Rollup merge of rust-lang#59462 - taiki-e:no-core, r=petrochenkov

Fix error in Rust 2018 + no_core environment

Minimized reproduction: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=5b9f6c3026ec9d856699fa6dbd4361f0

This is a fix for the error that occurred in rust-lang#58702.

r? @Centril

Author: Centril

src/librustc_resolve/lib.rs

@@ -1697,7 +1697,12 @@ impl<'a> hir::lowering::Resolver for Resolver<'a> {
         components: &[&str],
         is_value: bool
     ) -> hir::Path {
-        let segments = iter::once(keywords::PathRoot.ident())
+        let root = if crate_root.is_some() {
+            keywords::PathRoot
+        } else {
+            keywords::Crate
+        };
+        let segments = iter::once(root.ident())
             .chain(
                 crate_root.into_iter()
                     .chain(components.iter().cloned())

src/test/run-pass/no-core-2.rs

@@ -0,0 +1,18 @@
+#![allow(dead_code, unused_imports)]
+#![feature(no_core)]
+#![no_core]
+// edition:2018
+
+extern crate std;
+extern crate core;
+use core::{prelude::v1::*, *};
+
+fn foo() {
+    for _ in &[()] {}
+}
+
+fn bar() -> Option<()> {
+    None?
+}
+
+fn main() {}