diagnostics: make implicit Sized bounds explicit in E0277

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

@@ -1391,25 +1391,45 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             // return early in the caller.
 
             let mut label = || {
+                // Special case `Sized` as `old_pred` will be the trait itself instead of
+                // `Sized` when the trait bound is the source of the error.
+                let is_sized = match obligation.predicate.kind().skip_binder() {
+                    ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)) => {
+                        self.tcx.is_lang_item(trait_pred.def_id(), LangItem::Sized)
+                    }
+                    _ => false,
+                };
+
                 let msg = format!(
                     "the trait bound `{}` is not satisfied",
                     self.tcx.short_string(old_pred, err.long_ty_path()),
                 );
-                let self_ty_str =
-                    self.tcx.short_string(old_pred.self_ty().skip_binder(), err.long_ty_path());
+                let self_ty_str = self.tcx.short_string(old_pred.self_ty(), err.long_ty_path());
                 let trait_path = self
                     .tcx
                     .short_string(old_pred.print_modifiers_and_trait_path(), err.long_ty_path());
 
                 if has_custom_message {
+                    let msg = if is_sized {
+                        "the trait bound `Sized` is not satisfied".into()
+                    } else {
+                        msg
+                    };
                     err.note(msg);
                 } else {
                     err.messages = vec![(rustc_errors::DiagMessage::from(msg), Style::NoStyle)];
                 }
-                err.span_label(
-                    span,
-                    format!("the trait `{trait_path}` is not implemented for `{self_ty_str}`"),
-                );
+                if is_sized {
+                    err.span_label(
+                        span,
+                        format!("the trait `Sized` is not implemented for `{self_ty_str}`"),
+                    );
+                } else {
+                    err.span_label(
+                        span,
+                        format!("the trait `{trait_path}` is not implemented for `{self_ty_str}`"),
+                    );
+                }
             };
 
             let mut sugg_prefixes = vec![];
@@ -3578,10 +3598,27 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                                 "unsatisfied trait bound introduced in this `derive` macro",
                             );
                         } else if !data.span.is_dummy() && !data.span.overlaps(self_ty.span) {
-                            spans.push_span_label(
-                                data.span,
-                                "unsatisfied trait bound introduced here",
-                            );
+                            // `Sized` may be an explicit or implicit trait bound. If it is
+                            // implicit, mention it as such.
+                            if let Some(pred) = predicate.as_trait_clause()
+                                && self.tcx.is_lang_item(pred.def_id(), LangItem::Sized)
+                                && self
+                                    .tcx
+                                    .generics_of(data.impl_or_alias_def_id)
+                                    .own_params
+                                    .iter()
+                                    .any(|param| self.tcx.def_span(param.def_id) == data.span)
+                            {
+                                spans.push_span_label(
+                                    data.span,
+                                    "unsatisfied trait bound implicitly introduced here",
+                                );
+                            } else {
+                                spans.push_span_label(
+                                    data.span,
+                                    "unsatisfied trait bound introduced here",
+                                );
+                            }
                         }
                         err.span_note(spans, msg);
                         point_at_assoc_type_restriction(

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

@@ -88,7 +88,7 @@ note: required for `str` to implement `Foo<'_, '_, u8>`
 LL | impl<'a, 'b, T, S> Foo<'a, 'b, S> for T {}
    |              -     ^^^^^^^^^^^^^^     ^
    |              |
-   |              unsatisfied trait bound introduced here
+   |              unsatisfied trait bound implicitly introduced here
 
 error: aborting due to 5 previous errors
 

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

@@ -88,7 +88,7 @@ note: required for `str` to implement `Foo<'?0, '?1, u8>`
 LL | impl<'a, 'b, T, S> Foo<'a, 'b, S> for T {}
    |              -     ^^^^^^^^^^^^^^     ^
    |              |
-   |              unsatisfied trait bound introduced here
+   |              unsatisfied trait bound implicitly introduced here
 
