Tweak output of missing lifetime on associated type

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -1103,14 +1103,14 @@ fn check_region_bounds_on_impl_item<'tcx>(
         .expect("expected impl item to have generics or else we can't compare them")
         .span;
 
-    let mut generics_span = None;
+    let mut generics_span = tcx.def_span(trait_m.def_id);
     let mut bounds_span = vec![];
     let mut where_span = None;
 
     if let Some(trait_node) = tcx.hir_get_if_local(trait_m.def_id)
         && let Some(trait_generics) = trait_node.generics()
     {
-        generics_span = Some(trait_generics.span);
+        generics_span = trait_generics.span;
         // FIXME: we could potentially look at the impl's bounds to not point at bounds that
         // *are* present in the impl.
         for p in trait_generics.predicates {

compiler/rustc_hir_analysis/src/errors.rs

@@ -190,7 +190,7 @@ pub(crate) struct LifetimesOrBoundsMismatchOnTrait {
     #[label]
     pub span: Span,
     #[label(hir_analysis_generics_label)]
-    pub generics_span: Option<Span>,
+    pub generics_span: Span,
     #[label(hir_analysis_where_label)]
     pub where_span: Option<Span>,
     #[label(hir_analysis_bounds_label)]

compiler/rustc_hir_analysis/src/impl_wf_check.rs

@@ -12,11 +12,12 @@ use std::assert_matches::debug_assert_matches;
 
 use min_specialization::check_min_specialization;
 use rustc_data_structures::fx::FxHashSet;
+use rustc_errors::Applicability;
 use rustc_errors::codes::*;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::LocalDefId;
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
-use rustc_span::ErrorGuaranteed;
+use rustc_span::{ErrorGuaranteed, kw};
 
 use crate::constrained_generic_params as cgp;
 use crate::errors::UnconstrainedGenericParameter;
@@ -158,6 +159,27 @@ pub(crate) fn enforce_impl_lifetime_params_are_constrained(
                         const_param_note2: false,
                     });
                     diag.code(E0207);
+                    for p in &impl_generics.own_params {
+                        if p.name == kw::UnderscoreLifetime {
+                            let span = tcx.def_span(p.def_id);
+                            let Ok(snippet) = tcx.sess.source_map().span_to_snippet(span) else {
+                                continue;
+                            };
+
+                            let (span, sugg) = if &snippet == "'_" {
+                                (span, param.name.to_string())
+                            } else {
+                                (span.shrink_to_hi(), format!("{} ", param.name))
+                            };
+                            diag.span_suggestion_verbose(
+                                span,
+                                "consider using the named lifetime here instead of an implict \
+                                 lifetime",
+                                sugg,
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                    }
                     res = Err(diag.emit());
                 }
             }

compiler/rustc_resolve/messages.ftl

@@ -11,9 +11,9 @@ resolve_added_macro_use =
 resolve_ancestor_only =
     visibilities can only be restricted to ancestor modules
 
-resolve_anonymous_lifetime_non_gat_report_error =
-    in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
+resolve_anonymous_lifetime_non_gat_report_error = missing lifetime in associated type
     .label = this lifetime must come from the implemented type
+    .note = in the trait the associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
 
 resolve_arguments_macro_use_not_allowed = arguments to `macro_use` are not allowed here
 

compiler/rustc_resolve/src/errors.rs

@@ -897,6 +897,8 @@ pub(crate) struct AnonymousLifetimeNonGatReportError {
     #[primary_span]
     #[label]
     pub(crate) lifetime: Span,
+    #[note]
+    pub(crate) decl: MultiSpan,
 }
 
 #[derive(Subdiagnostic)]

compiler/rustc_resolve/src/late.rs

@@ -11,6 +11,7 @@ use std::borrow::Cow;
 use std::collections::BTreeSet;
 use std::collections::hash_map::Entry;
 use std::mem::{replace, swap, take};
+use std::ops::ControlFlow;
 
