Use root obligation on E0277 for some cases

When encountering trait bound errors that satisfy some heuristics that
tell us that the relevant trait for the user comes from the root
obligation and not the current obligation, we use the root predicate for
the main message.

This allows to talk about "X doesn't implement Pattern<'_>" over the
most specific case that just happened to fail, like  "char doesn't
implement Fn(&mut char)" in
`tests/ui/traits/suggest-dereferences/root-obligation.rs`

The heuristics are:

 - the type of the leaf predicate is (roughly) the same as the type
   from the root predicate, as a proxy for "we care about the root"
 - the leaf trait and the root trait are different, so as to avoid
   talking about `&mut T: Trait` and instead remain talking about
   `T: Trait` instead
 - the root trait is not `Unsize`, as to avoid talking about it in
   `tests/ui/coercion/coerce-issue-49593-box-never.rs`.

```
error[E0277]: the trait bound `&char: Pattern<'_>` is not satisfied
  --> $DIR/root-obligation.rs:6:38
   |
LL |         .filter(|c| "aeiou".contains(c))
   |                             -------- ^ the trait `Fn<(char,)>` is not implemented for `&char`, which is required by `&char: Pattern<'_>`
   |                             |
   |                             required by a bound introduced by this call
   |
   = note: required for `&char` to implement `FnOnce<(char,)>`
   = note: required for `&char` to implement `Pattern<'_>`
note: required by a bound in `core::str::<impl str>::contains`
  --> $SRC_DIR/core/src/str/mod.rs:LL:COL
help: consider dereferencing here
   |
LL |         .filter(|c| "aeiou".contains(*c))
   |                                      +
```

Fix rust-lang#79359, fix rust-lang#119983, fix rust-lang#118779, cc rust-lang#118415 (the suggestion needs
to change).

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

@@ -409,9 +409,44 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_predicate)) => {
                         let trait_predicate = bound_predicate.rebind(trait_predicate);
                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
-                        let trait_ref = trait_predicate.to_poly_trait_ref();
+                        let (main_trait_predicate, o) = if let ty::PredicateKind::Clause(
+                            ty::ClauseKind::Trait(root_pred)
+                        ) = root_obligation.predicate.kind().skip_binder()
+                            && trait_predicate.self_ty().skip_binder()
+                                == root_pred.self_ty().peel_refs()
+                            && trait_predicate.def_id() != root_pred.def_id()
+                            && Some(root_pred.def_id()) != self.tcx.lang_items().unsize_trait()
+                        {
+                            // Let's use the root obligation as the main message, as we care about
+                            // the most general case ("X doesn't implement Pattern<'_>") over the
+                            // most specific case that happened to fail ("char doesn't implement
+                            // Fn(&mut char)").
+                            //
+                            // We rely on a few heuristics to identify cases where this root
+                            // obligation is more important than the leaf obligation:
+                            //
+                            //  - the type of the leaf predicate is (roughly) the same as the type
+                            //    from the root predicate, as a proxy for "we care about the root"
+                            //  - the leaf trait and the root trait are different, so as to avoid
+                            //    talking about `&mut T: Trait` and instead remain talking about
+                            //    `T: Trait` instead
+                            //  - the root trait is not `Unsize`, as to avoid talking about it in
+                            //    `tests/ui/coercion/coerce-issue-49593-box-never.rs`.
+                            (
+                                self.resolve_vars_if_possible(
+                                    root_obligation.predicate.kind().rebind(root_pred),
+                                ),
+                                root_obligation,
+                            )
+                        } else {
+                            (trait_predicate, &obligation)
+                        };
+                        let trait_ref = main_trait_predicate.to_poly_trait_ref();
 
-                        if let Some(guar) = self.emit_specialized_closure_kind_error(&obligation, trait_ref) {
+                        if let Some(guar) = self.emit_specialized_closure_kind_error(
+                            &obligation,
+                            trait_ref,
+                        ) {
                             return guar;
                         }
 
