Tweak obligation error output

src/librustc/traits/error_reporting/mod.rs

@@ -1034,6 +1034,10 @@ pub fn report_object_safety_error(
     violations: Vec<ObjectSafetyViolation>,
 ) -> DiagnosticBuilder<'tcx> {
     let trait_str = tcx.def_path_str(trait_def_id);
+    let trait_span = tcx.hir().get_if_local(trait_def_id).and_then(|node| match node {
+        hir::Node::Item(item) => Some(item.ident.span),
+        _ => None,
+    });
     let span = tcx.sess.source_map().def_span(span);
     let mut err = struct_span_err!(
         tcx.sess,
@@ -1045,6 +1049,7 @@ pub fn report_object_safety_error(
     err.span_label(span, format!("the trait `{}` cannot be made into an object", trait_str));
 
     let mut reported_violations = FxHashSet::default();
+    let mut had_span_label = false;
     for violation in violations {
         if let ObjectSafetyViolation::SizedSelf(sp) = &violation {
             if !sp.is_empty() {
@@ -1055,15 +1060,28 @@ pub fn report_object_safety_error(
         }
         if reported_violations.insert(violation.clone()) {
             let spans = violation.spans();
+            let msg = if trait_span.is_none() || spans.is_empty() {
+                format!("the trait cannot be made into an object because {}", violation.error_msg())
+            } else {
+                had_span_label = true;
+                format!("...because {}", violation.error_msg())
+            };
             if spans.is_empty() {
-                err.note(&violation.error_msg());
+                err.note(&msg);
             } else {
                 for span in spans {
-                    err.span_label(span, violation.error_msg());
+                    err.span_label(span, &msg);
                 }
             }
+            if let (Some(_), Some(note)) = (trait_span, violation.solution()) {
+                // Only provide the help if its a local trait, otherwise it's not actionable.
+                err.help(&note);
+            }
         }
     }
+    if let (Some(trait_span), true) = (trait_span, had_span_label) {
+        err.span_label(trait_span, "this trait cannot be made into an object...");
+    }
 
     if tcx.sess.trait_methods_not_found.borrow().contains(&span) {
         // Avoid emitting error caused by non-existing method (#58734)

src/librustc/traits/object_safety.rs

@@ -43,12 +43,9 @@ pub enum ObjectSafetyViolation {
 impl ObjectSafetyViolation {
     pub fn error_msg(&self) -> Cow<'static, str> {
         match *self {
-            ObjectSafetyViolation::SizedSelf(_) => {
-                "traits that require `Self: Sized` cannot be made into an object".into()
-            }
+            ObjectSafetyViolation::SizedSelf(_) => "it requires `Self: Sized`".into(),
             ObjectSafetyViolation::SupertraitSelf => {
-                "the trait cannot use `Self` as a type parameter \
-                 in the supertraits or where-clauses"
+                "it cannot use `Self` as a type parameter in the supertraits or `where`-clauses"
                     .into()
             }
             ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod, _) => {
@@ -63,19 +60,45 @@ impl ObjectSafetyViolation {
                 name,
                 MethodViolationCode::WhereClauseReferencesSelf,
                 _,
-            ) => format!("method `{}` references the `Self` type in where clauses", name).into(),
+            ) => {
+                format!("method `{}` references the `Self` type in its `where` clause", name).into()
+            }
             ObjectSafetyViolation::Method(name, MethodViolationCode::Generic, _) => {
                 format!("method `{}` has generic type parameters", name).into()
             }
             ObjectSafetyViolation::Method(name, MethodViolationCode::UndispatchableReceiver, _) => {
                 format!("method `{}`'s `self` parameter cannot be dispatched on", name).into()
             }
+            ObjectSafetyViolation::AssocConst(_, DUMMY_SP) => {
+                "it cannot contain associated consts".into()
+            }
             ObjectSafetyViolation::AssocConst(name, _) => {
-                format!("the trait cannot contain associated consts like `{}`", name).into()
+                format!("it cannot contain associated consts like `{}`", name).into()
             }
         }
     }
 
