In some limited cases, suggest where bounds for non-type params

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -75,6 +75,36 @@ impl<'tcx> TyS<'tcx> {
     }
 }
 
+pub fn suggest_arbitrary_trait_bound(
+    generics: &hir::Generics<'_>,
+    err: &mut DiagnosticBuilder<'_>,
+    param_name: &str,
+    constraint: &str,
+) -> bool {
+    let param = generics.params.iter().find(|p| p.name.ident().as_str() == param_name);
+    match (param, param_name) {
+        (Some(_), "Self") => return false,
+        _ => {}
+    }
+    // Suggest a where clause bound for a non-type paremeter.
+    let (action, prefix) = if generics.where_clause.predicates.is_empty() {
+        ("introducing a", " where ")
+    } else {
+        ("extending the", ", ")
+    };
+    err.span_suggestion_verbose(
+        generics.where_clause.tail_span_for_suggestion(),
+        &format!(
+            "consider {} `where` bound, but there might be an alternative better way to express \
+             this requirement",
+            action,
+        ),
+        format!("{}{}: {}", prefix, param_name, constraint),
+        Applicability::MaybeIncorrect,
+    );
+    true
+}
+
 /// Suggest restricting a type param with a new bound.
 pub fn suggest_constraining_type_param(
     tcx: TyCtxt<'_>,

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -17,8 +17,8 @@ use rustc_hir::intravisit::Visitor;
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node};
 use rustc_middle::ty::{
-    self, suggest_constraining_type_param, AdtKind, DefIdTree, Infer, InferTy, ToPredicate, Ty,
-    TyCtxt, TypeFoldable, WithConstness,
+    self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind, DefIdTree,
+    Infer, InferTy, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness,
 };
 use rustc_middle::ty::{TypeAndMut, TypeckResults};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
@@ -334,7 +334,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let (param_ty, projection) = match self_ty.kind() {
             ty::Param(_) => (true, None),
             ty::Projection(projection) => (false, Some(projection)),
-            _ => return,
+            _ => (false, None),
         };
 
         // FIXME: Add check for trait bound that is already present, particularly `?Sized` so we
@@ -453,6 +453,26 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                     }
                 }
 
+                hir::Node::Item(hir::Item {
+                    kind:
+                        hir::ItemKind::Struct(_, generics)
+                        | hir::ItemKind::Enum(_, generics)
+                        | hir::ItemKind::Union(_, generics)
+                        | hir::ItemKind::Trait(_, _, generics, ..)
+                        | hir::ItemKind::Impl(hir::Impl { generics, .. })
+                        | hir::ItemKind::Fn(_, generics, _)
+                        | hir::ItemKind::TyAlias(_, generics)
+                        | hir::ItemKind::TraitAlias(generics, _)
+                        | hir::ItemKind::OpaqueTy(hir::OpaqueTy { generics, .. }),
+                    ..
+                }) if !param_ty => {
+                    // Missing generic type parameter bound.
+                    let param_name = self_ty.to_string();
+                    let constraint = trait_ref.print_only_trait_path().to_string();
+                    if suggest_arbitrary_trait_bound(generics, &mut err, &param_name, &constraint) {
+                        return;
+                    }
+                }
                 hir::Node::Crate(..) => return,
 
                 _ => {}

src/test/ui/partialeq_help.stderr

@@ -5,6 +5,10 @@ LL |     a == b;
    |       ^^ no implementation for `&T == T`
    |
    = help: the trait `PartialEq<T>` is not implemented for `&T`
+help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
+   |
+LL | fn foo<T: PartialEq>(a: &T, b: T) where &T: PartialEq<T> {
+   |                                   ^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/specialization/deafult-associated-type-bound-2.stderr

@@ -18,6 +18,10 @@ LL |     default type U = &'static B;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `&'static B == B`
    |
    = help: the trait `PartialEq<B>` is not implemented for `&'static B`
+help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
+   |
+LL | impl<B: 'static, T> X<B> for T where &'static B: PartialEq<B> {
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error; 1 warning emitted
 

src/test/ui/suggestions/suggest-change-mut.rs

@@ -2,7 +2,7 @@
 
 use std::io::{BufRead, BufReader, Read, Write};
 
-fn issue_81421<T: Read + Write>(mut stream: T) {
+fn issue_81421<T: Read + Write>(mut stream: T) { //~ HELP consider introducing a `where` bound
     let initial_message = format!("Hello world");
     let mut buffer: Vec<u8> = Vec::new();
     let bytes_written = stream.write_all(initial_message.as_bytes());

src/test/ui/suggestions/suggest-change-mut.stderr

@@ -9,6 +9,10 @@ help: consider removing the leading `&`-reference
    |
 LL |         let mut stream_reader = BufReader::new(stream);
    |                                               --
+help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
+   |
+LL | fn issue_81421<T: Read + Write>(mut stream: T) where &T: std::io::Read {
+   |                                                ^^^^^^^^^^^^^^^^^^^^^^^
 help: consider changing this borrow's mutability
    |
 LL |         let mut stream_reader = BufReader::new(&mut stream);

src/test/ui/traits/suggest-where-clause.stderr

@@ -35,6 +35,10 @@ LL |     <u64 as From<T>>::from;
    |     ^^^^^^^^^^^^^^^^^^^^^^ the trait `From<T>` is not implemented for `u64`
    |
    = note: required by `from`
+help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
+   |
+LL | fn check<T: Iterator, U: ?Sized>() where u64: From<T> {
+   |                                    ^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `u64: From<<T as Iterator>::Item>` is not satisfied
   --> $DIR/suggest-where-clause.rs:18:5
@@ -43,6 +47,10 @@ LL |     <u64 as From<<T as Iterator>::Item>>::from;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<<T as Iterator>::Item>` is not implemented for `u64`
    |
    = note: required by `from`
+help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
+   |
+LL | fn check<T: Iterator, U: ?Sized>() where u64: From<<T as Iterator>::Item> {
+   |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `Misc<_>: From<T>` is not satisfied
   --> $DIR/suggest-where-clause.rs:23:5