[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_hir_analysis/src/check/wfcheck.rs

@@ -1120,7 +1120,7 @@ fn check_type_defn<'tcx>(
                     } else {
                         // Evaluate the constant proactively, to emit an error if the constant has
                         // an unconditional error. We only do so if the const has no type params.
-                        let _ = tcx.const_eval_poly(def_id.into());
+                        let _ = tcx.const_eval_poly(def_id);
                     }
                 }
                 let field_id = field.did.expect_local();

compiler/rustc_lint/messages.ftl

@@ -740,6 +740,8 @@ lint_reserved_prefix = prefix `{$prefix}` is unknown
 lint_reserved_string = will be parsed as a guarded string in Rust 2024
     .suggestion = insert whitespace here to avoid this being parsed as a guarded string in Rust 2024
 
+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

@@ -58,7 +58,7 @@ use crate::lints::{
     BuiltinTrivialBounds, BuiltinTypeAliasBounds, BuiltinUngatedAsyncFnTrackCaller,
     BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub,
     BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub,
-    BuiltinWhileTrue, InvalidAsmLabel,
+    BuiltinWhileTrue, InvalidAsmLabel, SelfTypeConversionDiag,
 };
 use crate::nonstandard_style::{MethodLateContext, method_context};
 use crate::{
@@ -1304,9 +1304,9 @@ impl UnreachablePub {
                 cx.effective_visibilities.effective_vis(def_id).map(|effective_vis| {
                     effective_vis.at_level(rustc_middle::middle::privacy::Level::Reachable)
                 })
-                && let parent_parent = cx.tcx.parent_module_from_def_id(
-                    cx.tcx.parent_module_from_def_id(def_id.into()).into(),
-                )
+                && let parent_parent = cx
+                    .tcx
+                    .parent_module_from_def_id(cx.tcx.parent_module_from_def_id(def_id).into())
                 && *restricted_did == parent_parent.to_local_def_id()
                 && !restricted_did.to_def_id().is_crate_root()
             {
@@ -1589,6 +1589,7 @@ declare_lint_pass!(
     SoftLints => [
         WHILE_TRUE,
         NON_SHORTHAND_FIELD_PATTERNS,
+        SELF_TYPE_CONVERSION,
         UNSAFE_CODE,
         MISSING_DOCS,
         MISSING_COPY_IMPLEMENTATIONS,
@@ -3063,3 +3064,103 @@ 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:?}");
+            tracing::info!("ignoring {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!("found stripped {name:#?}");
+                }
+            }
+        }
+        // FIXME: also look at conditional cfgd uses so to account for things like
+        // `use std::io::repr_bitpacked` and `std::io::repr_unpacked`.
+    }
+
+    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) {
+            tracing::info!("ignored");
+            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

@@ -188,7 +188,7 @@ early_lint_methods!(
 late_lint_methods!(
     declare_combined_late_lint_pass,
     [
-        BuiltinCombinedModuleLateLintPass,
+        BuiltinCombinedModuleLateLintPass<'tcx>,
         [
             ForLoopsOverFallibles: ForLoopsOverFallibles,
             DefaultCouldBeDerived: DefaultCouldBeDerived::default(),
@@ -206,6 +206,7 @@ late_lint_methods!(
             UnitBindings: UnitBindings,
             NonUpperCaseGlobals: NonUpperCaseGlobals,
             NonShorthandFieldPatterns: NonShorthandFieldPatterns,
+            SelfTypeConversion<'tcx>: SelfTypeConversion::new(),
             UnusedAllocation: UnusedAllocation,
             // Depends on types used in type definitions
             MissingCopyImplementations: MissingCopyImplementations,
@@ -275,6 +276,7 @@ fn register_builtins(store: &mut LintStore) {
     store.register_lints(&foreign_modules::get_lints());
     store.register_lints(&HardwiredLints::lint_vec());
 
+    store.register_late_pass(move |_tcx| Box::new(SelfTypeConversion::new()));
     add_lint_group!(
         "nonstandard_style",
         NON_CAMEL_CASE_TYPES,
@@ -305,6 +307,7 @@ fn register_builtins(store: &mut LintStore) {
         UNUSED_PARENS,
         UNUSED_BRACES,
         REDUNDANT_SEMICOLONS,
+        // SELF_TYPE_CONVERSION,
         MAP_UNIT_FN
     );
 

compiler/rustc_lint/src/lints.rs

@@ -71,6 +71,13 @@ pub(crate) struct BuiltinNonShorthandFieldPatterns {
     pub prefix: &'static str,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_self_type_conversion)]
+pub(crate) struct SelfTypeConversionDiag<'t> {
+    pub source: Ty<'t>,
+    pub target: Ty<'t>,
+}
+
 #[derive(LintDiagnostic)]
 pub(crate) enum BuiltinUnsafe {
     #[diag(lint_builtin_allow_internal_unsafe)]

compiler/rustc_lint/src/passes.rs

@@ -95,13 +95,13 @@ 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,)*
@@ -115,12 +115,12 @@ 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_mir_transform/src/lint_tail_expr_drop_order.rs

