E0229: Suggest Moving Type Constraints to Type Parameter Declaration

Author: veera-sivarajan

compiler/rustc_hir/src/hir.rs

@@ -2456,6 +2456,15 @@ pub enum AssocItemConstraintKind<'hir> {
     Bound { bounds: &'hir [GenericBound<'hir>] },
 }
 
+impl<'hir> AssocItemConstraintKind<'hir> {
+    pub fn descr(&self) -> &'static str {
+        match self {
+            AssocItemConstraintKind::Equality { .. } => "binding",
+            AssocItemConstraintKind::Bound { .. } => "constraint",
+        }
+    }
+}
+
 #[derive(Debug, Clone, Copy, HashStable_Generic)]
 pub struct Ty<'hir> {
     pub hir_id: HirId,
@@ -3735,6 +3744,21 @@ impl<'hir> Node<'hir> {
         }
     }
 
+    /// Get a `hir::Impl` if the node is an impl block for the given `trait_def_id`.
+    pub fn impl_block_of_trait(self, trait_def_id: DefId) -> Option<&'hir Impl<'hir>> {
+        match self {
+            Node::Item(Item { kind: ItemKind::Impl(impl_block), .. })
+                if impl_block
+                    .of_trait
+                    .and_then(|trait_ref| trait_ref.trait_def_id())
+                    .is_some_and(|trait_id| trait_id == trait_def_id) =>
+            {
+                Some(impl_block)
+            }
+            _ => None,
+        }
+    }
+
     pub fn fn_sig(self) -> Option<&'hir FnSig<'hir>> {
         match self {
             Node::TraitItem(TraitItem { kind: TraitItemKind::Fn(fn_sig, _), .. })

compiler/rustc_hir_analysis/src/hir_ty_lowering/errors.rs

@@ -1217,7 +1217,6 @@ pub fn prohibit_assoc_item_constraint(
     // otherwise suggest the removal of the binding.
     if let Some((def_id, segment, _)) = segment
         && segment.args().parenthesized == hir::GenericArgsParentheses::No
-        && let hir::AssocItemConstraintKind::Equality { term } = constraint.kind
     {
         // Suggests removal of the offending binding
         let suggest_removal = |e: &mut Diag<'_>| {
@@ -1263,7 +1262,7 @@ pub fn prohibit_assoc_item_constraint(
             if let Ok(suggestion) = tcx.sess.source_map().span_to_snippet(removal_span) {
                 e.span_suggestion_verbose(
                     removal_span,
-                    "consider removing this associated item binding",
+                    format!("consider removing this associated item {}", constraint.kind.descr()),
                     suggestion,
                     Applicability::MaybeIncorrect,
                 );
@@ -1286,19 +1285,73 @@ pub fn prohibit_assoc_item_constraint(
         // Check if the type has a generic param with the same name
         // as the assoc type name in the associated item binding.
         let generics = tcx.generics_of(def_id);
-        let matching_param =
-            generics.own_params.iter().find(|p| p.name.as_str() == constraint.ident.as_str());
+        let matching_param = generics.own_params.iter().find(|p| p.name == constraint.ident.name);
 