+    pub fn solution(&self) -> Option<String> {
+        Some(match *self {
+            ObjectSafetyViolation::SizedSelf(_) | ObjectSafetyViolation::SupertraitSelf => {
+                return None;
+            }
+            ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod, _) => format!(
+                "consider turning `{}` into a method by giving it a `&self` argument or \
+                 constraining it with `where Self: Sized`",
+                name
+            ),
+            ObjectSafetyViolation::Method(name, MethodViolationCode::UndispatchableReceiver, _) => {
+                format!("consider changing method `{}`'s `self` parameter to be `&self`", name)
+                    .into()
+            }
+            ObjectSafetyViolation::AssocConst(name, _)
+            | ObjectSafetyViolation::Method(name, ..) => {
+                format!("consider moving `{}` to another trait", name)
+            }
+        })
+    }
+
     pub fn spans(&self) -> SmallVec<[Span; 1]> {
         // When `span` comes from a separate crate, it'll be `DUMMY_SP`. Treat it as `None` so
         // diagnostics use a `note` instead of a `span_label`.
@@ -190,7 +213,21 @@ fn object_safety_violations_for_trait(
                         tcx.def_path_str(trait_def_id)
                     ),
                 );
-                err.span_label(*span, violation.error_msg());
+                let node = tcx.hir().get_if_local(trait_def_id);
+                let msg = if let Some(hir::Node::Item(item)) = node {
+                    err.span_label(item.ident.span, "this trait cannot be made into an object...");
+                    format!("...because {}", violation.error_msg())
+                } else {
+                    format!(
+                        "the trait cannot be made into an object because {}",
+                        violation.error_msg()
+                    )
+                };
+                err.span_label(*span, &msg);
+                if let (Some(_), Some(note)) = (node, violation.solution()) {
+                    // Only provide the help if its a local trait, otherwise it's not actionable.
+                    err.help(&note);
+                }
                 err.emit();
                 false
             } else {

src/test/ui/associated-const/associated-const-in-trait.stderr

@@ -1,11 +1,15 @@
 error[E0038]: the trait `Trait` cannot be made into an object
   --> $DIR/associated-const-in-trait.rs:9:6
    |
+LL | trait Trait {
+   |       ----- this trait cannot be made into an object...
 LL |     const N: usize;
-   |           - the trait cannot contain associated consts like `N`
+   |           - ...because it cannot contain associated consts like `N`
 ...
 LL | impl dyn Trait {
    |      ^^^^^^^^^ the trait `Trait` cannot be made into an object
+   |
+   = help: consider moving `N` to another trait
 
 error: aborting due to previous error
 

src/test/ui/associated-item/issue-48027.stderr

@@ -1,11 +1,15 @@
 error[E0038]: the trait `Bar` cannot be made into an object
   --> $DIR/issue-48027.rs:6:6
    |
+LL | trait Bar {
+   |       --- this trait cannot be made into an object...
 LL |     const X: usize;
-   |           - the trait cannot contain associated consts like `X`
+   |           - ...because it cannot contain associated consts like `X`
 ...
 LL | impl dyn Bar {}
    |      ^^^^^^^ the trait `Bar` cannot be made into an object
+   |
+   = help: consider moving `X` to another trait
 
 error[E0283]: type annotations needed
   --> $DIR/issue-48027.rs:3:32

src/test/ui/coherence/coherence-impl-trait-for-trait-object-safe.stderr

@@ -2,9 +2,13 @@ error[E0038]: the trait `NotObjectSafe` cannot be made into an object
   --> $DIR/coherence-impl-trait-for-trait-object-safe.rs:7:6
    |
 LL | trait NotObjectSafe { fn eq(&self, other: Self); }
-   |                          -- method `eq` references the `Self` type in its parameters or return type
+   |       -------------      -- ...because method `eq` references the `Self` type in its parameters or return type
+   |       |
+   |       this trait cannot be made into an object...
 LL | impl NotObjectSafe for dyn NotObjectSafe { }
    |      ^^^^^^^^^^^^^ the trait `NotObjectSafe` cannot be made into an object
+   |
+   = help: consider moving `eq` to another trait
 
 error: aborting due to previous error
 

src/test/ui/did_you_mean/trait-object-reference-without-parens-suggestion.stderr

@@ -16,7 +16,7 @@ error[E0038]: the trait `std::marker::Copy` cannot be made into an object
 LL |     let _: &Copy + 'static;
    |            ^^^^^ the trait `std::marker::Copy` cannot be made into an object
    |
-   = note: traits that require `Self: Sized` cannot be made into an object
+   = note: the trait cannot be made into an object because it requires `Self: Sized`
 
