Rollup merge of #47944 - oberien:unboundediterator-trustedlen, r=bluss

Implement TrustedLen for Take<Repeat> and Take<RangeFrom>

This will allow optimization of simple `repeat(x).take(n).collect()` iterators, which are currently not vectorized and have capacity checks.

This will only support a few aggregates on `Repeat` and `RangeFrom`, which might be enough for simple cases, but doesn't optimize more complex ones. Namely, Cycle, StepBy, Filter, FilterMap, Peekable, SkipWhile, Skip, FlatMap, Fuse and Inspect are not marked `TrustedLen` when the inner iterator is infinite.

Previous discussion can be found in #47082

r? @alexcrichton

src/test/codegen/repeat-trusted-len.rs

@@ -0,0 +1,23 @@
+// 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.
+
+// compile-flags: -O
+// ignore-tidy-linelength
+
+#![crate_type = "lib"]
+
+use std::iter;
+
+// CHECK-LABEL: @repeat_take_collect
+#[no_mangle]
+pub fn repeat_take_collect() -> Vec<u8> {
+// CHECK: call void @llvm.memset.p0i8
+    iter::repeat(42).take(100000).collect()
+}