find the right error source when we can't unsize

Author: jdonszelmann

compiler/rustc_hir_typeck/src/coercion.rs

@@ -2055,22 +2055,39 @@ impl<'tcx> ProofTreeVisitor<'tcx> for CoerceVisitor<'_, 'tcx> {
             return ControlFlow::Continue(());
         };
 
+        // Make sure this predicate is referring to either an `Unsize` or `CoerceUnsized` trait,
+        // Otherwise there's nothing to do.
         if !self.fcx.tcx.is_lang_item(pred.def_id(), LangItem::Unsize)
             && !self.fcx.tcx.is_lang_item(pred.def_id(), LangItem::CoerceUnsized)
         {
             return ControlFlow::Continue(());
         }
 
         match goal.result() {
+            // If we prove the `Unsize` or `CoerceUnsized` goal, continue recursing.
             Ok(Certainty::Yes) => ControlFlow::Continue(()),
-            Err(NoSolution) => ControlFlow::Break(()),
+            Err(NoSolution) => {
+                // Even if we find no solution, continue recursing if we find a single candidate
+                // for which we're shallowly certain it holds to get the right error source.
+                if let [only_candidate] = &goal.candidates()[..]
+                    && only_candidate.shallow_certainty() == Certainty::Yes
+                {
+                    only_candidate.visit_nested_no_probe(self)
+                } else {
+                    ControlFlow::Break(())
+                }
+            }
             Ok(Certainty::Maybe { .. }) => {
                 // FIXME: structurally normalize?
                 if self.fcx.tcx.is_lang_item(pred.def_id(), LangItem::Unsize)
                     && let ty::Dynamic(..) = pred.skip_binder().trait_ref.args.type_at(1).kind()
                     && let ty::Infer(ty::TyVar(vid)) = *pred.self_ty().skip_binder().kind()
                     && self.fcx.type_var_is_sized(vid)
                 {
+                    // We get here when trying to unsize a type variable to a `dyn Trait`,
+                    // knowing that that variable is sized. Unsizing definitely has to happen in that case.
+                    // If the variable weren't sized, we may not need an unsizing coercion.
+                    // In general, we don't want to add coercions too eagerly since it makes error messages much worse.
                     ControlFlow::Continue(())
                 } else if let Some(cand) = goal.unique_applicable_candidate()
                     && cand.shallow_certainty() == Certainty::Yes

tests/ui/dst/dst-object-from-unsized-type.current.stderr

@@ -14,7 +14,7 @@ LL + fn test1<T: Foo>(t: &T) {
    |
 
 error[E0277]: the size for values of type `T` cannot be known at compilation time
-  --> $DIR/dst-object-from-unsized-type.rs:16:23
+  --> $DIR/dst-object-from-unsized-type.rs:17:23
    |
 LL | fn test2<T: ?Sized + Foo>(t: &T) {
    |          - this type parameter needs to be `Sized`
@@ -29,7 +29,7 @@ LL + fn test2<T: Foo>(t: &T) {
    |
 
 error[E0277]: the size for values of type `str` cannot be known at compilation time
-  --> $DIR/dst-object-from-unsized-type.rs:21:28
+  --> $DIR/dst-object-from-unsized-type.rs:23:28
    |
 LL |     let _: &[&dyn Foo] = &["hi"];
    |                            ^^^^ doesn't have a size known at compile-time
@@ -38,7 +38,7 @@ LL |     let _: &[&dyn Foo] = &["hi"];
    = note: required for the cast from `&'static str` to `&dyn Foo`
 
 error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
-  --> $DIR/dst-object-from-unsized-type.rs:26:23
+  --> $DIR/dst-object-from-unsized-type.rs:29:23
    |
 LL |     let _: &dyn Foo = x as &dyn Foo;
    |                       ^ doesn't have a size known at compile-time

