Rollup merge of rust-lang#121826 - estebank:e0277-root-obligation-2, r=oli-obk

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), cc rust-lang#121398 (doesn't fix the underlying issue).

Author: matthiaskrgr

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