[Experimental] <T as Into<T>>::into lint

Running crater to see how common that pattern is. The Lint would have to be at most warn-by-default because there are a handful of cases detected that are actually perfectly reasonable (`type` aliases with per-platform `cfg`, or macros) which are now at best half-heartedly handled.

I've detected a handful of cases where we're calling `.into()` unnecessarily in the `rustc` codebase as well, and changed those.

compiler/rustc_ast/src/ast.rs

@@ -3464,7 +3464,7 @@ impl From<ForeignItemKind> for ItemKind {
  fn from(foreign_item_kind: ForeignItemKind) -> ItemKind {
  match foreign_item_kind {
  ForeignItemKind::Static(box static_foreign_item) => {
- ItemKind::Static(Box::new(static_foreign_item.into()))
+ ItemKind::Static(Box::new(static_foreign_item))
  }
  ForeignItemKind::Fn(fn_kind) => ItemKind::Fn(fn_kind),
  ForeignItemKind::TyAlias(ty_alias_kind) => ItemKind::TyAlias(ty_alias_kind),
@@ -3478,9 +3478,7 @@ impl TryFrom<ItemKind> for ForeignItemKind {
 
  fn try_from(item_kind: ItemKind) -> Result<ForeignItemKind, ItemKind> {
  Ok(match item_kind {
- ItemKind::Static(box static_item) => {
- ForeignItemKind::Static(Box::new(static_item.into()))
- }
+ ItemKind::Static(box static_item) => ForeignItemKind::Static(Box::new(static_item)),
  ItemKind::Fn(fn_kind) => ForeignItemKind::Fn(fn_kind),
  ItemKind::TyAlias(ty_alias_kind) => ForeignItemKind::TyAlias(ty_alias_kind),
  ItemKind::MacCall(a) => ForeignItemKind::MacCall(a),

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -78,7 +78,7 @@ fn eval_body_using_ecx<'tcx, R: InterpretationResult<'tcx>>(
  ecx.push_stack_frame_raw(
  cid.instance,
  body,
- &ret.clone().into(),
+ &ret.clone(),
  StackPopCleanup::Root { cleanup: false },
  )?;
  ecx.storage_live_for_always_live_locals()?;

compiler/rustc_const_eval/src/interpret/call.rs

@@ -854,7 +854,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
  (Abi::Rust, fn_abi),
  &[FnArg::Copy(arg.into())],
  false,
- &ret.into(),
+ &ret,
  Some(target),
  unwind,
  )

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -2533,7 +2533,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  other_generic_param.name.ident() == generic_param.name.ident()
  },
  ) {
- idxs_matched.push(other_idx.into());
+ idxs_matched.push(other_idx);
  }
 
  if idxs_matched.is_empty() {

compiler/rustc_lint/messages.ftl

@@ -733,6 +733,8 @@ lint_reserved_prefix = prefix `{$prefix}` is unknown
  .label = unknown prefix
  .suggestion = insert whitespace here to avoid this being parsed as a prefix in Rust 2021
 
+lint_self_type_conversion = this conversion is useless `{$source}` to `{$target}`
+
 lint_shadowed_into_iter =
  this method call resolves to `<&{$target} as IntoIterator>::into_iter` (due to backwards compatibility), but will resolve to `<{$target} as IntoIterator>::into_iter` in Rust {$edition}
  .use_iter_suggestion = use `.iter()` instead of `.into_iter()` to avoid ambiguity