tests/ui/dst/dst-object-from-unsized-type.next.stderr

@@ -1,41 +1,57 @@
-error[E0308]: mismatched types
+error[E0277]: the trait bound `&T: CoerceUnsized<&dyn Foo>` is not satisfied in `T`
   --> $DIR/dst-object-from-unsized-type.rs:11:23
    |
 LL | fn test1<T: ?Sized + Foo>(t: &T) {
-   |          - found this type parameter
+   |          - this type parameter needs to be `Sized`
 LL |     let u: &dyn Foo = t;
-   |            --------   ^ expected `&dyn Foo`, found `&T`
-   |            |
-   |            expected due to this
-   |
-   = note: expected reference `&dyn Foo`
-              found reference `&T`
-   = help: type parameters must be constrained to match other types
-   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
+   |                       ^ within `T`, the trait `Sized` is not implemented for `T`
+   |
+   = note: required because it appears within the type `T`
+   = note: required for `&T` to implement `CoerceUnsized<&dyn Foo>`
+   = note: required for the cast from `&T` to `&dyn Foo`
+help: consider removing the `?Sized` bound to make the type parameter `Sized`
+   |
+LL - fn test1<T: ?Sized + Foo>(t: &T) {
+LL + fn test1<T: Foo>(t: &T) {
+   |
 
-error[E0605]: non-primitive cast: `&T` as `&dyn Foo`
-  --> $DIR/dst-object-from-unsized-type.rs:16:23
+error[E0277]: the trait bound `&T: CoerceUnsized<&dyn Foo>` is not satisfied in `T`
+  --> $DIR/dst-object-from-unsized-type.rs:17:23
    |
+LL | fn test2<T: ?Sized + Foo>(t: &T) {
+   |          - this type parameter needs to be `Sized`
 LL |     let v: &dyn Foo = t as &dyn Foo;
-   |                       ^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   |                       ^ within `T`, the trait `Sized` is not implemented for `T`
+   |
+   = note: required because it appears within the type `T`
+   = note: required for `&T` to implement `CoerceUnsized<&dyn Foo>`
+   = note: required for the cast from `&T` to `&dyn Foo`
+help: consider removing the `?Sized` bound to make the type parameter `Sized`
+   |
+LL - fn test2<T: ?Sized + Foo>(t: &T) {
+LL + fn test2<T: Foo>(t: &T) {
+   |
 
-error[E0308]: mismatched types
-  --> $DIR/dst-object-from-unsized-type.rs:21:28
+error[E0277]: the trait bound `&str: CoerceUnsized<&dyn Foo>` is not satisfied in `str`
+  --> $DIR/dst-object-from-unsized-type.rs:23:28
    |
 LL |     let _: &[&dyn Foo] = &["hi"];
-   |                            ^^^^ expected `&dyn Foo`, found `&str`
+   |                            ^^^^ within `str`, the trait `Sized` is not implemented for `str`
    |
-   = note: expected reference `&dyn Foo`
-              found reference `&'static str`
-   = help: `str` implements `Foo` so you could box the found value and coerce it to the trait object `Box<dyn Foo>`, you will have to change the expected type as well
+   = note: `str` is considered to contain a `[u8]` slice for auto trait purposes
+   = note: required for `&str` to implement `CoerceUnsized<&dyn Foo>`
+   = note: required for the cast from `&'static str` to `&dyn Foo`
 
-error[E0605]: non-primitive cast: `&[u8]` as `&dyn Foo`
-  --> $DIR/dst-object-from-unsized-type.rs:26:23
+error[E0277]: the trait bound `&[u8]: CoerceUnsized<&dyn Foo>` is not satisfied in `[u8]`
+  --> $DIR/dst-object-from-unsized-type.rs:29:23
    |
 LL |     let _: &dyn Foo = x as &dyn Foo;
-   |                       ^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   |                       ^ within `[u8]`, the trait `Sized` is not implemented for `[u8]`
+   |
+   = note: required because it appears within the type `[u8]`
+   = note: required for `&[u8]` to implement `CoerceUnsized<&dyn Foo>`
+   = note: required for the cast from `&[u8]` to `&dyn Foo`
 
 error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0308, E0605.
-For more information about an error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0277`.