 error: aborting due to 3 previous errors
 

src/test/ui/error-codes/E0033-teach.stderr

@@ -7,11 +7,15 @@ LL |     let trait_obj: &dyn SomeTrait = SomeTrait;
 error[E0038]: the trait `SomeTrait` cannot be made into an object
   --> $DIR/E0033-teach.rs:8:20
    |
+LL | trait SomeTrait {
+   |       --------- this trait cannot be made into an object...
 LL |     fn foo();
-   |        --- associated function `foo` has no `self` parameter
+   |        --- ...because associated function `foo` has no `self` parameter
 ...
 LL |     let trait_obj: &dyn SomeTrait = SomeTrait;
    |                    ^^^^^^^^^^^^^^ the trait `SomeTrait` cannot be made into an object
+   |
+   = help: consider turning `foo` into a method by giving it a `&self` argument or constraining it with `where Self: Sized`
 
 error[E0033]: type `&dyn SomeTrait` cannot be dereferenced
   --> $DIR/E0033-teach.rs:12:9

src/test/ui/error-codes/E0033.stderr

@@ -7,11 +7,15 @@ LL |     let trait_obj: &dyn SomeTrait = SomeTrait;
 error[E0038]: the trait `SomeTrait` cannot be made into an object
   --> $DIR/E0033.rs:6:20
    |
+LL | trait SomeTrait {
+   |       --------- this trait cannot be made into an object...
 LL |     fn foo();
-   |        --- associated function `foo` has no `self` parameter
+   |        --- ...because associated function `foo` has no `self` parameter
 ...
 LL |     let trait_obj: &dyn SomeTrait = SomeTrait;
    |                    ^^^^^^^^^^^^^^ the trait `SomeTrait` cannot be made into an object
+   |
+   = help: consider turning `foo` into a method by giving it a `&self` argument or constraining it with `where Self: Sized`
 
 error[E0033]: type `&dyn SomeTrait` cannot be dereferenced
   --> $DIR/E0033.rs:10:9

src/test/ui/error-codes/E0038.stderr

@@ -1,11 +1,15 @@
 error[E0038]: the trait `Trait` cannot be made into an object
   --> $DIR/E0038.rs:5:16
    |
+LL | trait Trait {
+   |       ----- this trait cannot be made into an object...
 LL |     fn foo(&self) -> Self;
-   |        --- method `foo` references the `Self` type in its parameters or return type
+   |        --- ...because method `foo` references the `Self` type in its parameters or return type
 ...
 LL | fn call_foo(x: Box<dyn Trait>) {
    |                ^^^^^^^^^^^^^^ the trait `Trait` cannot be made into an object
+   |
+   = help: consider moving `foo` to another trait
 
 error: aborting due to previous error
 

src/test/ui/feature-gates/feature-gate-object_safe_for_dispatch.stderr

@@ -2,43 +2,59 @@ error[E0038]: the trait `NonObjectSafe1` cannot be made into an object
   --> $DIR/feature-gate-object_safe_for_dispatch.rs:18:38
    |
 LL | trait NonObjectSafe1: Sized {}
-   |                       ----- traits that require `Self: Sized` cannot be made into an object
+   |       --------------  ----- ...because it requires `Self: Sized`
+   |       |
+   |       this trait cannot be made into an object...
 ...
 LL | fn takes_non_object_safe_ref<T>(obj: &dyn NonObjectSafe1) {
    |                                      ^^^^^^^^^^^^^^^^^^^ the trait `NonObjectSafe1` cannot be made into an object
 
 error[E0038]: the trait `NonObjectSafe2` cannot be made into an object
   --> $DIR/feature-gate-object_safe_for_dispatch.rs:22:36
    |
+LL | trait NonObjectSafe2 {
+   |       -------------- this trait cannot be made into an object...
 LL |     fn static_fn() {}
-   |        --------- associated function `static_fn` has no `self` parameter
+   |        --------- ...because associated function `static_fn` has no `self` parameter
 ...
 LL | fn return_non_object_safe_ref() -> &'static dyn NonObjectSafe2 {
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NonObjectSafe2` cannot be made into an object
+   |
+   = help: consider turning `static_fn` into a method by giving it a `&self` argument or constraining it with `where Self: Sized`
 
 error[E0038]: the trait `NonObjectSafe3` cannot be made into an object
   --> $DIR/feature-gate-object_safe_for_dispatch.rs:27:35
    |
+LL | trait NonObjectSafe3 {
+   |       -------------- this trait cannot be made into an object...
 LL |     fn foo<T>(&self);
-   |        --- method `foo` has generic type parameters
+   |        --- ...because method `foo` has generic type parameters
 ...
 LL | fn takes_non_object_safe_box(obj: Box<dyn NonObjectSafe3>) {
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^ the trait `NonObjectSafe3` cannot be made into an object
+   |
+   = help: consider moving `foo` to another trait
 
