Make it crystal clear what lint type_alias_bounds actually signifies

compiler/rustc_lint/messages.ftl

@@ -139,13 +139,18 @@ lint_builtin_special_module_name_used_main = found module declaration for main.r
 
 lint_builtin_trivial_bounds = {$predicate_kind_name} bound {$predicate} does not depend on any type or lifetime parameters
 
-lint_builtin_type_alias_bounds_help = use fully disambiguated paths (i.e., `<T as Trait>::Assoc`) to refer to associated types in type aliases
-
-lint_builtin_type_alias_generic_bounds = bounds on generic parameters are not enforced in type aliases
-    .suggestion = the bound will not be checked when the type alias is used, and should be removed
-
-lint_builtin_type_alias_where_clause = where clauses are not enforced in type aliases
-    .suggestion = the clause will not be checked when the type alias is used, and should be removed
+lint_builtin_type_alias_bounds_enable_feat_help = add `#![feature(lazy_type_alias)]` to the crate attributes to enable the desired semantics
+lint_builtin_type_alias_bounds_label = will not be checked at usage sites of the type alias
+lint_builtin_type_alias_bounds_limitation_note = this is a known limitation of the type checker that may be lifted in a future edition.
+    see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
+lint_builtin_type_alias_bounds_param_bounds = bounds on generic parameters in type aliases are not enforced
+    .suggestion = remove {$count ->
+        [one] this bound
+        *[other] these bounds
+    }
+lint_builtin_type_alias_bounds_qualify_assoc_tys_sugg = fully qualify this associated type
+lint_builtin_type_alias_bounds_where_clause = where clauses on type aliases are not enforced
+    .suggestion = remove this where clause
 
 lint_builtin_unpermitted_type_init_label = this code causes undefined behavior when executed
 lint_builtin_unpermitted_type_init_label_suggestion = help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done

compiler/rustc_lint/src/builtin.rs

@@ -31,12 +31,12 @@ use crate::{
         BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures, BuiltinKeywordIdents,
         BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc,
         BuiltinMutablesTransmutes, BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns,
-        BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds, BuiltinTypeAliasGenericBounds,
-        BuiltinTypeAliasGenericBoundsSuggestion, BuiltinTypeAliasWhereClause,
-        BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit,
-        BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub, BuiltinUnsafe,
-        BuiltinUnstableFeatures, BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub,
-        BuiltinWhileTrue, InvalidAsmLabel, SuggestChangingAssocTypes,
+        BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds, BuiltinTypeAliasBounds,
+        BuiltinTypeAliasParamBoundsSuggestion, BuiltinUngatedAsyncFnTrackCaller,
+        BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub,
+        BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment,
+        BuiltinUnusedDocCommentSub, BuiltinWhileTrue, InvalidAsmLabel,
+        TypeAliasBoundsQualifyAssocTysSugg,
     },
     EarlyContext, EarlyLintPass, LateContext, LateLintPass, Level, LintContext,
 };
@@ -1406,23 +1406,6 @@ declare_lint_pass!(
     TypeAliasBounds => [TYPE_ALIAS_BOUNDS]
 );
 
-impl TypeAliasBounds {
-    pub(crate) fn is_type_variable_assoc(qpath: &hir::QPath<'_>) -> bool {
-        match *qpath {
-            hir::QPath::TypeRelative(ty, _) => {
-                // If this is a type variable, we found a `T::Assoc`.
-                match ty.kind {
-                    hir::TyKind::Path(hir::QPath::Resolved(None, path)) => {
-                        matches!(path.res, Res::Def(DefKind::TyParam, _))
-                    }
-                    _ => false,
-                }
-            }
-            hir::QPath::Resolved(..) | hir::QPath::LangItem(..) => false,
-        }
-    }
-}
-
 impl<'tcx> LateLintPass<'tcx> for TypeAliasBounds {
     fn check_item(&mut self, cx: &LateContext<'_>, item: &hir::Item<'_>) {
         let hir::ItemKind::TyAlias(hir_ty, generics) = &item.kind else { return };
@@ -1437,7 +1420,6 @@ impl<'tcx> LateLintPass<'tcx> for TypeAliasBounds {
             return;
         }
 
