty) => ty,

src/librustc/traits/mod.rs

@@ -820,8 +820,7 @@ impl ObjectSafetyViolation {
                 MethodViolationCode::UndispatchableReceiver,
                 span,
             ) => (
-                format!("consider changing method `{}`'s `self` parameter to be `&self`", name)
-                    .into(),
+                format!("consider changing method `{}`'s `self` parameter to be `&self`", name),
                 Some(("&Self".to_string(), span)),
             ),
             ObjectSafetyViolation::AssocConst(name, _)

src/librustc_ast_lowering/expr.rs

@@ -831,8 +831,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 .last()
                 .cloned()
                 .map(|id| Ok(self.lower_node_id(id)))
-                .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope))
-                .into(),
+                .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
         };
         hir::Destination { label: destination.map(|(_, label)| label), target_id }
     }
@@ -841,7 +840,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         if self.is_in_loop_condition && opt_label.is_none() {
             hir::Destination {
                 label: None,
-                target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
+                target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
             }
         } else {
             self.lower_loop_destination(opt_label.map(|label| (id, label)))
@@ -912,7 +911,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 .collect(),
             asm: asm.asm,
             asm_str_style: asm.asm_str_style,
-            clobbers: asm.clobbers.clone().into(),
+            clobbers: asm.clobbers.clone(),
             volatile: asm.volatile,
             alignstack: asm.alignstack,
             dialect: asm.dialect,

src/librustc_codegen_ssa/mir/operand.rs

@@ -92,8 +92,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                 let a = Scalar::from(Pointer::new(
                     bx.tcx().alloc_map.lock().create_memory_alloc(data),
                     Size::from_bytes(start as u64),
-                ))
-                .into();
+                ));
                 let a_llval = bx.scalar_to_backend(
                     a,
                     a_scalar,

src/librustc_codegen_ssa/mir/rvalue.rs

@@ -387,7 +387,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
 
             mir::Rvalue::AddressOf(mutability, ref place) => {
                 let mk_ptr = move |tcx: TyCtxt<'tcx>, ty: Ty<'tcx>| {
-                    tcx.mk_ptr(ty::TypeAndMut { ty, mutbl: mutability.into() })
+                    tcx.mk_ptr(ty::TypeAndMut { ty, mutbl: mutability })
                 };
                 self.codegen_place_to_pointer(bx, place, mk_ptr)
             }

src/librustc_expand/mbe/quoted.rs

@@ -112,7 +112,7 @@ fn parse_tree(
                     sess.span_diagnostic.span_err(span.entire(), &msg);
                 }
                 // Parse the contents of the sequence itself
-                let sequence = parse(tts.into(), expect_matchers, sess);
+                let sequence = parse(tts, expect_matchers, sess);
                 // Get the Kleene operator and optional separator
                 let (separator, kleene) = parse_sep_and_kleene_op(trees, span.entire(), sess);
                 // Count the number of captured "names" (i.e., named metavars)
@@ -159,7 +159,7 @@ fn parse_tree(
         // descend into the delimited set and further parse it.
         tokenstream::TokenTree::Delimited(span, delim, tts) => TokenTree::Delimited(
             span,
-            Lrc::new(Delimited { delim, tts: parse(tts.into(), expect_matchers, sess) }),
+            Lrc::new(Delimited { delim, tts: parse(tts, expect_matchers, sess) }),
         ),
     }
 }

src/librustc_expand/mbe/transcribe.rs

@@ -155,8 +155,7 @@ pub(super) fn transcribe(
                     }
 
                     // Step back into the parent Delimited.
-                    let tree =
-                        TokenTree::Delimited(span, forest.delim, TokenStream::new(result).into());
+                    let tree = TokenTree::Delimited(span, forest.delim, TokenStream::new(result));
                     result = result_stack.pop().unwrap();
                     result.push(tree.into());
                 }

src/librustc_expand/proc_macro_server.rs

@@ -60,7 +60,7 @@ impl FromInternal<(TreeAndJoint, &'_ ParseSess, &'_ mut Vec<Self>)>
         let Token { kind, span } = match tree {
             tokenstream::TokenTree::Delimited(span, delim, tts) => {
                 let delimiter = Delimiter::from_internal(delim);
-                return TokenTree::Group(Group { delimiter, stream: tts.into(), span });
+                return TokenTree::Group(Group { delimiter, stream: tts, span });
             }
             tokenstream::TokenTree::Token(token) => token,
         };
@@ -196,12 +196,8 @@ impl ToInternal<TokenStream> for TokenTree<Group, Punct, Ident, Literal> {
         let (ch, joint, span) = match self {
             TokenTree::Punct(Punct { ch, joint, span }) => (ch, joint, span),
             TokenTree::Group(Group { delimiter, stream, span }) => {
-                return tokenstream::TokenTree::Delimited(
-                    span,
-                    delimiter.to_internal(),
-                    stream.into(),
-                )
-                .into();
+                return tokenstream::TokenTree::Delimited(span, delimiter.to_internal(), stream)
+                    .into();
             }
             TokenTree::Ident(self::Ident { sym, is_raw, span }) => {
                 return tokenstream::TokenTree::token(Ident(sym, is_raw), span).into();