@@ -450,7 +485,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             notes,
                             parent_label,
                             append_const_msg,
-                        } = self.on_unimplemented_note(trait_ref, &obligation);
+                        } = self.on_unimplemented_note(trait_ref, o);
                         let have_alt_message = message.is_some() || label.is_some();
                         let is_try_conversion = self.is_try_conversion(span, trait_ref.def_id());
                         let is_unsize =
@@ -473,7 +508,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         };
 
                         let err_msg = self.get_standard_error_message(
-                            &trait_predicate,
+                            &main_trait_predicate,
                             message,
                             predicate_is_const,
                             append_const_msg,

library/core/src/future/into_future.rs

@@ -100,6 +100,11 @@ use crate::future::Future;
 /// ```
 #[stable(feature = "into_future", since = "1.64.0")]
 #[rustc_diagnostic_item = "IntoFuture"]
+#[diagnostic::on_unimplemented(
+    label = "`{Self}` is not a future",
+    message = "`{Self}` is not a future",
+    note = "{Self} must be a future or must implement `IntoFuture` to be awaited"
+)]
 pub trait IntoFuture {
     /// The output that the future will produce on completion.
     #[stable(feature = "into_future", since = "1.64.0")]

library/core/src/iter/traits/collect.rs

@@ -236,6 +236,49 @@ pub trait FromIterator<A>: Sized {
 /// ```
 #[rustc_diagnostic_item = "IntoIterator"]
 #[rustc_skip_array_during_method_dispatch]
+#[rustc_on_unimplemented(
+    on(
+        _Self = "core::ops::range::RangeTo<Idx>",
+        label = "if you meant to iterate until a value, add a starting value",
+        note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \
+              bounded `Range`: `0..end`"
+    ),
+    on(
+        _Self = "core::ops::range::RangeToInclusive<Idx>",
+        label = "if you meant to iterate until a value (including it), add a starting value",
+        note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
+              to have a bounded `RangeInclusive`: `0..=end`"
+    ),
+    on(
+        _Self = "[]",
+        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
+    ),
+    on(_Self = "&[]", label = "`{Self}` is not an iterator; try calling `.iter()`"),
+    on(
+        _Self = "alloc::vec::Vec<T, A>",
+        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
+    ),
+    on(
+        _Self = "&str",
+        label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
+    ),
+    on(
+        _Self = "alloc::string::String",
+        label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
+    ),
+    on(
+        _Self = "{integral}",
+        note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
+              syntax `start..end` or the inclusive range syntax `start..=end`"
+    ),
+    on(
+        _Self = "{float}",
+        note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
+              syntax `start..end` or the inclusive range syntax `start..=end`"
+    ),
+    label = "`{Self}` is not an iterator",
+    message = "`{Self}` is not an iterator"
+)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait IntoIterator {
     /// The type of the elements being iterated over.

tests/ui/associated-types/substs-ppaux.normal.stderr

@@ -70,13 +70,12 @@ help: use parentheses to call this function
 LL |     let x: () = foo::<'static>();
    |                               ++
 
-error[E0277]: the size for values of type `str` cannot be known at compilation time
+error[E0277]: the trait bound `str: Foo<'_, '_, u8>` is not satisfied
   --> $DIR/substs-ppaux.rs:49:6
    |
 LL |     <str as Foo<u8>>::bar;
-   |      ^^^ doesn't have a size known at compile-time
+   |      ^^^ the trait `Sized` is not implemented for `str`, which is required by `str: Foo<'_, '_, u8>`
    |
-   = help: the trait `Sized` is not implemented for `str`, which is required by `str: Foo<'_, '_, u8>`
 note: required for `str` to implement `Foo<'_, '_, u8>`
   --> $DIR/substs-ppaux.rs:11:17
    |

tests/ui/associated-types/substs-ppaux.rs

@@ -47,6 +47,6 @@ fn foo<'z>() where &'z (): Sized {
     //[normal]~| found fn item `fn() {foo::<'static>}`
 
