[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

@@ -129,6 +129,8 @@ lint_builtin_no_mangle_static = declaration of a `no_mangle` static
 lint_builtin_non_shorthand_field_patterns = the `{$ident}:` in this pattern is redundant
     .suggestion = use shorthand field pattern
 
+lint_self_type_conversion = this conversion is useless `{$source}` to `{$target}`
+
 lint_builtin_overridden_symbol_name =
     the linker's behavior with multiple libraries exporting duplicate symbol names is undefined and Rust cannot provide guarantees when you manually override them
 

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 +3065,95 @@ 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;
+        }
+        // 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,

library/std/src/sys_common/process.rs

@@ -31,6 +31,7 @@ impl fmt::Debug for CommandEnv {
 
 impl CommandEnv {
     // Capture the current environment with these changes applied
+    #[cfg_attr(not(bootstrap), allow(self_type_conversion))]
     pub fn capture(&self) -> BTreeMap<EnvKey, OsString> {
         let mut result = BTreeMap::<EnvKey, OsString>::new();
         if !self.clear {