Give better error when collecting into &[T]

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

@@ -306,6 +306,14 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     }
                 }
             }
+
+            // `&[{integral}]` - `FromIterator` needs that.
+            if let ty::Ref(_, ref_ty, rustc_ast::Mutability::Not) = self_ty.kind()
+                && let ty::Slice(sty) = ref_ty.kind()
+                && sty.is_integral()
+            {
+                flags.push((sym::_Self, Some("&[{integral}]".to_owned())));
+            }
         });
 
         if let Ok(Some(command)) = OnUnimplementedDirective::of_item(self.tcx, def_id) {

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

@@ -94,6 +94,16 @@
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_on_unimplemented(
+    on(
+        _Self = "&[{A}]",
+        message = "a slice of type `{Self}` cannot be built since we need to store the elements somewhere",
+        label = "try explicitly collecting into a `Vec<{A}>`",
+    ),
+    on(
+        all(A = "{integer}", any(_Self = "&[{integral}]",)),
+        message = "a slice of type `{Self}` cannot be built since we need to store the elements somewhere",
+        label = "try explicitly collecting into a `Vec<{A}>`",
+    ),
     on(
         _Self = "[{A}]",
         message = "a slice of type `{Self}` cannot be built since `{Self}` has no definite size",

tests/ui/iterators/collect-into-slice.rs

@@ -14,4 +14,10 @@ fn main() {
     //~| NOTE doesn't have a size known at compile-time
     //~| NOTE doesn't have a size known at compile-time
     process_slice(&some_generated_vec);
+
+    let some_generated_vec = (0..10).collect();
+    //~^ ERROR a slice of type `&[i32]` cannot be built since we need to store the elements somewhere
+    //~| NOTE try explicitly collecting into a `Vec<{integer}>`
+    //~| NOTE required by a bound in `collect`
+    process_slice(some_generated_vec);
 }

tests/ui/iterators/collect-into-slice.stderr

@@ -28,6 +28,16 @@ LL |     let some_generated_vec = (0..10).collect();
 note: required by a bound in `collect`
   --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
 
-error: aborting due to 3 previous errors
+error[E0277]: a slice of type `&[i32]` cannot be built since we need to store the elements somewhere
+  --> $DIR/collect-into-slice.rs:18:38
+   |
+LL |     let some_generated_vec = (0..10).collect();
+   |                                      ^^^^^^^ try explicitly collecting into a `Vec<{integer}>`
+   |
+   = help: the trait `FromIterator<{integer}>` is not implemented for `&[i32]`
+note: required by a bound in `collect`
+  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0277`.