Make core_miri_test and alloc_miri_test empty when not collecting…

… doctests

alloc_miri_test/Cargo.toml

@@ -10,6 +10,11 @@ edition = "2018"
 name = "alloc_miri_test"
 path = "../liballoc/src/lib.rs"
 
+[features]
+# Set `cfg(feature = "miri-test")` to make the crate empty unless rustdoc is collecting doctests.
+default = ["miri-test"]
+miri-test = []
+
 [dependencies]
 
 [dev-dependencies]

core_miri_test/Cargo.toml

@@ -12,6 +12,11 @@ path = "../libcore/src/lib.rs"
 test = false
 bench = false
 
+[features]
+# Set `cfg(feature = "miri-test")` to make the crate empty unless rustdoc is collecting doctests.
+default = ["miri-test"]
+miri-test = []
+
 [[test]]
 name = "coretests"
 path = "../libcore/tests/lib.rs"

rust-src.diff

@@ -0,0 +1,24 @@
+diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
+index e8bd4bcb..cc518cc4 100644
+--- a/library/alloc/src/lib.rs
++++ b/library/alloc/src/lib.rs
+@@ -56,6 +56,7 @@
+ //! [`Rc`]: rc
+ //! [`RefCell`]: core::cell
+
++#![cfg(any(not(feature = "miri-test"), doctest))]
+ #![allow(unused_attributes)]
+ #![stable(feature = "alloc", since = "1.36.0")]
+ #![doc(
+diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
+index 3557dbad..6c089b0c 100644
+--- a/library/core/src/lib.rs
++++ b/library/core/src/lib.rs
+@@ -49,6 +49,7 @@
+ //
+ // This cfg won't affect doc tests.
+ #![cfg(not(test))]
++#![cfg(any(not(feature = "miri-test"), doctest))]
+ #![stable(feature = "core", since = "1.6.0")]
+ #![doc(
+     html_playground_url = "https://play.rust-lang.org/",