     <str as Foo<u8>>::bar;
-    //[verbose]~^ ERROR the size for values of type
-    //[normal]~^^ ERROR the size for values of type
+    //[verbose]~^ ERROR the trait bound `str: Foo<'?0, '?1, u8>` is not satisfied
+    //[normal]~^^ ERROR the trait bound `str: Foo<'_, '_, u8>` is not satisfied
 }

tests/ui/associated-types/substs-ppaux.verbose.stderr

@@ -70,13 +70,12 @@ help: use parentheses to call this function
 LL |     let x: () = foo::<'static>();
    |                               ++
 
-error[E0277]: the size for values of type `str` cannot be known at compilation time
+error[E0277]: the trait bound `str: Foo<'?0, '?1, u8>` is not satisfied
   --> $DIR/substs-ppaux.rs:49:6
    |
 LL |     <str as Foo<u8>>::bar;
-   |      ^^^ doesn't have a size known at compile-time
+   |      ^^^ the trait `Sized` is not implemented for `str`, which is required by `str: Foo<'?0, '?1, u8>`
    |
-   = help: the trait `Sized` is not implemented for `str`, which is required by `str: Foo<'?0, '?1, u8>`
 note: required for `str` to implement `Foo<'?0, '?1, u8>`
   --> $DIR/substs-ppaux.rs:11:17
    |

tests/ui/auto-traits/typeck-default-trait-impl-precedence.rs

@@ -17,5 +17,5 @@ impl Signed for i32 { }
 fn main() {
     is_defaulted::<&'static i32>();
     is_defaulted::<&'static u32>();
-    //~^ ERROR `u32: Signed` is not satisfied
+    //~^ ERROR the trait bound `&'static u32: Defaulted` is not satisfied
 }

tests/ui/auto-traits/typeck-default-trait-impl-precedence.stderr

@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `u32: Signed` is not satisfied
+error[E0277]: the trait bound `&'static u32: Defaulted` is not satisfied
   --> $DIR/typeck-default-trait-impl-precedence.rs:19:20
    |
 LL |     is_defaulted::<&'static u32>();
-   |                    ^^^^^^^^^^^^ the trait `Signed` is not implemented for `u32`, which is required by `&'static u32: Defaulted`
+   |                    ^^^^^^^^^^^^ the trait `Signed` is not implemented for `&'static u32`, which is required by `&'static u32: Defaulted`
    |
 note: required for `&'static u32` to implement `Defaulted`
   --> $DIR/typeck-default-trait-impl-precedence.rs:10:19

tests/ui/for/issue-20605.current.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the size for values of type `dyn Iterator<Item = &'a mut u8>` cannot be known at compilation time
+error[E0277]: `dyn Iterator<Item = &'a mut u8>` is not an iterator
   --> $DIR/issue-20605.rs:5:17
    |
 LL |     for item in *things { *item = 0 }

tests/ui/for/issue-20605.next.stderr

@@ -1,8 +1,10 @@
-error[E0277]: the trait bound `dyn Iterator<Item = &'a mut u8>: IntoIterator` is not satisfied
+error[E0277]: `dyn Iterator<Item = &'a mut u8>` is not an iterator
   --> $DIR/issue-20605.rs:5:17
    |
 LL |     for item in *things { *item = 0 }
-   |                 ^^^^^^^ the trait `IntoIterator` is not implemented for `dyn Iterator<Item = &'a mut u8>`
+   |                 ^^^^^^^ `dyn Iterator<Item = &'a mut u8>` is not an iterator
+   |
+   = help: the trait `IntoIterator` is not implemented for `dyn Iterator<Item = &'a mut u8>`
 
 error: the type `<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter` is not well-formed
   --> $DIR/issue-20605.rs:5:17