 use rustc_ast::visit::{
     AssocCtxt, BoundKind, FnCtxt, FnKind, Visitor, try_visit, visit_opt, walk_list,
@@ -20,20 +21,21 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
 use rustc_data_structures::unord::{UnordMap, UnordSet};
 use rustc_errors::codes::*;
 use rustc_errors::{
-    Applicability, DiagArgValue, ErrorGuaranteed, IntoDiagArg, StashKey, Suggestions,
+    Applicability, Diag, DiagArgValue, ErrorGuaranteed, IntoDiagArg, MultiSpan, StashKey,
+    Suggestions, pluralize,
 };
 use rustc_hir::def::Namespace::{self, *};
 use rustc_hir::def::{self, CtorKind, DefKind, LifetimeRes, NonMacroAttrKind, PartialRes, PerNS};
 use rustc_hir::def_id::{CRATE_DEF_ID, DefId, LOCAL_CRATE, LocalDefId};
 use rustc_hir::{MissingLifetimeKind, PrimTy, TraitCandidate};
 use rustc_middle::middle::resolve_bound_vars::Set1;
-use rustc_middle::ty::{DelegationFnSig, Visibility};
+use rustc_middle::ty::{AssocTag, DelegationFnSig, Visibility};
 use rustc_middle::{bug, span_bug};
 use rustc_session::config::{CrateType, ResolveDocLinks};
 use rustc_session::lint::{self, BuiltinLintDiag};
 use rustc_session::parse::feature_err;
 use rustc_span::source_map::{Spanned, respan};
-use rustc_span::{BytePos, Ident, Span, Symbol, SyntaxContext, kw, sym};
+use rustc_span::{BytePos, DUMMY_SP, Ident, Span, Symbol, SyntaxContext, kw, sym};
 use smallvec::{SmallVec, smallvec};
 use thin_vec::ThinVec;
 use tracing::{debug, instrument, trace};
@@ -372,11 +374,15 @@ enum LifetimeBinderKind {
     FnPtrType,
     PolyTrait,
     WhereBound,
+    // Item covers foreign items, ADTs, type aliases, trait associated items and
+    // trait alias associated items.
     Item,
     ConstItem,
     Function,
     Closure,
     ImplBlock,
+    // Covers only `impl` associated types.
+    ImplAssocType,
 }
 
 impl LifetimeBinderKind {
@@ -387,6 +393,7 @@ impl LifetimeBinderKind {
             PolyTrait => "bound",
             WhereBound => "bound",
             Item | ConstItem => "item",
+            ImplAssocType => "associated type",
             ImplBlock => "impl block",
             Function => "function",
             Closure => "closure",
@@ -721,6 +728,9 @@ struct DiagMetadata<'ast> {
     /// The current impl items (used to suggest).
     current_impl_items: Option<&'ast [Box<AssocItem>]>,
 
+    /// The current impl items (used to suggest).
+    current_impl_item: Option<&'ast AssocItem>,
+
     /// When processing impl trait
     currently_processing_impl_trait: Option<(TraitRef, Ty)>,
 
@@ -1887,9 +1897,35 @@ impl<'a, 'ast, 'ra, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                                     ty: ty.span,
                                 });
                             } else {
-                                self.r.dcx().emit_err(errors::AnonymousLifetimeNonGatReportError {
-                                    lifetime: lifetime.ident.span,
-                                });
+                                let decl = if !trait_id.is_local()
+                                    && let Some(assoc) = self.diag_metadata.current_impl_item
+                                    && let AssocItemKind::Type(_) = assoc.kind
+                                    && let assocs = self.r.tcx.associated_items(trait_id)
+                                    && let Some(ident) = assoc.kind.ident()
+                                    && let Some(assoc) = assocs.find_by_ident_and_kind(
+                                        self.r.tcx,
+                                        ident,
+                                        AssocTag::Type,
+                                        trait_id,
+                                    ) {
+                                    let mut decl: MultiSpan =
+                                        self.r.tcx.def_span(assoc.def_id).into();
+                                    decl.push_span_label(
+                                        self.r.tcx.def_span(trait_id),
+                                        String::new(),
+                                    );
+                                    decl
+                                } else {
+                                    DUMMY_SP.into()
+                                };
+                                let mut err = self.r.dcx().create_err(
+                                    errors::AnonymousLifetimeNonGatReportError {
+                                        lifetime: lifetime.ident.span,
+                                        decl,
+                                    },
+                                );
+                                self.point_at_impl_lifetimes(&mut err, i, lifetime.ident.span);
+                                err.emit();
                             }
                         } else {
                             self.r.dcx().emit_err(errors::ElidedAnonymousLifetimeReportError {
@@ -1926,6 +1962,97 @@ impl<'a, 'ast, 'ra, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
         self.report_missing_lifetime_specifiers(vec![missing_lifetime], None);
     }
 