@@ -691,7 +691,7 @@ impl Subdiagnostic for LocalLabel<'_> {
         for dtor in self.destructors {
             dtor.add_to_diag_with(diag, f);
         }
-        let msg = f(diag, crate::fluent_generated::mir_transform_label_local_epilogue.into());
+        let msg = f(diag, crate::fluent_generated::mir_transform_label_local_epilogue);
         diag.span_label(self.span, msg);
     }
 }

compiler/rustc_parse/src/parser/tests.rs

@@ -2366,8 +2366,7 @@ fn string_to_tts_1() {
                         token::Ident(sym::i32, IdentIsRaw::No),
                         sp(8, 11),
                     ),
-                ])
-                .into(),
+                ]),
             ),
             TokenTree::Delimited(
                 DelimSpan::from_pair(sp(13, 14), sp(18, 19)),
@@ -2383,8 +2382,7 @@ fn string_to_tts_1() {
                     ),
                     // `Alone` because the `;` is followed by whitespace.
                     TokenTree::token_alone(token::Semi, sp(16, 17)),
-                ])
-                .into(),
+                ]),
             ),
         ]);
 

compiler/rustc_span/src/symbol.rs

@@ -1119,6 +1119,7 @@ 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_trait_selection/src/traits/const_evaluatable.rs

@@ -79,7 +79,7 @@ pub fn is_const_evaluatable<'tcx>(
                     Err(
                         EvaluateConstErr::EvaluationFailure(e)
                         | EvaluateConstErr::InvalidConstParamTy(e),
-                    ) => Err(NotConstEvaluatable::Error(e.into())),
+                    ) => Err(NotConstEvaluatable::Error(e)),
                     Ok(_) => Ok(()),
                 }
             }
@@ -140,7 +140,7 @@ pub fn is_const_evaluatable<'tcx>(
             }
             Err(
                 EvaluateConstErr::EvaluationFailure(e) | EvaluateConstErr::InvalidConstParamTy(e),
-            ) => Err(NotConstEvaluatable::Error(e.into())),
+            ) => Err(NotConstEvaluatable::Error(e)),
             Ok(_) => Ok(()),
         }
     }

library/alloc/src/rc.rs

@@ -1789,7 +1789,7 @@ impl<T: ?Sized, A: Allocator> Rc<T, A> {
     /// let x: Rc<&str> = Rc::new("Hello, world!");
     /// {
     ///     let s = String::from("Oh, no!");
-    ///     let mut y: Rc<&str> = x.clone().into();
+    ///     let mut y: Rc<&str> = x.clone();
     ///     unsafe {
     ///         // this is Undefined Behavior, because x's inner type
     ///         // is &'long str, not &'short str

library/alloc/src/sync.rs

@@ -2468,7 +2468,7 @@ impl<T: ?Sized, A: Allocator> Arc<T, A> {
     /// let x: Arc<&str> = Arc::new("Hello, world!");
     /// {
     ///     let s = String::from("Oh, no!");
-    ///     let mut y: Arc<&str> = x.clone().into();
+    ///     let mut y: Arc<&str> = x.clone();
     ///     unsafe {
     ///         // this is Undefined Behavior, because x's inner type
     ///         // is &'long str, not &'short str

library/core/src/convert/mod.rs

@@ -446,6 +446,7 @@ pub trait AsMut<T: ?Sized> {
 #[doc(search_unbox)]
 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

@@ -2,7 +2,7 @@
 
 #![stable(feature = "io_safety", since = "1.63.0")]
 #![deny(unsafe_op_in_unsafe_fn)]
-
+#![cfg_attr(not(bootstrap), allow(self_type_conversion))]
 use super::raw::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
 use crate::marker::PhantomData;
 use crate::mem::ManuallyDrop;

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

@@ -285,6 +285,7 @@ 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

@@ -25,6 +25,7 @@ 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 {

library/std/src/thread/mod.rs

@@ -502,7 +502,7 @@ impl Builder {
 
         let id = ThreadId::new();
         let my_thread = match name {
-            Some(name) => Thread::new(id, name.into()),
+            Some(name) => Thread::new(id, name),
             None => Thread::new_unnamed(id),
         };
 

src/tools/clippy/tests/ui/needless_return_with_question_mark.fixed

@@ -30,7 +30,7 @@ fn main() -> Result<(), ()> {
     return Ok::<(), ()>(());
     Err(())?;
     Ok::<(), ()>(());
-    return Err(().into());
+    return Err(());
     external! {
         return Err(())?;
     }

src/tools/clippy/tests/ui/needless_return_with_question_mark.rs

@@ -30,7 +30,7 @@ fn main() -> Result<(), ()> {
     return Ok::<(), ()>(());
     Err(())?;
     Ok::<(), ()>(());
-    return Err(().into());
+    return Err(());
     external! {
         return Err(())?;
     }