compiler/rustc_lint/src/builtin.rs

@@ -66,7 +66,7 @@ use crate::lints::{
  BuiltinTrivialBounds, BuiltinTypeAliasBounds, BuiltinUngatedAsyncFnTrackCaller,
  BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub,
  BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub,
- BuiltinWhileTrue, InvalidAsmLabel,
+ BuiltinWhileTrue, InvalidAsmLabel, SelfTypeConversionDiag,
 };
 use crate::nonstandard_style::{method_context, MethodLateContext};
 use crate::{
@@ -1604,6 +1604,7 @@ declare_lint_pass!(
  SoftLints => [
  WHILE_TRUE,
  NON_SHORTHAND_FIELD_PATTERNS,
+ SELF_TYPE_CONVERSION,
  UNSAFE_CODE,
  MISSING_DOCS,
  MISSING_COPY_IMPLEMENTATIONS,
@@ -3064,3 +3065,100 @@ impl EarlyLintPass for SpecialModuleName {
  }
  }
 }
+
+declare_lint! {
+ /// The `self_type_conversion` lint detects when a call to `.into()` does not have any effect.
+ ///
+ /// ### Example
+ ///
+ /// ```rust,compile_fail
+ /// fn main() {
+ /// let () = ().into();
+ /// }
+ /// ```
+ ///
+ /// {{produces}}
+ ///
+ /// ### Explanation
+ ///
+ pub SELF_TYPE_CONVERSION,
+ Deny,
+ "",
+}
+
+pub struct SelfTypeConversion<'tcx> {
+ ignored_types: Vec<Ty<'tcx>>,
+}
+
+impl_lint_pass!(SelfTypeConversion<'_> => [SELF_TYPE_CONVERSION]);
+
+impl SelfTypeConversion<'_> {
+ pub fn new() -> Self {
+ Self { ignored_types: vec![] }
+ }
+}
+
+impl<'tcx> LateLintPass<'tcx> for SelfTypeConversion<'tcx> {
+ fn check_item_post(&mut self, cx: &LateContext<'tcx>, item: &hir::Item<'_>) {
+ let hir::ItemKind::Use(path, kind) = item.kind else { return };
+ tracing::info!("{:#?}", item);
+ tracing::info!(?path, ?kind);
+ for res in &path.res {
+ let Res::Def(DefKind::TyAlias, def_id) = res else { continue };
+ let ty = cx.tcx.type_of(def_id).instantiate_identity();
+ let name = cx.tcx.item_name(*def_id);
+ // println!("{ty:?} {name:?}");
+ self.ignored_types.push(ty);
+ for stripped in cx.tcx.stripped_cfg_items(def_id.krate) {
+ if stripped.name.name == name {
+ tracing::info!("{name:#?}");
+ }
+ }
+ }
+ }
+
+ fn check_expr_post(&mut self, cx: &LateContext<'tcx>, expr: &hir::Expr<'_>) {
+ let hir::ExprKind::MethodCall(_segment, rcvr, args, _) = expr.kind else { return };
+ if !args.is_empty() {
+ tracing::info!("non-empty args");
+ return;
+ }
+ let ty = cx.typeck_results().expr_ty(expr);
+ let rcvr_ty = cx.typeck_results().expr_ty(rcvr);
+ tracing::info!(?ty, ?rcvr_ty);
+
+ if ty != rcvr_ty {
+ tracing::info!("different types");
+ return;
+ }
+ if self.ignored_types.contains(&rcvr_ty) {
+ return;
+ }
+ let Some(def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id) else {
+ tracing::info!("no type dependent def id");
+ return;
+ };
+ tracing::info!(?def_id);
+ if Some(def_id) != cx.tcx.get_diagnostic_item(sym::into_fn) {
+ tracing::info!("not into_fn {:?}", cx.tcx.get_diagnostic_item(sym::into_fn));
+ return;
+ }
+ tracing::info!(?def_id);
+ tracing::info!(?expr);
+ if expr.span.macro_backtrace().next().is_some() {
+ return;
+ }
+ if cx.tcx.sess.source_map().span_to_embeddable_string(expr.span).contains("symbolize/gimli")
+ {
+ // HACK
+ return;
+ }
+ // println!("{:#?}", self.ignored_types);
+ cx.emit_span_lint(
+ SELF_TYPE_CONVERSION,
+ expr.span,
+ SelfTypeConversionDiag { source: rcvr_ty, target: ty },
+ );
+ // bug!("asdf");
+ }
+}

compiler/rustc_lint/src/lib.rs

@@ -185,7 +185,7 @@ early_lint_methods!(
 late_lint_methods!(
  declare_combined_late_lint_pass,
  [
- BuiltinCombinedModuleLateLintPass,
+ BuiltinCombinedModuleLateLintPass<'tcx>,
  [
  ForLoopsOverFallibles: ForLoopsOverFallibles,
  DerefIntoDynSupertrait: DerefIntoDynSupertrait,
@@ -203,6 +203,7 @@ late_lint_methods!(
  UnitBindings: UnitBindings,
  NonUpperCaseGlobals: NonUpperCaseGlobals,
  NonShorthandFieldPatterns: NonShorthandFieldPatterns,
+ SelfTypeConversion<'tcx>: SelfTypeConversion::new(),
  UnusedAllocation: UnusedAllocation,
  // Depends on types used in type definitions
  MissingCopyImplementations: MissingCopyImplementations,
@@ -268,6 +269,7 @@ fn register_builtins(store: &mut LintStore) {
  store.register_lints(&BuiltinCombinedModuleLateLintPass::get_lints());
  store.register_lints(&foreign_modules::get_lints());
 