src/librustc_infer/infer/canonical/canonicalizer.rs

@@ -669,7 +669,7 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
         } else {
             let var = self.canonical_var(info, const_var.into());
             self.tcx().mk_const(ty::Const {
-                val: ty::ConstKind::Bound(self.binder_index, var.into()),
+                val: ty::ConstKind::Bound(self.binder_index, var),
                 ty: self.fold_ty(const_var.ty),
             })
         }

src/librustc_infer/infer/outlives/verify.rs

@@ -140,7 +140,6 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         // Extend with bounds that we can find from the trait.
         let trait_bounds = self
             .projection_declared_bounds_from_trait(projection_ty)
-            .into_iter()
             .map(|r| VerifyBound::OutlivedBy(r));
 
         // see the extensive comment in projection_must_outlive

src/librustc_infer/traits/select.rs

@@ -3202,7 +3202,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     obligation.predicate.def_id(),
                     obligation.recursion_depth + 1,
                     a_last.expect_ty(),
-                    &[b_last.into()],
+                    &[b_last],
                 ));
             }
 

src/librustc_interface/util.rs

@@ -639,7 +639,7 @@ impl<'a, 'b> ReplaceBodyWithLoop<'a, 'b> {
                                     ast::GenericArg::Type(ty) => Some(ty),
                                     _ => None,
                                 });
-                                any_involves_impl_trait(types.into_iter())
+                                any_involves_impl_trait(types)
                                     || data.constraints.iter().any(|c| match c.kind {
                                         ast::AssocTyConstraintKind::Bound { .. } => true,
                                         ast::AssocTyConstraintKind::Equality { ref ty } => {

src/librustc_mir/borrow_check/region_infer/reverse_sccs.rs

@@ -56,7 +56,7 @@ impl RegionInferenceContext<'_> {
         let mut scc_regions = FxHashMap::default();
         let mut start = 0;
         for (scc, group) in &paired_scc_regions.into_iter().group_by(|(scc, _)| *scc) {
-            let group_size = group.into_iter().count();
+            let group_size = group.count();
             scc_regions.insert(scc, start..start + group_size);
             start += group_size;
         }

src/librustc_mir/const_eval/mod.rs

@@ -52,7 +52,7 @@ pub(crate) fn const_caller_location<'tcx>(
 
     let loc_place = ecx.alloc_caller_location(file, line, col);
     intern_const_alloc_recursive(&mut ecx, InternKind::Constant, loc_place, false).unwrap();
-    ConstValue::Scalar(loc_place.ptr.into())
+    ConstValue::Scalar(loc_place.ptr)
 }
 
 // this function uses `unwrap` copiously, because an already validated constant

src/librustc_mir/interpret/intrinsics.rs

@@ -67,7 +67,7 @@ crate fn eval_nullary_intrinsic<'tcx>(
             };
             ConstValue::from_machine_usize(n, &tcx)
         }
-        sym::type_id => ConstValue::from_u64(tcx.type_id_hash(tp_ty).into()),
+        sym::type_id => ConstValue::from_u64(tcx.type_id_hash(tp_ty)),
         other => bug!("`{}` is not a zero arg intrinsic", other),
     })
 }

src/librustc_mir/interpret/terminator.rs

@@ -293,7 +293,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                                 let (&untuple_arg, args) = args.split_last().unwrap();
                                 trace!("eval_fn_call: Will pass last argument by untupling");
                                 Cow::from(args.iter().map(|&a| Ok(a))
-                                .chain((0..untuple_arg.layout.fields.count()).into_iter()
+                                .chain((0..untuple_arg.layout.fields.count())
                                     .map(|i| self.operand_field(untuple_arg, i as u64))
                                 )
                                 .collect::<InterpResult<'_, Vec<OpTy<'tcx, M::PointerTag>>>>()?)

src/librustc_mir/transform/mod.rs

@@ -209,7 +209,7 @@ fn mir_const_qualif(tcx: TyCtxt<'_>, def_id: DefId) -> ConstQualifs {
 
