Feature gate FFI imports of LLVM intrinsics

Fixes #20313

Author: nagisa

src/doc/reference.md

@@ -2561,6 +2561,9 @@ The currently implemented features of the reference compiler are:
                 if the system linker is not used then specifying custom flags
                 doesn't have much meaning.
 
+* `link_llvm_intrinsics` – Allows linking to LLVM intrinsics via
+                           `#[link_name="llvm.*"]`.
+
 * `linkage` - Allows use of the `linkage` attribute, which is not portable.
 
 * `log_syntax` - Allows use of the `log_syntax` macro attribute, which is a
@@ -4149,11 +4152,11 @@ Unwinding the stack of a thread is done by the thread itself, on its own control
 stack. If a value with a destructor is freed during unwinding, the code for the
 destructor is run, also on the thread's control stack. Running the destructor
 code causes a temporary transition to a *running* state, and allows the
-destructor code to cause any subsequent state transitions. The original thread 
+destructor code to cause any subsequent state transitions. The original thread
 of unwinding and panicking thereby may suspend temporarily, and may involve
 (recursive) unwinding of the stack of a failed destructor. Nonetheless, the
 outermost unwinding activity will continue until the stack is unwound and the
-thread transitions to the *dead* state. There is no way to "recover" from thread 
+thread transitions to the *dead* state. There is no way to "recover" from thread
 panics. Once a thread has temporarily suspended its unwinding in the *panicking*
 state, a panic occurring from within this destructor results in *hard* panic.
 A hard panic currently results in the process aborting.

src/libsyntax/feature_gate.rs

@@ -56,6 +56,7 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("simd", Active),
     ("default_type_params", Active),
     ("quote", Active),
+    ("link_llvm_intrinsics", Active),
     ("linkage", Active),
     ("struct_inherit", Removed),
 
@@ -292,6 +293,16 @@ impl<'a, 'v> Visitor<'v> for Context<'a> {
                               "the `linkage` attribute is experimental \
                                and not portable across platforms")
         }
+
+        let links_to_llvm = match attr::first_attr_value_str_by_name(i.attrs[], "link_name") {
+            Some(val) => val.get().starts_with("llvm."),
+            _ => false
+        };
+        if links_to_llvm {
+            self.gate_feature("link_llvm_intrinsics", i.span,
+                              "linking to LLVM intrinsics is experimental");
+        }
+
         visit::walk_foreign_item(self, i)
     }
 

src/test/compile-fail/issue-20313.rs

@@ -0,0 +1,17 @@
+// Copyright 2014 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.
+
+extern {
+    #[link_name = "llvm.sqrt.f32"]
+    fn sqrt(x: f32) -> f32; //~ ERROR linking to LLVM intrinsics is experimental
+}
+
+fn main(){
+}

src/test/run-pass/issue-20313.rs

@@ -0,0 +1,18 @@
+// Copyright 2014 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.
+#![feature(link_llvm_intrinsics)]
+
+extern {
+    #[link_name = "llvm.sqrt.f32"]
+    fn sqrt(x: f32) -> f32;
+}
+
+fn main(){
+}