tests/ui/for/issue-20605.rs

@@ -3,8 +3,8 @@
 
 fn changer<'a>(mut things: Box<dyn Iterator<Item=&'a mut u8>>) {
     for item in *things { *item = 0 }
-    //[current]~^ ERROR the size for values of type
-    //[next]~^^ ERROR the trait bound `dyn Iterator<Item = &'a mut u8>: IntoIterator` is not satisfied
+    //[current]~^ ERROR `dyn Iterator<Item = &'a mut u8>` is not an iterator
+    //[next]~^^ ERROR `dyn Iterator<Item = &'a mut u8>` is not an iterator
     //[next]~| ERROR the type `<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter` is not well-formed
     //[next]~| ERROR the type `&mut <dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter` is not well-formed
     //[next]~| ERROR the type `Option<<<dyn Iterator<Item = &'a mut u8> as IntoIterator>::IntoIter as Iterator>::Item>` is not well-formed

tests/ui/kindck/kindck-impl-type-params-2.rs

@@ -11,5 +11,5 @@ fn take_param<T:Foo>(foo: &T) { }
 fn main() {
     let x: Box<_> = Box::new(3);
     take_param(&x);
-    //~^ ERROR the trait bound `Box<{integer}>: Copy` is not satisfied
+    //~^ ERROR the trait bound `Box<{integer}>: Foo` is not satisfied
 }

tests/ui/kindck/kindck-impl-type-params-2.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `Box<{integer}>: Copy` is not satisfied
+error[E0277]: the trait bound `Box<{integer}>: Foo` is not satisfied
   --> $DIR/kindck-impl-type-params-2.rs:13:16
    |
 LL |     take_param(&x);

tests/ui/kindck/kindck-inherited-copy-bound.curr.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `Box<{integer}>: Copy` is not satisfied
+error[E0277]: the trait bound `Box<{integer}>: Foo` is not satisfied
   --> $DIR/kindck-inherited-copy-bound.rs:21:16
    |
 LL |     take_param(&x);

tests/ui/kindck/kindck-inherited-copy-bound.object_safe_for_dispatch.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `Box<{integer}>: Copy` is not satisfied
+error[E0277]: the trait bound `Box<{integer}>: Foo` is not satisfied
   --> $DIR/kindck-inherited-copy-bound.rs:21:16
    |
 LL |     take_param(&x);

tests/ui/kindck/kindck-send-object.rs

@@ -10,7 +10,7 @@ trait Message : Send { }
 
 fn object_ref_with_static_bound_not_ok() {
     assert_send::<&'static (dyn Dummy + 'static)>();
-    //~^ ERROR `(dyn Dummy + 'static)` cannot be shared between threads safely [E0277]
+    //~^ ERROR `&'static (dyn Dummy + 'static)` cannot be sent between threads safely [E0277]
 }
 
 fn box_object_with_no_bound_not_ok<'a>() {

tests/ui/kindck/kindck-send-object.stderr

@@ -1,10 +1,10 @@
-error[E0277]: `(dyn Dummy + 'static)` cannot be shared between threads safely
+error[E0277]: `&'static (dyn Dummy + 'static)` cannot be sent between threads safely
   --> $DIR/kindck-send-object.rs:12:19
    |
 LL |     assert_send::<&'static (dyn Dummy + 'static)>();
-   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Dummy + 'static)` cannot be shared between threads safely
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `&'static (dyn Dummy + 'static)` cannot be sent between threads safely
    |
-   = help: the trait `Sync` is not implemented for `(dyn Dummy + 'static)`, which is required by `&'static (dyn Dummy + 'static): Send`
+   = help: the trait `Sync` is not implemented for `&'static (dyn Dummy + 'static)`, which is required by `&'static (dyn Dummy + 'static): Send`
    = note: required for `&'static (dyn Dummy + 'static)` to implement `Send`
 note: required by a bound in `assert_send`
   --> $DIR/kindck-send-object.rs:5:18

