Rollup merge of #50863 - oli-obk:const_len, r=SimonSapin,Gankro

Make `[T]::len` and `str::len` const fn

r? @Gankro

src/test/ui/const-eval/strlen.rs

@@ -0,0 +1,33 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// run-pass
+
+#![feature(const_str_len, const_str_as_bytes)]
+
+const S: &str = "foo";
+pub const B: &[u8] = S.as_bytes();
+
+pub fn foo() -> [u8; S.len()] {
+    let mut buf = [0; S.len()];
+    for (i, &c) in S.as_bytes().iter().enumerate() {
+        buf[i] = c;
+    }
+    buf
+}
+
+fn main() {
+    assert_eq!(&foo()[..], b"foo");
+    assert_eq!(foo().len(), S.len());
+    const LEN: usize = S.len();
+    assert_eq!(LEN, S.len());
+    assert_eq!(B, foo());
+    assert_eq!(B, b"foo");
+}