support using the in-tree compiler_builtins

Author: RalfJung

src/lib.rs

@@ -307,9 +307,7 @@ impl<'a> SysrootBuilder<'a> {
                 path = {src_dir_core:?}
                 [dependencies.alloc]
                 path = {src_dir_alloc:?}
-                [dependencies.compiler_builtins]
-                features = ["rustc-dep-of-std", "mem"]
-                version = "*"
+                features = ["compiler-builtins-mem"]
                 "#,
                 src_dir_core = src_dir.join("core"),
                 src_dir_alloc = src_dir.join("alloc"),