tests/ui/kindck/kindck-send-object1.rs

@@ -8,7 +8,7 @@ trait Dummy { }
 // careful with object types, who knows what they close over...
 fn test51<'a>() {
     assert_send::<&'a dyn Dummy>();
-    //~^ ERROR `(dyn Dummy + 'a)` cannot be shared between threads safely [E0277]
+    //~^ ERROR `&'a (dyn Dummy + 'a)` cannot be sent between threads safely [E0277]
 }
 fn test52<'a>() {
     assert_send::<&'a (dyn Dummy + Sync)>();

tests/ui/kindck/kindck-send-object1.stderr

@@ -1,10 +1,10 @@
-error[E0277]: `(dyn Dummy + 'a)` cannot be shared between threads safely
+error[E0277]: `&'a (dyn Dummy + 'a)` cannot be sent between threads safely
   --> $DIR/kindck-send-object1.rs:10:19
    |
 LL |     assert_send::<&'a dyn Dummy>();
-   |                   ^^^^^^^^^^^^^ `(dyn Dummy + 'a)` cannot be shared between threads safely
+   |                   ^^^^^^^^^^^^^ `&'a (dyn Dummy + 'a)` cannot be sent between threads safely
    |
-   = help: the trait `Sync` is not implemented for `(dyn Dummy + 'a)`, which is required by `&'a (dyn Dummy + 'a): Send`
+   = help: the trait `Sync` is not implemented for `&'a (dyn Dummy + 'a)`, which is required by `&'a (dyn Dummy + 'a): Send`
    = note: required for `&'a (dyn Dummy + 'a)` to implement `Send`
 note: required by a bound in `assert_send`
   --> $DIR/kindck-send-object1.rs:5:18

tests/ui/kindck/kindck-send-object2.rs

@@ -5,7 +5,7 @@ trait Dummy { }
 
 fn test50() {
     assert_send::<&'static dyn Dummy>();
-    //~^ ERROR `(dyn Dummy + 'static)` cannot be shared between threads safely [E0277]
+    //~^ ERROR `&'static (dyn Dummy + 'static)` cannot be sent between threads safely [E0277]
 }
 
 fn test53() {

tests/ui/kindck/kindck-send-object2.stderr

@@ -1,10 +1,10 @@
-error[E0277]: `(dyn Dummy + 'static)` cannot be shared between threads safely
+error[E0277]: `&'static (dyn Dummy + 'static)` cannot be sent between threads safely
   --> $DIR/kindck-send-object2.rs:7:19
    |
 LL |     assert_send::<&'static dyn Dummy>();
-   |                   ^^^^^^^^^^^^^^^^^^ `(dyn Dummy + 'static)` cannot be shared between threads safely
+   |                   ^^^^^^^^^^^^^^^^^^ `&'static (dyn Dummy + 'static)` cannot be sent between threads safely
    |
-   = help: the trait `Sync` is not implemented for `(dyn Dummy + 'static)`, which is required by `&'static (dyn Dummy + 'static): Send`
+   = help: the trait `Sync` is not implemented for `&'static (dyn Dummy + 'static)`, which is required by `&'static (dyn Dummy + 'static): Send`
    = note: required for `&'static (dyn Dummy + 'static)` to implement `Send`
 note: required by a bound in `assert_send`
   --> $DIR/kindck-send-object2.rs:3:18

tests/ui/rfcs/rfc-2397-do-not-recommend/feature-gate-do_not_recommend.rs

@@ -17,5 +17,5 @@ fn stuff<T: Bar>(_: T) {}
 
 fn main() {
     stuff(1u8);
-    //~^ the trait bound `u8: Foo` is not satisfied
+    //~^ the trait bound `u8: Bar` is not satisfied
 }

tests/ui/rfcs/rfc-2397-do-not-recommend/feature-gate-do_not_recommend.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `u8: Foo` is not satisfied
+error[E0277]: the trait bound `u8: Bar` is not satisfied
   --> $DIR/feature-gate-do_not_recommend.rs:19:11
    |
 LL |     stuff(1u8);