-
         // FIXME(generic_const_exprs): Revisit this before stabilization.
         // See also `tests/ui/const-generics/generic_const_exprs/type-alias-bounds.rs`.
         let ty = cx.tcx.type_of(item.owner_id).instantiate_identity();
@@ -1455,6 +1437,8 @@ impl<'tcx> LateLintPass<'tcx> for TypeAliasBounds {
         let mut where_spans = Vec::new();
         let mut inline_spans = Vec::new();
         let mut inline_sugg = Vec::new();
+        let mut affects_object_lifetime_defaults = false;
+
         for p in generics.predicates {
             let span = p.span();
             if p.in_where_clause() {
@@ -1465,31 +1449,61 @@ impl<'tcx> LateLintPass<'tcx> for TypeAliasBounds {
                 }
                 inline_sugg.push((span, String::new()));
             }
+
+            // FIXME(fmease): Move this into a "diagnostic decorator" for increased laziness
+            // Bounds of the form `T: 'a` where `T` is a type param of
+            // the type alias affect object lifetime defaults.
+            if !affects_object_lifetime_defaults
+                && let hir::WherePredicate::BoundPredicate(pred) = p
+                && pred.bounds.iter().any(|bound| matches!(bound, hir::GenericBound::Outlives(_)))
+                && pred.bound_generic_params.is_empty()
+                && let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = pred.bounded_ty.kind
+                && let Res::Def(DefKind::TyParam, _) = path.res
+            {
+                affects_object_lifetime_defaults = true;
+            }
         }
 
-        let mut suggested_changing_assoc_types = false;
-        if !where_spans.is_empty() {
-            let sub = (!suggested_changing_assoc_types).then(|| {
-                suggested_changing_assoc_types = true;
-                SuggestChangingAssocTypes { ty: hir_ty }
-            });
+        // FIXME(fmease): Add a disclaimer (in the form of a multi-span note) that the removal of
+        //                type-param-outlives-bounds affects OLDs and explicit object lifetime
+        //                bounds might be required [...].
+        // FIXME(fmease): The applicability should also depend on the outcome of the HIR walker
+        //                inside of `TypeAliasBoundsQualifyAssocTysSugg`: Whether it found a
+        //                shorthand projection or not.
+        let applicability = if affects_object_lifetime_defaults {
+            Applicability::MaybeIncorrect
+        } else {
+            Applicability::MachineApplicable
+        };
+
+        let mut qualify_assoc_tys_sugg = Some(TypeAliasBoundsQualifyAssocTysSugg { ty: hir_ty });
+        let enable_feat_help = cx.tcx.sess.is_nightly_build().then_some(());
+
+        if let [.., label_sp] = *where_spans {
             cx.emit_span_lint(
                 TYPE_ALIAS_BOUNDS,
                 where_spans,
-                BuiltinTypeAliasWhereClause { suggestion: generics.where_clause_span, sub },
+                BuiltinTypeAliasBounds::WhereClause {
+                    label: label_sp,
+                    enable_feat_help,
+                    suggestion: (generics.where_clause_span, applicability),
+                    qualify_assoc_tys_sugg: qualify_assoc_tys_sugg.take(),
+                },
             );
         }
-
-        if !inline_spans.is_empty() {
-            let suggestion = BuiltinTypeAliasGenericBoundsSuggestion { suggestions: inline_sugg };
-            let sub = (!suggested_changing_assoc_types).then(|| {
-                suggested_changing_assoc_types = true;
-                SuggestChangingAssocTypes { ty: hir_ty }
-            });
+        if let [.., label_sp] = *inline_spans {
             cx.emit_span_lint(
                 TYPE_ALIAS_BOUNDS,
                 inline_spans,
-                BuiltinTypeAliasGenericBounds { suggestion, sub },
+                BuiltinTypeAliasBounds::ParamBounds {
+                    label: label_sp,
+                    enable_feat_help,
+                    suggestion: BuiltinTypeAliasParamBoundsSuggestion {
+                        suggestions: inline_sugg,
+                        applicability,
+                    },
+                    qualify_assoc_tys_sugg,
+                },
             );
         }
     }

compiler/rustc_lint/src/lints.rs

@@ -9,7 +9,7 @@ use rustc_errors::{
     ElidedLifetimeInPathSubdiag, EmissionGuarantee, LintDiagnostic, MultiSpan, SubdiagMessageOp,
     Subdiagnostic, SuggestionStyle,
 };
-use rustc_hir::{def::Namespace, def_id::DefId};
+use rustc_hir::{self as hir, def::Namespace, def_id::DefId};
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::{
     inhabitedness::InhabitedPredicate, Clause, PolyExistentialTraitRef, Ty, TyCtxt,
@@ -22,9 +22,7 @@ use rustc_span::{
     Span, Symbol,
 };
 
-use crate::{
-    builtin::InitError, builtin::TypeAliasBounds, errors::OverruledAttributeSub, LateContext,
-};
+use crate::{builtin::InitError, errors::OverruledAttributeSub, LateContext};
 
