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

Partially address #81971.

5 files changed, 21 insertions, 1 deletions

diff --git a/src/test/ui/partialeq_help.stderr b/src/test/ui/partialeq_help.stderr
index 6decbef1af4..e14e17c1622 100644
--- a/src/test/ui/partialeq_help.stderr
+++ b/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
 
diff --git a/src/test/ui/specialization/deafult-associated-type-bound-2.stderr b/src/test/ui/specialization/deafult-associated-type-bound-2.stderr
index 2bc14dbe3b2..0e8a774bce3 100644
--- a/src/test/ui/specialization/deafult-associated-type-bound-2.stderr
+++ b/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
 
diff --git a/src/test/ui/suggestions/suggest-change-mut.rs b/src/test/ui/suggestions/suggest-change-mut.rs
index 8b465aae66b..a2bc6fd09b5 100644
--- a/src/test/ui/suggestions/suggest-change-mut.rs
+++ b/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());
diff --git a/src/test/ui/suggestions/suggest-change-mut.stderr b/src/test/ui/suggestions/suggest-change-mut.stderr
index cb156f7c787..9b8181647a0 100644
--- a/src/test/ui/suggestions/suggest-change-mut.stderr
+++ b/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);
diff --git a/src/test/ui/traits/suggest-where-clause.stderr b/src/test/ui/traits/suggest-where-clause.stderr
index b50017afa4d..86d589ffa9e 100644
--- a/src/test/ui/traits/suggest-where-clause.stderr
+++ b/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