Rollup merge of #70562 - lcnr:const-arr_len, r=Centril

infer array len from pattern

closes #70529

This still errors in the following case

```rust
#![feature(const_generics)]
fn arr<const N: usize>() -> [u8; N] {
    todo!()
}

fn main() {
    match arr() {
        [5, ..] => (),
        //~^ ERROR cannot pattern-match on an array without a fixed length
        [_, _] => (),
    }
}
```
Considering that this should be rare and is harder to implement I would merge this PR without *fixing* the above.

1 files changed, 11 insertions, 0 deletions

diff --git a/src/test/ui/array-slice-vec/infer_array_len.stderr b/src/test/ui/array-slice-vec/infer_array_len.stderr
new file mode 100644
index 00000000000..6eed4ce4f0c
--- /dev/null
+++ b/src/test/ui/array-slice-vec/infer_array_len.stderr
@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed
+  --> $DIR/infer_array_len.rs:19:9
+   |
+LL |     let [_, _] = a.into();
+   |         ^^^^^^ consider giving this pattern a type
+   |
+   = note: type must be known at this point
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.