Suggest collecting into Vec<_> when collecting into [_]

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

@@ -81,6 +81,32 @@
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_on_unimplemented(
+ on(
+ _Self = "[{A}]",
+ message = "a value of type `{Self}` cannot be built since `{Self}` has no definite size",
+ label = "try explicitly collecting into a `Vec<{A}>`",
+ ),
+ on(
+ all(
+ A = "{integer}",
+ any(
+ _Self = "[i8]",
+ _Self = "[i16]",
+ _Self = "[i32]",
+ _Self = "[i64]",
+ _Self = "[i128]",
+ _Self = "[isize]",
+ _Self = "[u8]",
+ _Self = "[u16]",
+ _Self = "[u32]",
+ _Self = "[u64]",
+ _Self = "[u128]",
+ _Self = "[usize]"
+ )
+ ),
+ message = "a value of type `{Self}` cannot be built since `{Self}` has no definite size",
+ label = "try explicitly collecting into a `Vec<{A}>`",
+ ),
  message = "a value of type `{Self}` cannot be built from an iterator \
  over elements of type `{A}`",
  label = "value of type `{Self}` cannot be built from `std::iter::Iterator<Item={A}>`"

src/test/ui/iterators/collect-into-slice.rs

@@ -0,0 +1,15 @@
+fn process_slice(data: &[i32]) {
+ //~^ NOTE required by a bound in this
+ todo!()
+}
+
+fn main() {
+ let some_generated_vec = (0..10).collect();
+ //~^ ERROR the size for values of type `[i32]` cannot be known at compilation time
+ //~| ERROR a value of type `[i32]` cannot be built since `[i32]` has no definite size
+ //~| NOTE try explicitly collecting into a `Vec<{integer}>`
+ //~| NOTE required by a bound in `collect`
+ //~| NOTE all local variables must have a statically known size
+ //~| NOTE doesn't have a size known at compile-time
+ process_slice(&some_generated_vec);
+}

src/test/ui/iterators/collect-into-slice.stderr

@@ -0,0 +1,26 @@
+error[E0277]: the size for values of type `[i32]` cannot be known at compilation time
+ --> $DIR/collect-into-slice.rs:7:9
+ |
+LL | let some_generated_vec = (0..10).collect();
+ | ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+ |
+ = help: the trait `Sized` is not implemented for `[i32]`
+ = note: all local variables must have a statically known size
+ = help: unsized locals are gated as an unstable feature
+
+error[E0277]: a value of type `[i32]` cannot be built since `[i32]` has no definite size
+ --> $DIR/collect-into-slice.rs:7:38
+ |
+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
+ |
+LL | fn collect<B: FromIterator<Self::Item>>(self) -> B
+ | ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `collect`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.