Use root obligation on E0277 for some cases

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -166,7 +166,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return false;
         }
 
-        match ty.kind() {
+        match ty.peel_refs().kind() {
             ty::Param(param) => {
                 let generics = self.tcx.generics_of(self.body_id);
                 let generic_param = generics.type_param(&param, self.tcx);
@@ -184,7 +184,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     }
                 }
             }
-            ty::Alias(ty::AliasKind::Opaque, _) => {
+            ty::Slice(..) | ty::Adt(..) | ty::Alias(ty::AliasKind::Opaque, _) => {
                 for unsatisfied in unsatisfied_predicates.iter() {
                     if is_iterator_predicate(unsatisfied.0, self.tcx) {
                         return true;

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

@@ -416,9 +416,59 @@ 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();
 
-                        if let Some(guar) = self.emit_specialized_closure_kind_error(&obligation, trait_ref) {
+                        // Let's use the root obligation as the main message, when we care about the
+                        // most general case ("X doesn't implement Pattern<'_>") over the 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:
+                        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().has_escaping_bound_vars()
+                            && !root_pred.self_ty().has_escaping_bound_vars()
+                            // 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"
+                            // FIXME: this doesn't account for trivial derefs, but works as a first
+                            // approximation.
+                            && (
+                                // `T: Trait` && `&&T: OtherTrait`, we want `OtherTrait`
+                                self.can_eq(
+                                    obligation.param_env,
+                                    trait_predicate.self_ty().skip_binder(),
+                                    root_pred.self_ty().peel_refs(),
+                                )
+                                // `&str: Iterator` && `&str: IntoIterator`, we want `IntoIterator`
+                                || self.can_eq(
+                                    obligation.param_env,
+                                    trait_predicate.self_ty().skip_binder(),
+                                    root_pred.self_ty(),
+                                )
+                            )
+                            // 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
+                            && trait_predicate.def_id() != root_pred.def_id()
+                            // The root trait is not `Unsize`, as to avoid talking about it in
+                            // `tests/ui/coercion/coerce-issue-49593-box-never.rs`.
+                            && Some(root_pred.def_id()) != self.tcx.lang_items().unsize_trait()
+                        {
+                            (
+                                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,
+                        ) {
                             return guar;
                         }
 
@@ -459,8 +509,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                             notes,
                             parent_label,
                             append_const_msg,
-                        } = self.on_unimplemented_note(trait_ref, &obligation, &mut long_ty_file);
-
+                        } = self.on_unimplemented_note(trait_ref, o, &mut long_ty_file);
                         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 =
@@ -483,7 +532,7 @@ 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/future.rs

@@ -30,8 +30,7 @@ use crate::task::{Context, Poll};
 #[lang = "future_trait"]
 #[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"
+    message = "`{Self}` is not a future"
 )]
 pub trait Future {
     /// The type of value produced on completion.

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.

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

@@ -28,42 +28,11 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
 #[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`"
+        note = "you might have meant to use a bounded `Range`"
     ),
     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`"
+        note = "you might have meant to use a bounded `RangeInclusive`"
     ),
     label = "`{Self}` is not an iterator",
     message = "`{Self}` is not an iterator"

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/async-await/async-error-span.stderr

@@ -5,7 +5,6 @@ LL | fn get_future() -> impl Future<Output = ()> {
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not a future
    |
    = help: the trait `Future` is not implemented for `()`
-   = note: () must be a future or must implement `IntoFuture` to be awaited
 
 error[E0282]: type annotations needed
   --> $DIR/async-error-span.rs:13:9

tests/ui/async-await/coroutine-not-future.stderr

@@ -49,7 +49,6 @@ LL |     takes_future(returns_coroutine());
    |     required by a bound introduced by this call
    |
    = help: the trait `Future` is not implemented for `impl Coroutine<Yield = (), Return = ()>`
-   = note: impl Coroutine<Yield = (), Return = ()> must be a future or must implement `IntoFuture` to be awaited
 note: required by a bound in `takes_future`
   --> $DIR/coroutine-not-future.rs:17:26
    |
@@ -69,7 +68,6 @@ LL | |     });
    | |_____^ `{coroutine@$DIR/coroutine-not-future.rs:41:18: 41:23}` is not a future
    |
    = help: the trait `Future` is not implemented for `{coroutine@$DIR/coroutine-not-future.rs:41:18: 41:23}`
-   = note: {coroutine@$DIR/coroutine-not-future.rs:41:18: 41:23} must be a future or must implement `IntoFuture` to be awaited
 note: required by a bound in `takes_future`
   --> $DIR/coroutine-not-future.rs:17:26
    |

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/coroutine/gen_block_is_no_future.stderr

@@ -5,7 +5,6 @@ LL | fn foo() -> impl std::future::Future {
    |             ^^^^^^^^^^^^^^^^^^^^^^^^ `{gen block@$DIR/gen_block_is_no_future.rs:5:5: 5:21}` is not a future
    |
    = help: the trait `Future` is not implemented for `{gen block@$DIR/gen_block_is_no_future.rs:5:5: 5:21}`
-   = note: {gen block@$DIR/gen_block_is_no_future.rs:5:5: 5:21} must be a future or must implement `IntoFuture` to be awaited
 
 error: aborting due to 1 previous error
 

tests/ui/feature-gates/feature-gate-trivial_bounds.stderr

@@ -74,7 +74,6 @@ LL | fn use_for() where i32: Iterator {
    |                    ^^^^^^^^^^^^^ `i32` is not an iterator
    |
    = help: the trait `Iterator` is not implemented for `i32`
-   = 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`
    = help: see issue #48214
    = help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
 

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/impl-trait/issues/issue-83919.stderr

@@ -5,7 +5,6 @@ LL |     fn get_fut(&self) -> Self::Fut {
    |                          ^^^^^^^^^ `{integer}` is not a future
    |
    = help: the trait `Future` is not implemented for `{integer}`
-   = note: {integer} must be a future or must implement `IntoFuture` to be awaited
 
 error: aborting due to 1 previous error
 

tests/ui/issues-71798.stderr

@@ -14,7 +14,6 @@ LL |     *x
    |     -- return type was inferred to be `u32` here
    |
    = help: the trait `Future` is not implemented for `u32`
-   = note: u32 must be a future or must implement `IntoFuture` to be awaited
 
 error: aborting due to 2 previous errors
 

tests/ui/iterators/bound.stderr

@@ -5,7 +5,6 @@ LL | struct T(S<u8>);
    |          ^^^^^ `u8` is not an iterator
    |
    = help: the trait `Iterator` is not implemented for `u8`
-   = 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`
 note: required by a bound in `S`
   --> $DIR/bound.rs:1:13
    |

tests/ui/iterators/vec-on-unimplemented.fixed

@@ -0,0 +1,5 @@
+//@ run-rustfix
+fn main() {
+    let _ = vec![true, false].into_iter().map(|v| !v).collect::<Vec<_>>();
+    //~^ ERROR no method named `map` found for struct `Vec<bool>` in the current scope
+}

tests/ui/iterators/vec-on-unimplemented.rs

@@ -1,4 +1,5 @@
+//@ run-rustfix
 fn main() {
-    vec![true, false].map(|v| !v).collect::<Vec<_>>();
-    //~^ ERROR `Vec<bool>` is not an iterator
+    let _ = vec![true, false].map(|v| !v).collect::<Vec<_>>();
+    //~^ ERROR no method named `map` found for struct `Vec<bool>` in the current scope
 }