+    fn point_at_impl_lifetimes(&mut self, err: &mut Diag<'_>, i: usize, lifetime: Span) {
+        let Some((rib, span)) = self.lifetime_ribs[..i]
+            .iter()
+            .rev()
+            .find_map(|rib| match rib.kind {
+                LifetimeRibKind::Generics { span, kind: LifetimeBinderKind::ImplBlock, .. } => {
+                    Some((rib, span))
+                }
+                _ => None,
+            })
+        else {
+            return;
+        };
+        if !rib.bindings.is_empty() {
+            err.span_label(
+                span,
+                format!(
+                    "there {} named lifetime{} specified on the impl block you could use",
+                    if rib.bindings.len() == 1 { "is a" } else { "are" },
+                    pluralize!(rib.bindings.len()),
+                ),
+            );
+            if rib.bindings.len() == 1 {
+                err.span_suggestion_verbose(
+                    lifetime.shrink_to_hi(),
+                    "consider using the lifetime from the impl block",
+                    format!("{} ", rib.bindings.keys().next().unwrap()),
+                    Applicability::MaybeIncorrect,
+                );
+            }
+        } else {
+            struct AnonRefFinder;
+            impl<'ast> Visitor<'ast> for AnonRefFinder {
+                type Result = ControlFlow<Span>;
+
+                fn visit_ty(&mut self, ty: &'ast ast::Ty) -> Self::Result {
+                    if let ast::TyKind::Ref(None, mut_ty) = &ty.kind {
+                        return ControlFlow::Break(mut_ty.ty.span.shrink_to_lo());
+                    }
+                    visit::walk_ty(self, ty)
+                }
+
+                fn visit_lifetime(
+                    &mut self,
+                    lt: &'ast ast::Lifetime,
+                    _cx: visit::LifetimeCtxt,
+                ) -> Self::Result {
+                    if lt.ident.name == kw::UnderscoreLifetime {
+                        return ControlFlow::Break(lt.ident.span);
+                    }
+                    visit::walk_lifetime(self, lt)
+                }
+            }
+
+            if let Some(ty) = &self.diag_metadata.current_self_type
+                && let ControlFlow::Break(sp) = AnonRefFinder.visit_ty(ty)
+            {
+                err.multipart_suggestion_verbose(
+                    "add a lifetime to the impl block and use it in the self type and associated \
+                     type",
+                    vec![
+                        (span, "<'a>".to_string()),
+                        (sp, "'a ".to_string()),
+                        (lifetime.shrink_to_hi(), "'a ".to_string()),
+                    ],
+                    Applicability::MaybeIncorrect,
+                );
+            } else if let Some(item) = &self.diag_metadata.current_item
+                && let ItemKind::Impl(impl_) = &item.kind
+                && let Some(of_trait) = &impl_.of_trait
+                && let ControlFlow::Break(sp) = AnonRefFinder.visit_trait_ref(&of_trait.trait_ref)
+            {
+                err.multipart_suggestion_verbose(
+                    "add a lifetime to the impl block and use it in the trait and associated type",
+                    vec![
+                        (span, "<'a>".to_string()),
+                        (sp, "'a".to_string()),
+                        (lifetime.shrink_to_hi(), "'a ".to_string()),
+                    ],
+                    Applicability::MaybeIncorrect,
+                );
+            } else {
+                err.span_label(
+                    span,
+                    "you could add a lifetime on the impl block, if the trait or the self type \
+                     could have one",
+                );
+            }
+        }
+    }
+
     #[instrument(level = "debug", skip(self))]
     fn resolve_elided_lifetime(&mut self, anchor_id: NodeId, span: Span) {
         let id = self.r.next_node_id();
@@ -3261,6 +3388,8 @@ impl<'a, 'ast, 'ra, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
     ) {
         use crate::ResolutionError::*;
         self.resolve_doc_links(&item.attrs, MaybeExported::ImplItem(trait_id.ok_or(&item.vis)));
+        let prev = self.diag_metadata.current_impl_item.take();
+        self.diag_metadata.current_impl_item = Some(&item);
         match &item.kind {
             AssocItemKind::Const(box ast::ConstItem {
                 ident,
@@ -3360,7 +3489,7 @@ impl<'a, 'ast, 'ra, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                     &generics.params,
                     RibKind::AssocItem,
                     item.id,
-                    LifetimeBinderKind::Item,
+                    LifetimeBinderKind::ImplAssocType,
                     generics.span,
                     |this| {
                         this.with_lifetime_rib(LifetimeRibKind::AnonymousReportError, |this| {
@@ -3409,6 +3538,7 @@ impl<'a, 'ast, 'ra, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                 panic!("unexpanded macro in resolve!")
             }
         }
+        self.diag_metadata.current_impl_item = prev;
     }
 
     fn check_trait_item<F>(

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -3154,6 +3154,9 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                     {
                         continue;
                     }
+                    if let LifetimeBinderKind::ImplAssocType = kind {
+                        continue;
+                    }
 
                     if !span.can_be_used_for_suggestions()
                         && suggest_note

tests/ui/impl-header-lifetime-elision/assoc-type.rs

@@ -9,14 +9,27 @@ trait MyTrait {
 
 impl MyTrait for &i32 {
     type Output = &i32;
-    //~^ ERROR in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
+    //~^ ERROR missing lifetime in associated type
 }
 
 impl MyTrait for &u32 {
     type Output = &'_ i32;
     //~^ ERROR `'_` cannot be used here
 }
 
+impl<'a> MyTrait for &f64 {
+    type Output = &f64;
+    //~^ ERROR missing lifetime in associated type
+}
+
+trait OtherTrait<'a> {
+    type Output;
+}
+impl OtherTrait<'_> for f64 {
+    type Output = &f64;
+    //~^ ERROR missing lifetime in associated type
+}
+
 // This is what you have to do:
 impl<'a> MyTrait for &'a f32 {
     type Output = &'a f32;