Rollup merge of #111612 - ChayimFriedman2:collect-into-slice-ref, r=p…

…etrochenkov

Give better error when collecting into `&[T]`

The detection of slice reference of `{integral}` in `rustc_on_unimplemented` is hacky, but a proper solution requires changing `FmtPrinter` to add a parameter to print integers as `{integral}` and I didn't want to change it just for `rustc_on_unimplemented`. I can do that if requested, though.

I'm open to better wording; this is the best I could come up with.

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",