     // We return the qualifs in the return place for every MIR body, even though it is only used
     // when deciding to promote a reference to a `const` for now.
-    validator.qualifs_in_return_place().into()
+    validator.qualifs_in_return_place()
 }
 
 fn mir_const(tcx: TyCtxt<'_>, def_id: DefId) -> &Steal<BodyAndCache<'_>> {

src/librustc_mir/util/aggregate.rs

@@ -49,7 +49,6 @@ pub fn expand_aggregate<'tcx>(
     };
 
     operands
-        .into_iter()
         .enumerate()
         .map(move |(i, (op, ty))| {
             let lhs_field = if let AggregateKind::Array(_) = kind {

src/librustc_mir_build/build/matches/mod.rs

@@ -1942,8 +1942,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let tcx = self.hir.tcx();
         let debug_source_info = SourceInfo { span: source_info.span, scope: visibility_scope };
         let binding_mode = match mode {
-            BindingMode::ByValue => ty::BindingMode::BindByValue(mutability.into()),
-            BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability.into()),
+            BindingMode::ByValue => ty::BindingMode::BindByValue(mutability),
+            BindingMode::ByRef(_) => ty::BindingMode::BindByReference(mutability),
         };
         debug!("declare_binding: user_ty={:?}", user_ty);
         let local = LocalDecl::<'tcx> {

src/librustc_mir_build/build/mod.rs

@@ -882,7 +882,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                             span: tcx_hir.span(var_id),
                         },
                         place: Place {
-                            local: closure_env_arg.into(),
+                            local: closure_env_arg,
                             projection: tcx.intern_place_elems(&projs),
                         },
                     });
@@ -927,7 +927,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         self.local_decls[local].local_info = if let Some(kind) = self_binding {
                             LocalInfo::User(ClearCrossCrate::Set(BindingForm::ImplicitSelf(*kind)))
                         } else {
-                            let binding_mode = ty::BindingMode::BindByValue(mutability.into());
+                            let binding_mode = ty::BindingMode::BindByValue(mutability);
                             LocalInfo::User(ClearCrossCrate::Set(BindingForm::Var(
                                 VarBindingForm {
                                     binding_mode,

src/librustc_parse/lexer/mod.rs

@@ -327,8 +327,7 @@ impl<'a> StringReader<'a> {
         match kind {
             rustc_lexer::LiteralKind::Char { terminated } => {
                 if !terminated {
-                    self.fatal_span_(start, suffix_start, "unterminated character literal".into())
-                        .raise()
+                    self.fatal_span_(start, suffix_start, "unterminated character literal").raise()
                 }
                 let content_start = start + BytePos(1);
                 let content_end = suffix_start - BytePos(1);
@@ -338,12 +337,8 @@ impl<'a> StringReader<'a> {
             }
             rustc_lexer::LiteralKind::Byte { terminated } => {
                 if !terminated {
-                    self.fatal_span_(
-                        start + BytePos(1),
-                        suffix_start,
-                        "unterminated byte constant".into(),
-                    )
-                    .raise()
+                    self.fatal_span_(start + BytePos(1), suffix_start, "unterminated byte constant")
+                        .raise()
                 }
                 let content_start = start + BytePos(2);
                 let content_end = suffix_start - BytePos(1);
@@ -353,7 +348,7 @@ impl<'a> StringReader<'a> {
             }
             rustc_lexer::LiteralKind::Str { terminated } => {
                 if !terminated {
-                    self.fatal_span_(start, suffix_start, "unterminated double quote string".into())
+                    self.fatal_span_(start, suffix_start, "unterminated double quote string")
                         .raise()
                 }
                 let content_start = start + BytePos(1);
@@ -367,7 +362,7 @@ impl<'a> StringReader<'a> {
                     self.fatal_span_(
                         start + BytePos(1),
                         suffix_start,
-                        "unterminated double quote byte string".into(),
+                        "unterminated double quote byte string",
                     )
                     .raise()
                 }

src/librustc_parse/lexer/tokentrees.rs

@@ -212,7 +212,7 @@ impl<'a> TokenTreesReader<'a> {
                     _ => {}
                 }
 