tests/ui/dst/dst-object-from-unsized-type.rs

@@ -9,22 +9,26 @@ impl Foo for [u8] {}
 
 fn test1<T: ?Sized + Foo>(t: &T) {
     let u: &dyn Foo = t;
-    //~^ ERROR the size for values of type
+    //[current]~^ ERROR the size for values of type
+    //[next]~^^ ERROR the trait bound `&T: CoerceUnsized<&dyn Foo>` is not satisfied in `T`
 }
 
 fn test2<T: ?Sized + Foo>(t: &T) {
     let v: &dyn Foo = t as &dyn Foo;
-    //~^ ERROR the size for values of type
+    //[current]~^ ERROR the size for values of type
+    //[next]~^^ ERROR the trait bound `&T: CoerceUnsized<&dyn Foo>` is not satisfied in `T`
 }
 
 fn test3() {
     let _: &[&dyn Foo] = &["hi"];
-    //~^ ERROR the size for values of type
+    //[current]~^ ERROR the size for values of type
+    //[next]~^^ ERROR the trait bound `&str: CoerceUnsized<&dyn Foo>` is not satisfied in `str`
 }
 
 fn test4(x: &[u8]) {
     let _: &dyn Foo = x as &dyn Foo;
-    //~^ ERROR the size for values of type
+    //[current]~^ ERROR the size for values of type
+    //[next]~^^ ERROR the trait bound `&[u8]: CoerceUnsized<&dyn Foo>` is not satisfied in `[u8]`
 }
 
 fn main() { }

tests/ui/traits/trait-upcasting/higher-ranked-upcasting-ub.next.stderr

@@ -1,14 +1,13 @@
-error[E0308]: mismatched types
+error[E0277]: the trait bound `&dyn for<'a> Subtrait<'a, 'a>: CoerceUnsized<&dyn for<'a, 'b> Supertrait<'a, 'b>>` is not satisfied
   --> $DIR/higher-ranked-upcasting-ub.rs:22:5
    |
-LL | fn unsound(x: &dyn for<'a> Subtrait<'a, 'a>) -> &dyn for<'a, 'b> Supertrait<'a, 'b> {
-   |                                                 ----------------------------------- expected `&dyn for<'a, 'b> Supertrait<'a, 'b>` because of return type
 LL |     x
-   |     ^ expected trait `Supertrait`, found trait `Subtrait`
+   |     ^ the trait `Unsize<dyn for<'a, 'b> Supertrait<'a, 'b>>` is not implemented for `dyn for<'a> Subtrait<'a, 'a>`
    |
-   = note: expected reference `&dyn for<'a, 'b> Supertrait<'a, 'b>`
-              found reference `&dyn for<'a> Subtrait<'a, 'a>`
+   = note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information
+   = note: required for `&dyn for<'a> Subtrait<'a, 'a>` to implement `CoerceUnsized<&dyn for<'a, 'b> Supertrait<'a, 'b>>`
+   = note: required for the cast from `&dyn for<'a> Subtrait<'a, 'a>` to `&dyn for<'a, 'b> Supertrait<'a, 'b>`
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/trait-upcasting/higher-ranked-upcasting-ub.rs

@@ -19,8 +19,10 @@ impl<'a> Supertrait<'a, 'a> for () {
 }
 impl<'a> Subtrait<'a, 'a> for () {}
 fn unsound(x: &dyn for<'a> Subtrait<'a, 'a>) -> &dyn for<'a, 'b> Supertrait<'a, 'b> {
-    x //~ ERROR mismatched types
+    x
     //[current]~^ ERROR mismatched types
+    //[current]~| ERROR mismatched types
+    //[next]~^^^ ERROR the trait bound `&dyn for<'a> Subtrait<'a, 'a>: CoerceUnsized<&dyn for<'a, 'b> Supertrait<'a, 'b>>` is not satisfied
 }
 
 fn transmute<'a, 'b>(x: &'a str) -> &'b str {