+ store.register_late_pass(move |_tcx| Box::new(SelfTypeConversion::new()));
  add_lint_group!(
  "nonstandard_style",
  NON_CAMEL_CASE_TYPES,
@@ -298,6 +300,7 @@ fn register_builtins(store: &mut LintStore) {
  UNUSED_PARENS,
  UNUSED_BRACES,
  REDUNDANT_SEMICOLONS,
+ // SELF_TYPE_CONVERSION,
  MAP_UNIT_FN
  );
 

compiler/rustc_lint/src/lints.rs

@@ -72,6 +72,13 @@ pub struct BuiltinNonShorthandFieldPatterns {
  pub prefix: &'static str,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_self_type_conversion)]
+pub struct SelfTypeConversionDiag<'t> {
+ pub source: Ty<'t>,
+ pub target: Ty<'t>,
+}
+
 #[derive(LintDiagnostic)]
 pub enum BuiltinUnsafe {
  #[diag(lint_builtin_allow_internal_unsafe)]

compiler/rustc_lint/src/passes.rs

@@ -93,13 +93,13 @@ macro_rules! expand_combined_late_lint_pass_methods {
 /// runtime.
 #[macro_export]
 macro_rules! declare_combined_late_lint_pass {
- ([$v:vis $name:ident, [$($pass:ident: $constructor:expr,)*]], $methods:tt) => (
+ ([$v:vis $name:ident$(<$lt:lifetime>)?, [$($pass:ident$(<$inner_lt:lifetime>)?: $constructor:expr,)*]], $methods:tt) => (
  #[allow(non_snake_case)]
- $v struct $name {
- $($pass: $pass,)*
+ $v struct $name$(<$lt>)? {
+ $($pass: $pass$(<$inner_lt>)?,)*
  }
 
- impl $name {
+ impl$(<$lt>)? $name$(<$lt>)? {
  $v fn new() -> Self {
  Self {
  $($pass: $constructor,)*
@@ -113,12 +113,12 @@ macro_rules! declare_combined_late_lint_pass {
  }
  }
 
- impl<'tcx> $crate::LateLintPass<'tcx> for $name {
+ impl<'tcx> $crate::LateLintPass<'tcx> for $name$(<$lt>)? {
  $crate::expand_combined_late_lint_pass_methods!([$($pass),*], $methods);
  }
 
  #[allow(rustc::lint_pass_impl_without_macro)]
- impl $crate::LintPass for $name {
+ impl$(<$lt>)? $crate::LintPass for $name$(<$lt>)? {
  fn name(&self) -> &'static str {
  panic!()
  }

compiler/rustc_middle/src/ty/consts.rs

@@ -384,7 +384,7 @@ impl<'tcx> Const<'tcx> {
  Ok((tcx.type_of(unevaluated.def).instantiate(tcx, unevaluated.args), c))
  }
  Ok(Err(bad_ty)) => Err(Either::Left(bad_ty)),
- Err(err) => Err(Either::Right(err.into())),
+ Err(err) => Err(Either::Right(err)),
  }
  }
  ConstKind::Value(ty, val) => Ok((ty, val)),

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -1526,7 +1526,7 @@ pub trait PrettyPrinter<'tcx>: Printer<'tcx> + fmt::Write {
 
  let precedence = |binop: rustc_middle::mir::BinOp| {
  use rustc_ast::util::parser::AssocOp;
- AssocOp::from_ast_binop(binop.to_hir_binop().into()).precedence()
+ AssocOp::from_ast_binop(binop.to_hir_binop()).precedence()
  };
  let op_precedence = precedence(op);
  let formatted_op = op.to_hir_binop().as_str();

compiler/rustc_parse/src/parser/item.rs

@@ -1586,7 +1586,7 @@ impl<'a> Parser<'a> {
  (thin_vec![], Recovered::Yes(guar))
  }
  };
- VariantData::Struct { fields, recovered: recovered.into() }
+ VariantData::Struct { fields, recovered }
  } else if this.check(&token::OpenDelim(Delimiter::Parenthesis)) {
  let body = match this.parse_tuple_struct_body() {
  Ok(body) => body,
@@ -1670,7 +1670,7 @@ impl<'a> Parser<'a> {
  class_name.span,
  generics.where_clause.has_where_token,
  )?;
- VariantData::Struct { fields, recovered: recovered.into() }
+ VariantData::Struct { fields, recovered }
  }
  // No `where` so: `struct Foo<T>;`
  } else if self.eat(&token::Semi) {
@@ -1682,7 +1682,7 @@ impl<'a> Parser<'a> {
  class_name.span,
  generics.where_clause.has_where_token,
  )?;
- VariantData::Struct { fields, recovered: recovered.into() }
+ VariantData::Struct { fields, recovered }
  // Tuple-style struct definition with optional where-clause.
  } else if self.token == token::OpenDelim(Delimiter::Parenthesis) {
  let body = VariantData::Tuple(self.parse_tuple_struct_body()?, DUMMY_NODE_ID);
@@ -1711,14 +1711,14 @@ impl<'a> Parser<'a> {
  class_name.span,
  generics.where_clause.has_where_token,
  )?;
- VariantData::Struct { fields, recovered: recovered.into() }
+ VariantData::Struct { fields, recovered }
  } else if self.token == token::OpenDelim(Delimiter::Brace) {
  let (fields, recovered) = self.parse_record_struct_body(
  "union",
  class_name.span,
  generics.where_clause.has_where_token,
  )?;
- VariantData::Struct { fields, recovered: recovered.into() }
+ VariantData::Struct { fields, recovered }
  } else {
  let token_str = super::token_descr(&self.token);
  let msg = format!("expected `where` or `{{` after union name, found {token_str}");

