Warn/error on self ctor from outer item in inner item

4 files changed, 113 insertions, 1 deletions

diff --git a/compiler/rustc_hir_typeck/messages.ftl b/compiler/rustc_hir_typeck/messages.ftl
index 72b95a9603d..6f499947d5c 100644
--- a/compiler/rustc_hir_typeck/messages.ftl
+++ b/compiler/rustc_hir_typeck/messages.ftl
@@ -137,6 +137,10 @@ hir_typeck_rpit_change_return_type = you could change the return type to be a bo
 hir_typeck_rustcall_incorrect_args =
     functions with the "rust-call" ABI must take a single non-self tuple argument
 
+hir_typeck_self_ctor_from_outer_item = can't reference `Self` constructor from outer item
+    .label = the inner item doesn't inherit generics from this impl, so `Self` is invalid to reference
+    .suggestion = replace `Self` with the actual type
+
 hir_typeck_struct_expr_non_exhaustive =
     cannot create non-exhaustive {$what} using struct expression
 
diff --git a/compiler/rustc_hir_typeck/src/errors.rs b/compiler/rustc_hir_typeck/src/errors.rs
index f250b909596..3f12f252654 100644
--- a/compiler/rustc_hir_typeck/src/errors.rs
+++ b/compiler/rustc_hir_typeck/src/errors.rs
@@ -651,3 +651,31 @@ pub enum SuggestBoxingForReturnImplTrait {
         ends: Vec<Span>,
     },
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_typeck_self_ctor_from_outer_item, code = E0401)]
+pub struct SelfCtorFromOuterItem {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub impl_span: Span,
+    #[subdiagnostic]
+    pub sugg: Option<ReplaceWithName>,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(hir_typeck_self_ctor_from_outer_item)]
+pub struct SelfCtorFromOuterItemLint {
+    #[label]
+    pub impl_span: Span,
+    #[subdiagnostic]
+    pub sugg: Option<ReplaceWithName>,
+}
+
+#[derive(Subdiagnostic)]
+#[suggestion(hir_typeck_suggestion, code = "{name}", applicability = "machine-applicable")]
+pub struct ReplaceWithName {
+    #[primary_span]
+    pub span: Span,
+    pub name: String,
+}
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
index 6e8ef044452..dc927e66765 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs
@@ -1,5 +1,5 @@
 use crate::callee::{self, DeferredCallResolution};
-use crate::errors::CtorIsPrivate;
+use crate::errors::{self, CtorIsPrivate};
 use crate::method::{self, MethodCallee, SelfSource};
 use crate::rvalue_scopes;
 use crate::{BreakableCtxt, Diverges, Expectation, FnCtxt, LoweredTy};
@@ -21,6 +21,7 @@ use rustc_hir_analysis::hir_ty_lowering::{
 use rustc_infer::infer::canonical::{Canonical, OriginalQueryValues, QueryResponse};
 use rustc_infer::infer::error_reporting::TypeAnnotationNeeded::E0282;
 use rustc_infer::infer::{DefineOpaqueTypes, InferResult};
+use rustc_lint::builtin::SELF_CONSTRUCTOR_FROM_OUTER_ITEM;
 use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::fold::TypeFoldable;
@@ -1162,6 +1163,43 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 span,
                 tcx.at(span).type_of(impl_def_id).instantiate_identity(),
             );
+
+            // Firstly, check that this SelfCtor even comes from the item we're currently
+            // typechecking. This can happen because we never validated the resolution of
+            // SelfCtors, and when we started doing so, we noticed regressions. After
+            // sufficiently long time, we can remove this check and turn it into a hard
+            // error in `validate_res_from_ribs` -- it's just difficult to tell whether the
+            // self type has any generic types during rustc_resolve, which is what we use
+            // to determine if this is a hard error or warning.
+            if std::iter::successors(Some(self.body_id.to_def_id()), |def_id| {
+                self.tcx.generics_of(def_id).parent
+            })
+            .all(|def_id| def_id != impl_def_id)
+            {
+                let sugg = ty.normalized.ty_adt_def().map(|def| errors::ReplaceWithName {
+                    span: path_span,
+                    name: self.tcx.item_name(def.did()).to_ident_string(),
+                });
+                if ty.raw.has_param() {
+                    let guar = self.tcx.dcx().emit_err(errors::SelfCtorFromOuterItem {
+                        span: path_span,
+                        impl_span: tcx.def_span(impl_def_id),
+                        sugg,
+                    });
+                    return (Ty::new_error(self.tcx, guar), res);
+                } else {
+                    self.tcx.emit_node_span_lint(
+                        SELF_CONSTRUCTOR_FROM_OUTER_ITEM,
+                        hir_id,
+                        path_span,
+                        errors::SelfCtorFromOuterItemLint {
+                            impl_span: tcx.def_span(impl_def_id),
+                            sugg,
+                        },
+                    );
+                }
+            }
+
             match ty.normalized.ty_adt_def() {
                 Some(adt_def) if adt_def.has_ctor() => {
                     let (ctor_kind, ctor_def_id) = adt_def.non_enum_variant().ctor.unwrap();
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 53694545772..3ddf7f9e6c0 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -90,6 +90,7 @@ declare_lint_pass! {
         RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX,
         RUST_2021_PRELUDE_COLLISIONS,
         RUST_2024_INCOMPATIBLE_PAT,
+        SELF_CONSTRUCTOR_FROM_OUTER_ITEM,
         SEMICOLON_IN_EXPRESSIONS_FROM_MACROS,
         SINGLE_USE_LIFETIMES,
         SOFT_UNSTABLE,
@@ -3150,6 +3151,47 @@ declare_lint! {
 }
 
 declare_lint! {
+    /// The `self_constructor_from_outer_item` lint detects cases where the `Self` constructor
+    /// was silently allowed due to a bug in the resolver, and which may produce surprising
+    /// and unintended behavior.
+    ///
+    /// Using a `Self` type alias from an outer item was never intended, but was silently allowed.
+    /// This is deprecated -- and is a hard error when the `Self` type alias references generics
+    /// that are not in scope.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(self_constructor_from_outer_item)]
+    ///
+    /// struct S0(usize);
+    ///
+    /// impl S0 {
+    ///     fn foo() {
+    ///         const C: S0 = Self(0);
+    ///         fn bar() -> S0 {
+    ///             Self(0)
+    ///         }
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// The `Self` type alias should not be reachable because nested items are not associated with
+    /// the scope of the parameters from the parent item.
+    pub SELF_CONSTRUCTOR_FROM_OUTER_ITEM,
+    Warn,
+    "detect unsupported use of `Self` from outer item",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps,
+        reference: "issue #124186 <https://github.com/rust-lang/rust/issues/124186>",
+    };
+}
+
+declare_lint! {
     /// The `semicolon_in_expressions_from_macros` lint detects trailing semicolons
     /// in macro bodies when the macro is invoked in expression position.
     /// This was previous accepted, but is being phased out.