 // array_into_iter.rs
 #[derive(LintDiagnostic)]
@@ -263,84 +261,104 @@ pub struct BuiltinUnreachablePub<'a> {
     pub help: Option<()>,
 }
 
-pub struct SuggestChangingAssocTypes<'a, 'b> {
-    pub ty: &'a rustc_hir::Ty<'b>,
+#[derive(LintDiagnostic)]
+#[diag(lint_macro_expr_fragment_specifier_2024_migration)]
+pub struct MacroExprFragment2024 {
+    #[suggestion(code = "expr_2021", applicability = "machine-applicable")]
+    pub suggestion: Span,
+}
+
+#[derive(LintDiagnostic)]
+pub enum BuiltinTypeAliasBounds<'a, 'hir> {
+    #[diag(lint_builtin_type_alias_bounds_where_clause)]
+    #[note(lint_builtin_type_alias_bounds_limitation_note)]
+    WhereClause {
+        #[label(lint_builtin_type_alias_bounds_label)]
+        label: Span,
+        #[help(lint_builtin_type_alias_bounds_enable_feat_help)]
+        enable_feat_help: Option<()>,
+        #[suggestion(code = "")]
+        suggestion: (Span, Applicability),
+        #[subdiagnostic]
+        qualify_assoc_tys_sugg: Option<TypeAliasBoundsQualifyAssocTysSugg<'a, 'hir>>,
+    },
+    #[diag(lint_builtin_type_alias_bounds_param_bounds)]
+    #[note(lint_builtin_type_alias_bounds_limitation_note)]
+    ParamBounds {
+        #[label(lint_builtin_type_alias_bounds_label)]
+        label: Span,
+        #[help(lint_builtin_type_alias_bounds_enable_feat_help)]
+        enable_feat_help: Option<()>,
+        #[subdiagnostic]
+        suggestion: BuiltinTypeAliasParamBoundsSuggestion,
+        #[subdiagnostic]
+        qualify_assoc_tys_sugg: Option<TypeAliasBoundsQualifyAssocTysSugg<'a, 'hir>>,
+    },
+}
+
+pub struct BuiltinTypeAliasParamBoundsSuggestion {
+    pub suggestions: Vec<(Span, String)>,
+    pub applicability: Applicability,
 }
 
-impl<'a, 'b> Subdiagnostic for SuggestChangingAssocTypes<'a, 'b> {
+impl Subdiagnostic for BuiltinTypeAliasParamBoundsSuggestion {
     fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
         self,
         diag: &mut Diag<'_, G>,
         _f: &F,
     ) {
-        // Access to associates types should use `<T as Bound>::Assoc`, which does not need a
-        // bound. Let's see if this type does that.
-
-        // We use a HIR visitor to walk the type.
-        use rustc_hir::intravisit::{self, Visitor};
-        struct WalkAssocTypes<'a, 'b, G: EmissionGuarantee> {
-            err: &'a mut Diag<'b, G>,
-        }
-        impl<'a, 'b, G: EmissionGuarantee> Visitor<'_> for WalkAssocTypes<'a, 'b, G> {
-            fn visit_qpath(
-                &mut self,
-                qpath: &rustc_hir::QPath<'_>,
-                id: rustc_hir::HirId,
-                span: Span,
-            ) {
-                if TypeAliasBounds::is_type_variable_assoc(qpath) {
-                    self.err.span_help(span, fluent::lint_builtin_type_alias_bounds_help);
-                }
-                intravisit::walk_qpath(self, qpath, id)
-            }
-        }
-
-        // Let's go for a walk!
-        let mut visitor = WalkAssocTypes { err: diag };
-        visitor.visit_ty(self.ty);
+        diag.arg("count", self.suggestions.len());
+        diag.multipart_suggestion(fluent::lint_suggestion, self.suggestions, self.applicability);
     }
 }
 
-#[derive(LintDiagnostic)]
-#[diag(lint_builtin_type_alias_where_clause)]
-pub struct BuiltinTypeAliasWhereClause<'a, 'b> {
-    #[suggestion(code = "", applicability = "machine-applicable")]
-    pub suggestion: Span,
-    #[subdiagnostic]
-    pub sub: Option<SuggestChangingAssocTypes<'a, 'b>>,
-}
-
-#[derive(LintDiagnostic)]
-#[diag(lint_builtin_type_alias_generic_bounds)]
-pub struct BuiltinTypeAliasGenericBounds<'a, 'b> {
-    #[subdiagnostic]
-    pub suggestion: BuiltinTypeAliasGenericBoundsSuggestion,
-    #[subdiagnostic]
-    pub sub: Option<SuggestChangingAssocTypes<'a, 'b>>,
-}
-
-#[derive(LintDiagnostic)]
-#[diag(lint_macro_expr_fragment_specifier_2024_migration)]
-pub struct MacroExprFragment2024 {
-    #[suggestion(code = "expr_2021", applicability = "machine-applicable")]
-    pub suggestion: Span,
+pub struct TypeAliasBoundsQualifyAssocTysSugg<'a, 'hir> {
+    pub ty: &'a hir::Ty<'hir>,
 }
 