compiler/rustc_span/src/symbol.rs

@@ -1045,6 +1045,7 @@ symbols! {
  integer_: "integer", // underscore to avoid clashing with the function `sym::integer` below
  integral,
  into_async_iter_into_iter,
+ into_fn,
  into_future,
  into_iter,
  intra_doc_pointers,

compiler/rustc_symbol_mangling/src/v0.rs

@@ -382,7 +382,7 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
  let consts = [
  start.unwrap_or(self.tcx.consts.unit),
  end.unwrap_or(self.tcx.consts.unit),
- ty::Const::from_bool(self.tcx, include_end).into(),
+ ty::Const::from_bool(self.tcx, include_end),
  ];
  // HACK: Represent as tuple until we have something better.
  // HACK: constants are used in arrays, even if the types don't match.

compiler/rustc_trait_selection/src/traits/project.rs

@@ -406,7 +406,7 @@ pub(super) fn opt_normalize_projection_term<'a, 'b, 'tcx>(
  debug!("opt_normalize_projection_type: found error");
  let result = normalize_to_error(selcx, param_env, projection_term, cause, depth);
  obligations.extend(result.obligations);
- return Ok(Some(result.value.into()));
+ return Ok(Some(result.value));
  }
  }
 
@@ -476,7 +476,7 @@ pub(super) fn opt_normalize_projection_term<'a, 'b, 'tcx>(
  }
  let result = normalize_to_error(selcx, param_env, projection_term, cause, depth);
  obligations.extend(result.obligations);
- Ok(Some(result.value.into()))
+ Ok(Some(result.value))
  }
  }
 }

library/core/src/convert/mod.rs

@@ -445,6 +445,7 @@ pub trait AsMut<T: ?Sized> {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait Into<T>: Sized {
  /// Converts this type into the (usually inferred) input type.
+ #[rustc_diagnostic_item = "into_fn"]
  #[must_use]
  #[stable(feature = "rust1", since = "1.0.0")]
  fn into(self) -> T;

library/std/src/os/fd/owned.rs

@@ -328,7 +328,7 @@ impl From<crate::net::TcpStream> for OwnedFd {
  /// Takes ownership of a [`TcpStream`](crate::net::TcpStream)'s socket file descriptor.
  #[inline]
  fn from(tcp_stream: crate::net::TcpStream) -> OwnedFd {
- tcp_stream.into_inner().into_socket().into_inner().into_inner().into()
+ tcp_stream.into_inner().into_socket().into_inner().into_inner()
  }
 }
 
@@ -355,7 +355,7 @@ impl From<crate::net::TcpListener> for OwnedFd {
  /// Takes ownership of a [`TcpListener`](crate::net::TcpListener)'s socket file descriptor.
  #[inline]
  fn from(tcp_listener: crate::net::TcpListener) -> OwnedFd {
- tcp_listener.into_inner().into_socket().into_inner().into_inner().into()
+ tcp_listener.into_inner().into_socket().into_inner().into_inner()
  }
 }
 
@@ -382,7 +382,7 @@ impl From<crate::net::UdpSocket> for OwnedFd {
  /// Takes ownership of a [`UdpSocket`](crate::net::UdpSocket)'s file descriptor.
  #[inline]
  fn from(udp_socket: crate::net::UdpSocket) -> OwnedFd {
- udp_socket.into_inner().into_socket().into_inner().into_inner().into()
+ udp_socket.into_inner().into_socket().into_inner().into_inner()
  }
 }
 

library/std/src/sys/pal/unix/process/process_unix.rs

@@ -282,6 +282,7 @@ impl Command {
  // have the drop glue anyway because this code never returns (the
  // child will either exec() or invoke libc::exit)
  #[cfg(not(any(target_os = "tvos", target_os = "watchos")))]
+ #[cfg_attr(not(bootstrap), allow(self_type_conversion))]
  unsafe fn do_exec(
  &mut self,
  stdio: ChildPipes,