tests/ui/suggestions/issue-104961.fixed

@@ -2,12 +2,12 @@
 
 fn foo(x: &str) -> bool {
     x.starts_with(&("hi".to_string() + " you"))
-    //~^ ERROR expected a `FnMut(char)` closure, found `String`
+    //~^ ERROR the trait bound `String: Pattern<'_>` is not satisfied [E0277]
 }
 
 fn foo2(x: &str) -> bool {
     x.starts_with(&"hi".to_string())
-    //~^ ERROR expected a `FnMut(char)` closure, found `String`
+    //~^ ERROR the trait bound `String: Pattern<'_>` is not satisfied [E0277]
 }
 
 fn main() {

tests/ui/suggestions/issue-104961.rs

@@ -2,12 +2,12 @@
 
 fn foo(x: &str) -> bool {
     x.starts_with("hi".to_string() + " you")
-    //~^ ERROR expected a `FnMut(char)` closure, found `String`
+    //~^ ERROR the trait bound `String: Pattern<'_>` is not satisfied [E0277]
 }
 
 fn foo2(x: &str) -> bool {
     x.starts_with("hi".to_string())
-    //~^ ERROR expected a `FnMut(char)` closure, found `String`
+    //~^ ERROR the trait bound `String: Pattern<'_>` is not satisfied [E0277]
 }
 
 fn main() {

tests/ui/suggestions/issue-104961.stderr

@@ -1,12 +1,11 @@
-error[E0277]: expected a `FnMut(char)` closure, found `String`
+error[E0277]: the trait bound `String: Pattern<'_>` is not satisfied
   --> $DIR/issue-104961.rs:4:19
    |
 LL |     x.starts_with("hi".to_string() + " you")
    |       ----------- ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Pattern<'_>` is not implemented for `String`
    |       |
    |       required by a bound introduced by this call
    |
-   = note: the trait bound `String: Pattern<'_>` is not satisfied
    = note: required for `String` to implement `Pattern<'_>`
 note: required by a bound in `core::str::<impl str>::starts_with`
   --> $SRC_DIR/core/src/str/mod.rs:LL:COL
@@ -15,15 +14,14 @@ help: consider borrowing here
 LL |     x.starts_with(&("hi".to_string() + " you"))
    |                   ++                         +
 
-error[E0277]: expected a `FnMut(char)` closure, found `String`
+error[E0277]: the trait bound `String: Pattern<'_>` is not satisfied
   --> $DIR/issue-104961.rs:9:19
    |
 LL |     x.starts_with("hi".to_string())
    |       ----------- ^^^^^^^^^^^^^^^^ the trait `Pattern<'_>` is not implemented for `String`
    |       |
    |       required by a bound introduced by this call
    |
-   = note: the trait bound `String: Pattern<'_>` is not satisfied
    = note: required for `String` to implement `Pattern<'_>`
 note: required by a bound in `core::str::<impl str>::starts_with`
   --> $SRC_DIR/core/src/str/mod.rs:LL:COL

tests/ui/suggestions/issue-62843.stderr

@@ -1,12 +1,11 @@
-error[E0277]: expected a `FnMut(char)` closure, found `String`
+error[E0277]: the trait bound `String: Pattern<'_>` is not satisfied
   --> $DIR/issue-62843.rs:4:32
    |
 LL |     println!("{:?}", line.find(pattern));
    |                           ---- ^^^^^^^ the trait `Pattern<'_>` is not implemented for `String`
    |                           |
    |                           required by a bound introduced by this call
    |
-   = note: the trait bound `String: Pattern<'_>` is not satisfied
    = note: required for `String` to implement `Pattern<'_>`
 note: required by a bound in `core::str::<impl str>::find`
   --> $SRC_DIR/core/src/str/mod.rs:LL:COL