From fea0015f9320f59c46821957a01f688f79a02546 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 1 Dec 2021 18:51:27 -0800
Subject: [PATCH] Suggest collecting into `Vec<_>` when collecting into `[_]`

---
 library/core/src/iter/traits/collect.rs       | 26 +++++++++++++++++++
 src/test/ui/iterators/collect-into-slice.rs   | 15 +++++++++++
 .../ui/iterators/collect-into-slice.stderr    | 26 +++++++++++++++++++
 3 files changed, 67 insertions(+)
 create mode 100644 src/test/ui/iterators/collect-into-slice.rs
 create mode 100644 src/test/ui/iterators/collect-into-slice.stderr

diff --git a/library/core/src/iter/traits/collect.rs b/library/core/src/iter/traits/collect.rs
index fc14620a2df84..637d7bc44885e 100644
--- a/library/core/src/iter/traits/collect.rs
+++ b/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}>`"
diff --git a/src/test/ui/iterators/collect-into-slice.rs b/src/test/ui/iterators/collect-into-slice.rs
new file mode 100644
index 0000000000000..905752dec7478
--- /dev/null
+++ b/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);
+}
diff --git a/src/test/ui/iterators/collect-into-slice.stderr b/src/test/ui/iterators/collect-into-slice.stderr
new file mode 100644
index 0000000000000..521f239451d2f
--- /dev/null
+++ b/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`.