 error[E0038]: the trait `NonObjectSafe4` cannot be made into an object
   --> $DIR/feature-gate-object_safe_for_dispatch.rs:31:35
    |
+LL | trait NonObjectSafe4 {
+   |       -------------- this trait cannot be made into an object...
 LL |     fn foo(&self, &Self);
-   |        --- method `foo` references the `Self` type in its parameters or return type
+   |        --- ...because method `foo` references the `Self` type in its parameters or return type
 ...
 LL | fn return_non_object_safe_rc() -> std::rc::Rc<dyn NonObjectSafe4> {
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `NonObjectSafe4` cannot be made into an object
+   |
+   = help: consider moving `foo` to another trait
 
 error[E0038]: the trait `NonObjectSafe1` cannot be made into an object
   --> $DIR/feature-gate-object_safe_for_dispatch.rs:38:6
    |
 LL | trait NonObjectSafe1: Sized {}
-   |                       ----- traits that require `Self: Sized` cannot be made into an object
+   |       --------------  ----- ...because it requires `Self: Sized`
+   |       |
+   |       this trait cannot be made into an object...
 ...
 LL | impl Trait for dyn NonObjectSafe1 {}
    |      ^^^^^ the trait `NonObjectSafe1` cannot be made into an object

src/test/ui/impl-trait/object-unsafe-trait-in-return-position-dyn-trait.stderr

@@ -1,20 +1,28 @@
 error[E0038]: the trait `NotObjectSafe` cannot be made into an object
   --> $DIR/object-unsafe-trait-in-return-position-dyn-trait.rs:21:13
    |
+LL | trait NotObjectSafe {
+   |       ------------- this trait cannot be made into an object...
 LL |     fn foo() -> Self;
-   |        --- associated function `foo` has no `self` parameter
+   |        --- ...because associated function `foo` has no `self` parameter
 ...
 LL | fn car() -> dyn NotObjectSafe {
    |             ^^^^^^^^^^^^^^^^^ the trait `NotObjectSafe` cannot be made into an object
+   |
+   = help: consider turning `foo` into a method by giving it a `&self` argument or constraining it with `where Self: Sized`
 
 error[E0038]: the trait `NotObjectSafe` cannot be made into an object
   --> $DIR/object-unsafe-trait-in-return-position-dyn-trait.rs:28:13
    |
+LL | trait NotObjectSafe {
+   |       ------------- this trait cannot be made into an object...
 LL |     fn foo() -> Self;
-   |        --- associated function `foo` has no `self` parameter
+   |        --- ...because associated function `foo` has no `self` parameter
 ...
 LL | fn cat() -> Box<dyn NotObjectSafe> {
    |             ^^^^^^^^^^^^^^^^^^^^^^ the trait `NotObjectSafe` cannot be made into an object
+   |
+   = help: consider turning `foo` into a method by giving it a `&self` argument or constraining it with `where Self: Sized`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-18959.stderr

@@ -2,10 +2,14 @@ error[E0038]: the trait `Bar` cannot be made into an object
   --> $DIR/issue-18959.rs:11:11
    |
 LL | pub trait Foo { fn foo<T>(&self, ext_thing: &T); }
-   |                    --- method `foo` has generic type parameters
+   |                    --- ...because method `foo` has generic type parameters
+LL | pub trait Bar: Foo { }
+   |           --- this trait cannot be made into an object...
 ...
 LL | fn foo(b: &dyn Bar) {
    |           ^^^^^^^^ the trait `Bar` cannot be made into an object
+   |
+   = help: consider moving `foo` to another trait
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-19380.stderr

@@ -1,11 +1,15 @@
 error[E0038]: the trait `Qiz` cannot be made into an object
   --> $DIR/issue-19380.rs:11:3
    |
+LL | trait Qiz {
+   |       --- this trait cannot be made into an object...
 LL |   fn qiz();
-   |      --- associated function `qiz` has no `self` parameter
+   |      --- ...because associated function `qiz` has no `self` parameter
 ...
 LL |   foos: &'static [&'static (dyn Qiz + 'static)]
    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Qiz` cannot be made into an object
+   |
+   = help: consider turning `qiz` into a method by giving it a `&self` argument or constraining it with `where Self: Sized`
 
 error: aborting due to previous error