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, 21 insertions, 0 deletions

diff --git a/src/test/ui/array-slice-vec/infer_array_len.rs b/src/test/ui/array-slice-vec/infer_array_len.rs
new file mode 100644
index 00000000000..22fe7cb8838
--- /dev/null
+++ b/src/test/ui/array-slice-vec/infer_array_len.rs
@@ -0,0 +1,21 @@
+// see issue #70529
+struct A;
+
+impl From<A> for [u8; 2] {
+    fn from(a: A) -> Self {
+        [0; 2]
+    }
+}
+
+impl From<A> for [u8; 3] {
+    fn from(a: A) -> Self {
+        [0; 3]
+    }
+}
+
+
+fn main() {
+    let a = A;
+    let [_, _] = a.into();
+    //~^ ERROR type annotations needed
+}