-                Ok(TokenTree::Delimited(delim_span, delim, tts.into()).into())
+                Ok(TokenTree::Delimited(delim_span, delim, tts).into())
             }
             token::CloseDelim(delim) => {
                 // An unexpected closing delimiter (i.e., there is no

src/librustc_parse/lib.rs

@@ -420,7 +420,7 @@ fn prepend_attrs(
         builder.push(tokenstream::TokenTree::Delimited(
             delim_span,
             token::DelimToken::Bracket,
-            brackets.build().into(),
+            brackets.build(),
         ));
     }
     builder.push(tokens.clone());

src/librustc_parse/parser/mod.rs

@@ -263,8 +263,7 @@ impl TokenCursor {
             ]
             .iter()
             .cloned()
-            .collect::<TokenStream>()
-            .into(),
+            .collect::<TokenStream>(),
         );
 
         self.stack.push(mem::replace(
@@ -389,7 +388,7 @@ impl<'a> Parser<'a> {
             root_module_name: None,
             expected_tokens: Vec::new(),
             token_cursor: TokenCursor {
-                frame: TokenCursorFrame::new(DelimSpan::dummy(), token::NoDelim, &tokens.into()),
+                frame: TokenCursorFrame::new(DelimSpan::dummy(), token::NoDelim, &tokens),
                 stack: Vec::new(),
             },
             desugar_doc_comments,
@@ -1006,7 +1005,7 @@ impl<'a> Parser<'a> {
                 );
                 self.set_token(Token::new(TokenKind::CloseDelim(frame.delim), frame.span.close));
                 self.bump();
-                TokenTree::Delimited(frame.span, frame.delim, frame.tree_cursor.stream.into())
+                TokenTree::Delimited(frame.span, frame.delim, frame.tree_cursor.stream)
             }
             token::CloseDelim(_) | token::Eof => unreachable!(),
             _ => {

src/librustc_parse/parser/stmt.rs

@@ -169,7 +169,7 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_local_mk(&mut self, lo: Span, attrs: AttrVec) -> PResult<'a, Stmt> {
-        let local = self.parse_local(attrs.into())?;
+        let local = self.parse_local(attrs)?;
         Ok(self.mk_stmt(lo.to(self.prev_span), StmtKind::Local(local)))
     }
 

src/librustc_passes/loops.rs

@@ -77,7 +77,7 @@ impl<'a, 'hir> Visitor<'hir> for CheckLoopVisitor<'a, 'hir> {
                     return;
                 }
 
-                let loop_id = match label.target_id.into() {
+                let loop_id = match label.target_id {
                     Ok(loop_id) => loop_id,
                     Err(hir::LoopIdError::OutsideLoopScope) => hir::DUMMY_HIR_ID,
                     Err(hir::LoopIdError::UnlabeledCfInWhileCondition) => {

src/librustc_traits/generic_types.rs

@@ -24,7 +24,6 @@ crate fn fn_ptr(
 ) -> Ty<'tcx> {
     let inputs_and_output = tcx.mk_type_list(
         (0..arity_and_output)
-            .into_iter()
             .map(|i| ty::BoundVar::from(i))
             // DebruijnIndex(1) because we are going to inject these in a `PolyFnSig`
             .map(|var| tcx.mk_ty(ty::Bound(ty::DebruijnIndex::from(1usize), var.into()))),
@@ -37,7 +36,6 @@ crate fn fn_ptr(
 crate fn type_list(tcx: TyCtxt<'tcx>, arity: usize) -> SubstsRef<'tcx> {
     tcx.mk_substs(
         (0..arity)
-            .into_iter()
             .map(|i| ty::BoundVar::from(i))
             .map(|var| tcx.mk_ty(ty::Bound(ty::INNERMOST, var.into())))
             .map(|ty| GenericArg::from(ty)),

src/librustc_typeck/outlives/implicit_infer.rs

@@ -312,6 +312,6 @@ pub fn check_explicit_predicates<'tcx>(
 
         let predicate = outlives_predicate.subst(tcx, substs);
         debug!("predicate = {:?}", &predicate);
-        insert_outlives_predicate(tcx, predicate.0.into(), predicate.1, span, required_predicates);
+        insert_outlives_predicate(tcx, predicate.0, predicate.1, span, required_predicates);
     }
 }

src/libstd/sys/unix/process/process_common.rs

@@ -287,9 +287,7 @@ impl CStringArray {
 
 fn construct_envp(env: BTreeMap<OsString, OsString>, saw_nul: &mut bool) -> CStringArray {
     let mut result = CStringArray::with_capacity(env.len());
-    for (k, v) in env {
-        let mut k: OsString = k.into();
-
+    for (mut k, v) in env {
         // Reserve additional space for '=' and null terminator
         k.reserve_exact(v.len() + 2);
         k.push("=");

src/libsyntax/attr/mod.rs

@@ -529,7 +529,7 @@ impl MetaItemKind {
                     TokenTree::Delimited(
                         DelimSpan::from_single(span),
                         token::Paren,
-                        TokenStream::new(tokens).into(),
+                        TokenStream::new(tokens),
                     )
                     .into(),
                 ]