Auto merge of #43838 - eddyb:stable-rvalue-promotion, r=arielb1

Stabilize rvalue promotion to 'static.

Closes #38865.

Documentation PR at rust-lang/reference#98.

Author: bors

src/doc/reference

@@ -873,10 +873,10 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
         let promotable = self.tcx.rvalue_promotable_to_static.borrow().get(&id).cloned()
                                    .unwrap_or(false);
 
-        // When the corresponding feature isn't toggled, only promote `[T; 0]`.
+        // Always promote `[T; 0]` (even when e.g. borrowed mutably).
         let promotable = match expr_ty.sty {
             ty::TyArray(_, 0) => true,
-            _ => promotable && self.tcx.sess.features.borrow().rvalue_static_promotion,
+            _ => promotable,
         };
 
         // Compute maximum lifetime of this rvalue. This is 'static if

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

@@ -345,9 +345,6 @@ declare_features! (
     // Allows `repr(align(u16))` struct attribute (RFC 1358)
     (active, repr_align, "1.17.0", Some(33626)),
 
-    // See rust-lang/rfcs#1414. Allows code like `let x: &'static u32 = &42` to work.
-    (active, rvalue_static_promotion, "1.15.1", Some(38865)),
-
     // Used to preserve symbols (see llvm.used)
     (active, used, "1.18.0", Some(40289)),
 
@@ -457,6 +454,8 @@ declare_features! (
     (accepted, associated_consts, "1.20.0", Some(29646)),
     // Usage of the `compile_error!` macro
     (accepted, compile_error, "1.20.0", Some(40872)),
+    // See rust-lang/rfcs#1414. Allows code like `let x: &'static u32 = &42` to work.
+    (accepted, rvalue_static_promotion, "1.21.0", Some(38865)),
 );
 
 // If you change this, please modify src/doc/unstable-book as well. You must

src/librustc/middle/mem_categorization.rs

@@ -11,10 +11,12 @@
 // Test lifetimes are linked properly when we take reference
 // to interior.
 
+fn id<T>(x: T) -> T { x }
+
 struct Foo(isize);
 
 fn foo<'a>() -> &'a isize {
-    let &Foo(ref x) = &Foo(3); //~ ERROR borrowed value does not live long enough
+    let &Foo(ref x) = &id(Foo(3)); //~ ERROR borrowed value does not live long enough
     x
 }
 

src/libsyntax/feature_gate.rs

@@ -10,7 +10,9 @@
 
 // This file must never have a trailing newline
 
+fn id<T>(x: T) -> T { x }
+
 fn main() {
     let x = Some(3);
-    let y = x.as_ref().unwrap_or(&5); //~ ERROR: borrowed value does not live long enough
+    let y = x.as_ref().unwrap_or(&id(5)); //~ ERROR: borrowed value does not live long enough
 }

src/test/compile-fail/borrowck/borrowck-borrow-from-temporary.rs

@@ -10,9 +10,11 @@
 
 #![feature(fn_traits)]
 
+fn id<T>(x: T) -> T { x }
+
 pub fn foo<'a, F: Fn(&'a ())>(bar: F) {
     bar.call((
-        &(), //~ ERROR borrowed value does not live long enough
+        &id(()), //~ ERROR borrowed value does not live long enough
     ));
 }
 fn main() {}

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

@@ -8,9 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+fn id<T>(x: T) -> T { x }
+
 const FOO: usize = 3;
 
-fn foo() -> &'static usize { &FOO }
+fn foo() -> &'static usize { &id(FOO) }
 //~^ ERROR: borrowed value does not live long enough
 
 fn main() {

src/test/compile-fail/issue-11493.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// Regression test for issue #27591.
+// Regression test for issue #27592.
 
 fn write<'a, F: ::std::ops::FnOnce()->::std::fmt::Arguments<'a> + 'a>(fcn: F) {
     use std::fmt::Write;
@@ -23,7 +23,7 @@ impl ::std::fmt::Write for Stream {
 }
 
 fn main() {
-    write(|| format_args!("{}", "Hello world"));
+    write(|| format_args!("{}", String::from("Hello world")));
     //~^ ERROR borrowed value does not live long enough
     //~| ERROR borrowed value does not live long enough
 }