 error: aborting due to 5 previous errors
 

tests/ui/error-codes/E0277-4.rs

@@ -0,0 +1,50 @@
+use std::fmt::Display;
+
+trait ImplicitTrait {
+    fn foo(&self);
+}
+
+trait ExplicitTrait {
+    fn foo(&self);
+}
+
+trait DisplayTrait {
+    fn foo(&self);
+}
+
+trait UnimplementedTrait {
+    fn foo(&self);
+}
+
+// Implicitly requires `T: Sized`.
+impl<T> ImplicitTrait for T {
+    fn foo(&self) {}
+}
+
+// Explicitly requires `T: Sized`.
+impl<T: Sized> ExplicitTrait for T {
+    fn foo(&self) {}
+}
+
+// Requires `T: Display`.
+impl<T: Display> DisplayTrait for T {
+    fn foo(&self) {}
+}
+
+fn main() {
+    // `[u8]` does not implement `Sized`.
+    let x: &[u8] = &[];
+    ImplicitTrait::foo(x);
+    //~^ ERROR: the trait bound `[u8]: ImplicitTrait` is not satisfied [E0277]
+    ExplicitTrait::foo(x);
+    //~^ ERROR: the trait bound `[u8]: ExplicitTrait` is not satisfied [E0277]
+
+    // `UnimplementedTrait` has no implementations.
+    UnimplementedTrait::foo(x);
+    //~^ ERROR: the trait bound `[u8]: UnimplementedTrait` is not satisfied [E0277]
+
+    // `[u8; 0]` implements `Sized` but not `Display`.
+    let x: &[u8; 0] = &[];
+    DisplayTrait::foo(x);
+    //~^ ERROR: the trait bound `[u8; 0]: DisplayTrait` is not satisfied [E0277]
+}

tests/ui/error-codes/E0277-4.stderr

@@ -0,0 +1,77 @@
+error[E0277]: the trait bound `[u8]: ImplicitTrait` is not satisfied
+  --> $DIR/E0277-4.rs:37:24
+   |
+LL |     ImplicitTrait::foo(x);
+   |     ------------------ ^ the trait `Sized` is not implemented for `[u8]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required for `[u8]` to implement `ImplicitTrait`
+  --> $DIR/E0277-4.rs:20:9
+   |
+LL | impl<T> ImplicitTrait for T {
+   |      -  ^^^^^^^^^^^^^     ^
+   |      |
+   |      unsatisfied trait bound implicitly introduced here
+help: consider borrowing here
+   |
+LL |     ImplicitTrait::foo(&x);
+   |                        +
+LL |     ImplicitTrait::foo(&mut x);
+   |                        ++++
+
+error[E0277]: the trait bound `[u8]: ExplicitTrait` is not satisfied
+  --> $DIR/E0277-4.rs:39:24
+   |
+LL |     ExplicitTrait::foo(x);
+   |     ------------------ ^ the trait `Sized` is not implemented for `[u8]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required for `[u8]` to implement `ExplicitTrait`
+  --> $DIR/E0277-4.rs:25:16
+   |
+LL | impl<T: Sized> ExplicitTrait for T {
+   |         -----  ^^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+help: consider borrowing here
+   |
+LL |     ExplicitTrait::foo(&x);
+   |                        +
+LL |     ExplicitTrait::foo(&mut x);
+   |                        ++++
+
+error[E0277]: the trait bound `[u8]: UnimplementedTrait` is not satisfied
+  --> $DIR/E0277-4.rs:43:29
+   |
+LL |     UnimplementedTrait::foo(x);
+   |     ----------------------- ^ the trait `UnimplementedTrait` is not implemented for `[u8]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+help: this trait has no implementations, consider adding one
+  --> $DIR/E0277-4.rs:15:1
+   |
+LL | trait UnimplementedTrait {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0277]: the trait bound `[u8; 0]: DisplayTrait` is not satisfied
+  --> $DIR/E0277-4.rs:48:23
+   |
+LL |     DisplayTrait::foo(x);
+   |     ----------------- ^ the trait `std::fmt::Display` is not implemented for `[u8; 0]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required for `[u8; 0]` to implement `DisplayTrait`
+  --> $DIR/E0277-4.rs:30:18
+   |
+LL | impl<T: Display> DisplayTrait for T {
+   |         -------  ^^^^^^^^^^^^     ^
+   |         |
+   |         unsatisfied trait bound introduced here
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/recursion/issue-23122-2.stderr

