Add some tests for the current `#![cfg(FALSE)]` crate behavior

tests/ui/cfg/auxiliary/cfg_false_lib.rs

@@ -0,0 +1,6 @@
+// It is unclear whether a fully unconfigured crate should link to standard library,
+// or what its `no_std`/`no_core`/`compiler_builtins` status, more precisely.
+// Currently the usual standard library prelude is added to such crates,
+// and therefore they link to libstd.
+
+#![cfg(FALSE)]

tests/ui/cfg/cfg-false-feature.rs

@@ -0,0 +1,20 @@
+// It is unclear which features should be in effect in a fully unconfigured crate (issue #104633).
+// Currently none on the features are in effect, so we get the feature gates reported.
+
+// check-pass
+// compile-flags: --crate-type lib
+
+#![feature(decl_macro)]
+#![cfg(FALSE)]
+#![feature(box_syntax)]
+
+macro mac() {} //~ WARN `macro` is experimental
+               //~| WARN unstable syntax can change at any point in the future
+
+trait A = Clone; //~ WARN trait aliases are experimental
+                 //~| WARN unstable syntax can change at any point in the future
+
+fn main() {
+    let box _ = Box::new(0); //~ WARN box pattern syntax is experimental
+                             //~| WARN unstable syntax can change at any point in the future
+}

tests/ui/cfg/cfg-false-feature.stderr

@@ -0,0 +1,35 @@
+warning: trait aliases are experimental
+  --> $DIR/cfg-false-feature.rs:14:1
+   |
+LL | trait A = Clone;
+   | ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #41517 <https://github.com/rust-lang/rust/issues/41517> for more information
+   = help: add `#![feature(trait_alias)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: `macro` is experimental
+  --> $DIR/cfg-false-feature.rs:11:1
+   |
+LL | macro mac() {}
+   | ^^^^^^^^^^^^^^
+   |
+   = note: see issue #39412 <https://github.com/rust-lang/rust/issues/39412> for more information
+   = help: add `#![feature(decl_macro)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: box pattern syntax is experimental
+  --> $DIR/cfg-false-feature.rs:18:9
+   |
+LL |     let box _ = Box::new(0);
+   |         ^^^^^
+   |
+   = note: see issue #29641 <https://github.com/rust-lang/rust/issues/29641> for more information
+   = help: add `#![feature(box_patterns)]` to the crate attributes to enable
+   = warning: unstable syntax can change at any point in the future, causing a hard error!
+   = note: for more information, see issue #65860 <https://github.com/rust-lang/rust/issues/65860>
+
+warning: 3 warnings emitted
+

tests/ui/cfg/cfg_false_no_std.rs

@@ -0,0 +1,11 @@
+// Currently no error because the panic handler is supplied by libstd linked though the empty
+// library, but the desirable behavior is unclear (see comments in cfg_false_lib.rs).
+
+// check-pass
+// aux-build: cfg_false_lib.rs
+
+#![no_std]
+
+extern crate cfg_false_lib as _;
+
+fn main() {}