Rollup merge of #40441 - tschottdorf:promotable-rfc, r=eddyb

Add feature gate for rvalue-static-promotion

Probably needs more tests (which ones?) and there may be other things that need to be done. Also not sure whether the version that introduces the flag is really `1.15.1`.

See https://github.com/rust-lang/rfcs/pull/1414.

Updates #38865.

src/doc/unstable-book/src/SUMMARY.md

@@ -71,6 +71,7 @@
 - [repr_simd](repr-simd.md)
 - [rustc_attrs](rustc-attrs.md)
 - [rustc_diagnostic_macros](rustc-diagnostic-macros.md)
+- [rvalue_static_promotion](rvalue-static-promotion.md)
 - [sanitizer_runtime](sanitizer-runtime.md)
 - [simd](simd.md)
 - [simd_ffi](simd-ffi.md)

src/doc/unstable-book/src/rvalue-static-promotion.md

@@ -0,0 +1,23 @@
+# `rvalue_static_promotion`
+
+The tracking issue for this feature is: [#38865]
+
+[#38865]: https://github.com/rust-lang/rust/issues/38865
+
+------------------------
+
+The `rvalue_static_promotion` feature allows directly creating `'static` references to
+constant `rvalue`s, which in particular allowing for more concise code in the common case
+in which a `'static` reference is all that's needed.
+
+
+## Examples
+
+```rust
+#![feature(rvalue_static_promotion)]
+
+fn main() {
+    let DEFAULT_VALUE: &'static u32 = &42;
+    assert_eq!(*DEFAULT_VALUE, 42);
+}
+```

src/librustc/middle/mem_categorization.rs

@@ -843,11 +843,10 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
         let promotable = self.tcx().rvalue_promotable_to_static.borrow().get(&id).cloned()
                                    .unwrap_or(false);
 
-        // Only promote `[T; 0]` before an RFC for rvalue promotions
-        // is accepted.
+        // When the corresponding feature isn't toggled, only promote `[T; 0]`.
         let promotable = match expr_ty.sty {
             ty::TyArray(_, 0) => true,
-            _ => promotable & false
+            _ => promotable && self.tcx().sess.features.borrow().rvalue_static_promotion,
         };
 
         // Compute maximum lifetime of this rvalue. This is 'static if

src/libsyntax/feature_gate.rs

@@ -342,6 +342,9 @@ declare_features! (
 
     // Allows the `catch {...}` expression
     (active, catch_expr, "1.17.0", Some(31436)),
+
+    // See rust-lang/rfcs#1414. Allows code like `let x: &'static u32 = &42` to work.
+    (active, rvalue_static_promotion, "1.15.1", Some(38865)),
 );
 
 declare_features! (

src/test/compile-fail/feature-gate-rvalue_static_promotion.rs

@@ -0,0 +1,15 @@
+// Copyright 2017 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.
+
+#[allow(unused_variables)]
+fn main() {
+    let x: &'static u32 = &42; //~ error: does not live long enough
+    let y: &'static Option<u32> = &None; //~ error: does not live long enough
+}

src/test/run-pass/rvalue-static-promotion.rs

@@ -0,0 +1,17 @@
+// Copyright 2017 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(rvalue_static_promotion)]
+
+#[allow(unused_variables)]
+fn main() {
+    let x: &'static u32 = &42;
+    let y: &'static Option<u32> = &None;
+}