@@ -11,7 +11,7 @@ note: required for `GetNext<<<<... as Next>::Next as Next>::Next as Next>::Next>
 LL | impl<T: Next> Next for GetNext<T> {
    |      -        ^^^^     ^^^^^^^^^^
    |      |
-   |      unsatisfied trait bound introduced here
+   |      unsatisfied trait bound implicitly introduced here
    = note: the full name for the type has been written to '$TEST_BUILD_DIR/issue-23122-2.long-type-$LONG_TYPE_HASH.txt'
    = note: consider using `--verbose` to print the full type name to the console
 

tests/ui/suggestions/slice-issue-87994.stderr

@@ -17,11 +17,10 @@ error[E0277]: `[i32]` is not an iterator
   --> $DIR/slice-issue-87994.rs:3:12
    |
 LL |   for _ in v[1..] {
-   |            ^^^^^^ the trait `IntoIterator` is not implemented for `[i32]`
+   |            ^^^^^^ the trait `Sized` is not implemented for `[i32]`
    |
-   = note: the trait bound `[i32]: IntoIterator` is not satisfied
+   = note: the trait bound `Sized` is not satisfied
    = note: required for `[i32]` to implement `IntoIterator`
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 help: consider borrowing here
    |
 LL |   for _ in &v[1..] {
@@ -48,11 +47,10 @@ error[E0277]: `[K]` is not an iterator
   --> $DIR/slice-issue-87994.rs:11:13
    |
 LL |   for i2 in v2[1..] {
-   |             ^^^^^^^ the trait `IntoIterator` is not implemented for `[K]`
+   |             ^^^^^^^ the trait `Sized` is not implemented for `[K]`
    |
-   = note: the trait bound `[K]: IntoIterator` is not satisfied
+   = note: the trait bound `Sized` is not satisfied
    = note: required for `[K]` to implement `IntoIterator`
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 help: consider borrowing here
    |
 LL |   for i2 in &v2[1..] {

tests/ui/traits/dyn-iterator-deref-in-for-loop.current.stderr

@@ -2,9 +2,9 @@ error[E0277]: `dyn Iterator<Item = &'a mut u8>` is not an iterator
   --> $DIR/dyn-iterator-deref-in-for-loop.rs:9:17
    |
 LL |     for item in *things {
-   |                 ^^^^^^^ the trait `IntoIterator` is not implemented for `dyn Iterator<Item = &'a mut u8>`
+   |                 ^^^^^^^ the trait `Sized` is not implemented for `dyn Iterator<Item = &'a mut u8>`
    |
-   = note: the trait bound `dyn Iterator<Item = &'a mut u8>: IntoIterator` is not satisfied
+   = note: the trait bound `Sized` is not satisfied
    = note: required for `dyn Iterator<Item = &'a mut u8>` to implement `IntoIterator`
 help: consider mutably borrowing here
    |

tests/ui/traits/dyn-iterator-deref-in-for-loop.next.stderr

@@ -2,9 +2,9 @@ error[E0277]: `dyn Iterator<Item = &'a mut u8>` is not an iterator
   --> $DIR/dyn-iterator-deref-in-for-loop.rs:9:17
    |
 LL |     for item in *things {
-   |                 ^^^^^^^ the trait `IntoIterator` is not implemented for `dyn Iterator<Item = &'a mut u8>`
+   |                 ^^^^^^^ the trait `Sized` is not implemented for `dyn Iterator<Item = &'a mut u8>`
    |
-   = note: the trait bound `dyn Iterator<Item = &'a mut u8>: IntoIterator` is not satisfied
+   = note: the trait bound `Sized` is not satisfied
    = note: required for `dyn Iterator<Item = &'a mut u8>` to implement `IntoIterator`
 help: consider mutably borrowing here
    |

tests/ui/traits/issue-18400.stderr

@@ -11,7 +11,7 @@ note: required for `{integer}` to implement `Set<&[_]>`
 LL | impl<'a, T, S> Set<&'a [T]> for S where
    |          -     ^^^^^^^^^^^^     ^
    |          |
-   |          unsatisfied trait bound introduced here
+   |          unsatisfied trait bound implicitly introduced here
    = note: 128 redundant requirements hidden
    = note: required for `{integer}` to implement `Set<&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[&[_]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]>`