Address comments

compiler/rustc_ast_lowering/src/item.rs

@@ -1378,7 +1378,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let mut params: SmallVec<[hir::GenericParam<'hir>; 4]> =
             self.lower_generic_params_mut(&generics.params).collect();
         let has_where_clause_predicates = !generics.where_clause.predicates.is_empty();
-        let has_where_clause_token = generics.where_clause.has_where_token;
         let where_clause_span = self.lower_span(generics.where_clause.span);
         let span = self.lower_span(generics.span);
         let res = f(self);
@@ -1397,7 +1396,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
             params: self.arena.alloc_from_iter(params),
             predicates: self.arena.alloc_from_iter(predicates),
             has_where_clause_predicates,
-            has_where_clause_token,
             where_clause_span,
             span,
         });

compiler/rustc_ast_lowering/src/lib.rs

@@ -1316,7 +1316,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     params: lifetime_defs,
                     predicates: &[],
                     has_where_clause_predicates: false,
-                    has_where_clause_token: false,
                     where_clause_span: lctx.lower_span(span),
                     span: lctx.lower_span(span),
                 }),
@@ -1639,7 +1638,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     params: generic_params,
                     predicates: &[],
                     has_where_clause_predicates: false,
-                    has_where_clause_token: false,
                     where_clause_span: this.lower_span(span),
                     span: this.lower_span(span),
                 }),

compiler/rustc_hir/src/hir.rs

@@ -536,7 +536,6 @@ pub struct Generics<'hir> {
     pub params: &'hir [GenericParam<'hir>],
     pub predicates: &'hir [WherePredicate<'hir>],
     pub has_where_clause_predicates: bool,
-    pub has_where_clause_token: bool,
     pub where_clause_span: Span,
     pub span: Span,
 }
@@ -547,7 +546,6 @@ impl<'hir> Generics<'hir> {
             params: &[],
             predicates: &[],
             has_where_clause_predicates: false,
-            has_where_clause_token: false,
             where_clause_span: DUMMY_SP,
             span: DUMMY_SP,
         };
@@ -583,10 +581,6 @@ impl<'hir> Generics<'hir> {
         }
     }
 
-    pub fn where_clause_span(&self) -> Option<Span> {
-        if self.predicates.is_empty() { None } else { Some(self.where_clause_span) }
-    }
-
     /// `Span` where further predicates would be suggested, accounting for trailing commas, like
     ///  in `fn foo<T>(t: T) where T: Foo,` so we don't suggest two trailing commas.
     pub fn tail_span_for_predicate_suggestion(&self) -> Span {
@@ -607,10 +601,11 @@ impl<'hir> Generics<'hir> {
     pub fn add_where_or_trailing_comma(&self) -> &'static str {
         if self.has_where_clause_predicates {
             ","
-        } else if self.has_where_clause_token {
-            ""
-        } else {
+        } else if self.where_clause_span.is_empty() {
             " where"
+        } else {
+            // No where clause predicates, but we have `where` token
+            ""
         }
     }
 

compiler/rustc_lint/src/builtin.rs

@@ -2295,9 +2295,7 @@ impl<'tcx> LateLintPass<'tcx> for ExplicitOutlivesRequirements {
             // (including the `where`)
             if hir_generics.has_where_clause_predicates && dropped_predicate_count == num_predicates
             {
-                let where_span = hir_generics
-                    .where_clause_span()
-                    .expect("span of (nonempty) where clause should exist");
+                let where_span = hir_generics.where_clause_span;
                 // Extend the where clause back to the closing `>` of the
                 // generics, except for tuple struct, which have the `where`
                 // after the fields of the struct.

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -76,21 +76,21 @@ impl<'tcx> Ty<'tcx> {
 }
 
 pub trait IsSuggestable<'tcx> {
+    /// Whether this makes sense to suggest in a diagnostic.
+    ///
+    /// We filter out certain types and constants since they don't provide
+    /// meaningful rendered suggestions when pretty-printed. We leave some
+    /// nonsense, such as region vars, since those render as `'_` and are
+    /// usually okay to reinterpret as elided lifetimes.
     fn is_suggestable(self, tcx: TyCtxt<'tcx>) -> bool;
-
-    fn is_suggestable_modulo_impl_trait(self, tcx: TyCtxt<'tcx>, bound_str: &str) -> bool;
 }
 
 impl<'tcx, T> IsSuggestable<'tcx> for T
 where
     T: TypeFoldable<'tcx>,
 {
     fn is_suggestable(self, tcx: TyCtxt<'tcx>) -> bool {
-        self.visit_with(&mut IsSuggestableVisitor { tcx, bound_str: None }).is_continue()
-    }
-
-    fn is_suggestable_modulo_impl_trait(self, tcx: TyCtxt<'tcx>, bound_str: &str) -> bool {
-        self.visit_with(&mut IsSuggestableVisitor { tcx, bound_str: Some(bound_str) }).is_continue()
+        self.visit_with(&mut IsSuggestableVisitor { tcx }).is_continue()
     }
 }
 
@@ -119,7 +119,7 @@ pub fn suggest_arbitrary_trait_bound<'tcx>(
         &format!(
             "consider {} `where` clause, but there might be an alternative better way to express \
              this requirement",
-            if generics.has_where_clause_token { "extending the" } else { "introducing a" },
+            if generics.where_clause_span.is_empty() { "introducing a" } else { "extending the" },
         ),
         format!("{} {}: {}", generics.add_where_or_trailing_comma(), param_name, constraint),
         Applicability::MaybeIncorrect,
@@ -417,12 +417,11 @@ impl<'v> hir::intravisit::Visitor<'v> for StaticLifetimeVisitor<'v> {
     }
 }
 
-pub struct IsSuggestableVisitor<'tcx, 's> {
+pub struct IsSuggestableVisitor<'tcx> {
     tcx: TyCtxt<'tcx>,
-    bound_str: Option<&'s str>,
 }
 
-impl<'tcx> TypeVisitor<'tcx> for IsSuggestableVisitor<'tcx, '_> {
+impl<'tcx> TypeVisitor<'tcx> for IsSuggestableVisitor<'tcx> {
     type BreakTy = ();
 
     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
@@ -462,12 +461,13 @@ impl<'tcx> TypeVisitor<'tcx> for IsSuggestableVisitor<'tcx, '_> {
             }
 
             Param(param) => {
-                if let Some(found_bound_str) =
-                    param.name.as_str().strip_prefix("impl ").map(|s| s.trim_start())
-                {
-                    if self.bound_str.map_or(true, |bound_str| bound_str != found_bound_str) {
-                        return ControlFlow::Break(());
-                    }
+                // FIXME: It would be nice to make this not use string manipulation,
+                // but it's pretty hard to do this, since `ty::ParamTy` is missing
+                // sufficient info to determine if it is synthetic, and we don't
+                // always have a convenient way of getting `ty::Generics` at the call
+                // sites we invoke `IsSuggestable::is_suggestable`.
+                if param.name.as_str().starts_with("impl ") {
+                    return ControlFlow::Break(());
                 }
             }
 

compiler/rustc_middle/src/ty/generics.rs

@@ -39,6 +39,13 @@ impl GenericParamDefKind {
             GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => true,
         }
     }
+
+    pub fn is_synthetic(&self) -> bool {
+        match self {
+            GenericParamDefKind::Type { synthetic, .. } => *synthetic,
+            _ => false,
+        }
+    }
 }
 
 #[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]