         // Now emit the appropriate suggestion
         if let Some(matching_param) = matching_param {
-            match (&matching_param.kind, term) {
-                (GenericParamDefKind::Type { .. }, hir::Term::Ty(ty)) => {
-                    suggest_direct_use(&mut err, ty.span);
-                }
-                (GenericParamDefKind::Const { .. }, hir::Term::Const(c)) => {
+            match (constraint.kind, &matching_param.kind) {
+                (
+                    hir::AssocItemConstraintKind::Equality { term: hir::Term::Ty(ty) },
+                    GenericParamDefKind::Type { .. },
+                ) => suggest_direct_use(&mut err, ty.span),
+                (
+                    hir::AssocItemConstraintKind::Equality { term: hir::Term::Const(c) },
+                    GenericParamDefKind::Const { .. },
+                ) => {
                     let span = tcx.hir().span(c.hir_id);
                     suggest_direct_use(&mut err, span);
                 }
+                (hir::AssocItemConstraintKind::Bound { bounds }, _) => {
+                    // Suggest `impl<T: Bound> Trait<T> for Foo` when finding
+                    // `impl Trait<T: Bound> for Foo`
+
+                    // Get the parent impl block based on the binding we have
+                    // and the trait DefId
+                    let impl_block = tcx
+                        .hir()
+                        .parent_iter(constraint.hir_id)
+                        .find_map(|(_, node)| node.impl_block_of_trait(def_id));
+
+                    let type_with_constraints =
+                        tcx.sess.source_map().span_to_snippet(constraint.span);
+
+                    if let Some(impl_block) = impl_block
+                        && let Ok(type_with_constraints) = type_with_constraints
+                    {
+                        // Filter out the lifetime parameters because
+                        // they should be declared before the type parameter
+                        let lifetimes: String = bounds
+                            .iter()
+                            .filter_map(|bound| {
+                                if let hir::GenericBound::Outlives(lifetime) = bound {
+                                    Some(format!("{lifetime}, "))
+                                } else {
+                                    None
+                                }
+                            })
+                            .collect();
+                        // Figure out a span and suggestion string based on
+                        // whether there are any existing parameters
+                        let param_decl = if let Some(param_span) =
+                            impl_block.generics.span_for_param_suggestion()
+                        {
+                            (param_span, format!(", {lifetimes}{type_with_constraints}"))
+                        } else {
+                            (
+                                impl_block.generics.span.shrink_to_lo(),
+                                format!("<{lifetimes}{type_with_constraints}>"),
+                            )
+                        };
+                        let suggestions =
+                            vec![param_decl, (constraint.span, format!("{}", matching_param.name))];
+
+                        err.multipart_suggestion_verbose(
+                            format!("declare the type parameter right after the `impl` keyword"),
+                            suggestions,
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                }
                 _ => suggest_removal(&mut err),
             }
         } else {

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -531,6 +531,7 @@ pub(crate) fn check_generic_arg_count(
 
         let num_default_params = expected_max - expected_min;
 
+        let mut all_params_are_binded = false;
         let gen_args_info = if provided > expected_max {
             invalid_args.extend((expected_max..provided).map(|i| i + args_offset));
             let num_redundant_args = provided - expected_max;
@@ -547,6 +548,20 @@ pub(crate) fn check_generic_arg_count(
         } else {
             let num_missing_args = expected_max - provided;
 
+            let constraint_names: Vec<_> =
+                gen_args.constraints.iter().map(|b| b.ident.name).collect();
+            let param_names: Vec<_> = gen_params
+                .own_params
+                .iter()
+                .filter(|param| !has_self || param.index != 0) // Assumes `Self` will always be the first parameter
+                .map(|param| param.name)
+                .collect();
+            if constraint_names == param_names {
+                // We set this to true and delay emitting `WrongNumberOfGenericArgs`
+                // to provide a succinct error for cases like issue #113073
+                all_params_are_binded = true;
+            };
+
             GenericArgsInfo::MissingTypesOrConsts {
                 num_missing_args,
                 num_default_params,
@@ -567,7 +582,7 @@ pub(crate) fn check_generic_arg_count(
                 def_id,
             )
             .diagnostic()
-            .emit()
+            .emit_unless(all_params_are_binded)
         });
 
         Err(reported)

tests/ui/associated-type-bounds/no-gat-position.rs

@@ -5,7 +5,7 @@ pub trait Iter {
 
     fn next<'a>(&'a mut self) -> Option<Self::Item<'a, As1: Copy>>;
     //~^ ERROR associated item constraints are not allowed here
-    //~| HELP consider removing this associated item binding
+    //~| HELP consider removing this associated item constraint
 }
 
 impl Iter for () {

tests/ui/associated-type-bounds/no-gat-position.stderr

@@ -4,7 +4,7 @@ error[E0229]: associated item constraints are not allowed here
 LL |     fn next<'a>(&'a mut self) -> Option<Self::Item<'a, As1: Copy>>;
    |                                                        ^^^^^^^^^ associated item constraint not allowed here
    |
-help: consider removing this associated item binding
+help: consider removing this associated item constraint
    |
 LL |     fn next<'a>(&'a mut self) -> Option<Self::Item<'a, As1: Copy>>;
    |                                                      ~~~~~~~~~~~

tests/ui/async-await/return-type-notation/rtn-in-impl-signature.stderr

@@ -13,7 +13,7 @@ error[E0229]: associated item constraints are not allowed here
 LL | impl Super1<'_, bar(): Send> for () {}
    |                 ^^^^^^^^^^^ associated item constraint not allowed here
    |
-help: consider removing this associated item binding
+help: consider removing this associated item constraint
    |
 LL | impl Super1<'_, bar(): Send> for () {}
    |               ~~~~~~~~~~~~~