-pub struct BuiltinTypeAliasGenericBoundsSuggestion {
-    pub suggestions: Vec<(Span, String)>,
-}
-
-impl Subdiagnostic for BuiltinTypeAliasGenericBoundsSuggestion {
+impl<'a, 'hir> Subdiagnostic for TypeAliasBoundsQualifyAssocTysSugg<'a, 'hir> {
     fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
         self,
         diag: &mut Diag<'_, G>,
         _f: &F,
     ) {
-        diag.multipart_suggestion(
-            fluent::lint_suggestion,
-            self.suggestions,
-            Applicability::MachineApplicable,
-        );
+        // We perform the walk in here instead of in `<TypeAliasBounds as LateLintPass>` to
+        // avoid doing throwaway work in case the lint ends up getting suppressed.
+
+        use hir::intravisit::Visitor;
+        struct ProbeShorthandAssocTys<'a, 'b, G: EmissionGuarantee> {
+            diag: &'a mut Diag<'b, G>,
+        }
+        impl<'a, 'b, G: EmissionGuarantee> Visitor<'_> for ProbeShorthandAssocTys<'a, 'b, G> {
+            fn visit_qpath(&mut self, qpath: &hir::QPath<'_>, id: hir::HirId, _: Span) {
+                // Look for "type-parameter shorthand-associated-types". I.e., paths of the
+                // form `T::Assoc` with `T` type param. These are reliant on trait bounds.
+                // Suggest fully qualifying them via `<T as /* Trait */>::Assoc`.
+                //
+                // Instead of attempting to figure out the necessary trait ref, just use a
+                // placeholder. Since we don't record type-dependent resolutions for non-body
+                // items like type aliases, we can't simply deduce the corresp. trait from
+                // the HIR path alone without rerunning parts of HIR ty lowering here
+                // (namely `probe_single_ty_param_bound_for_assoc_ty`) which is infeasible.
+                //
+                // (We could employ some simple heuristics but that's likely not worth it).
+                if let hir::QPath::TypeRelative(qself, _) = qpath
+                    && let hir::TyKind::Path(hir::QPath::Resolved(None, path)) = qself.kind
+                    && let hir::def::Res::Def(hir::def::DefKind::TyParam, _) = path.res
+                {
+                    self.diag.multipart_suggestion(
+                        fluent::lint_builtin_type_alias_bounds_qualify_assoc_tys_sugg,
+                        vec![
+                            (qself.span.shrink_to_lo(), "<".into()),
+                            (qself.span.shrink_to_hi(), " as /* Trait */>".into()),
+                        ],
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                hir::intravisit::walk_qpath(self, qpath, id)
+            }
+        }
+        ProbeShorthandAssocTys { diag }.visit_ty(self.ty);
     }
 }
 

tests/ui/associated-inherent-types/type-alias-bounds.rs

@@ -19,7 +19,7 @@
 // automatically lead to full wfchecking and lint TAB getting suppressed.
 
 pub type Alias<T: Bound> = (Source<T>::Assoc,);
-//~^ WARN bounds on generic parameters are not enforced in type aliases
+//~^ WARN bounds on generic parameters in type aliases are not enforced
 
 pub struct Source<T>(T);
 pub trait Bound {}

tests/ui/associated-inherent-types/type-alias-bounds.stderr

@@ -1,15 +1,16 @@
-warning: bounds on generic parameters are not enforced in type aliases
+warning: bounds on generic parameters in type aliases are not enforced
   --> $DIR/type-alias-bounds.rs:21:19
    |
 LL | pub type Alias<T: Bound> = (Source<T>::Assoc,);
-   |                   ^^^^^
+   |                 --^^^^^
+   |                 | |
+   |                 | will not be checked at usage sites of the type alias
+   |                 help: remove this bound
    |
+   = note: this is a known limitation of the type checker that may be lifted in a future edition.
+           see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
+   = help: add `#![feature(lazy_type_alias)]` to the crate attributes to enable the desired semantics
    = note: `#[warn(type_alias_bounds)]` on by default
-help: the bound will not be checked when the type alias is used, and should be removed
-   |
-LL - pub type Alias<T: Bound> = (Source<T>::Assoc,);
-LL + pub type Alias<T> = (Source<T>::Assoc,);
-   |
 
 warning: 1 warning emitted