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
 }

src/test/compile-fail/issue-17545.rs

@@ -12,19 +12,23 @@
 // are treated as rvalues and their lifetime is not bounded to
 // the static scope.
 
+fn id<T>(x: T) -> T { x }
+
 struct Test;
 
 enum MyEnum {
     Variant1
 }
 
 fn structLifetime<'a>() -> &'a Test {
-  let testValue = &Test; //~ ERROR borrowed value does not live long enough
+  let testValue = &id(Test);
+  //~^ ERROR borrowed value does not live long enough
   testValue
 }
 
 fn variantLifetime<'a>() -> &'a MyEnum {
-  let testValue = &MyEnum::Variant1; //~ ERROR borrowed value does not live long enough
+  let testValue = &id(MyEnum::Variant1);
+  //~^ ERROR borrowed value does not live long enough
   testValue
 }
 

src/test/compile-fail/issue-17718-constants-not-static.rs

@@ -8,8 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+fn id<T>(x: T) -> T { x }
+
 fn f(_x: &isize) -> &isize {
-    return &3; //~ ERROR borrowed value does not live long enough
+    return &id(3); //~ ERROR borrowed value does not live long enough
 }
 
 fn main() {

src/test/compile-fail/issue-27592.rs

@@ -8,13 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+fn id<T>(x: T) -> T { x }
+
 fn foo(cond: bool) {
     // Here we will infer a type that uses the
     // region of the if stmt then block:
     let mut x;
 
     if cond {
-        x = &3; //~ ERROR borrowed value does not live long enough
+        x = &id(3); //~ ERROR borrowed value does not live long enough
         assert_eq!(*x, 3);
     }
 }

src/test/compile-fail/regions-lifetime-of-struct-or-enum-variant.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+fn id<T>(x: T) -> T { x }
+
 struct StateMachineIter<'a> {
     statefn: &'a StateMachineFunc<'a>
 }
@@ -23,19 +25,19 @@ impl<'a> Iterator for StateMachineIter<'a> {
 }
 
 fn state1(self_: &mut StateMachineIter) -> Option<&'static str> {
-    self_.statefn = &(state2 as StateMachineFunc);
+    self_.statefn = &id(state2 as StateMachineFunc);
     //~^ ERROR borrowed value does not live long enough
     return Some("state1");
 }
 
 fn state2(self_: &mut StateMachineIter) -> Option<(&'static str)> {
-    self_.statefn = &(state3 as StateMachineFunc);
+    self_.statefn = &id(state3 as StateMachineFunc);
     //~^ ERROR borrowed value does not live long enough
     return Some("state2");
 }
 
 fn state3(self_: &mut StateMachineIter) -> Option<(&'static str)> {
-    self_.statefn = &(finished as StateMachineFunc);
+    self_.statefn = &id(finished as StateMachineFunc);
     //~^ ERROR borrowed value does not live long enough
     return Some("state3");
 }
@@ -46,7 +48,8 @@ fn finished(_: &mut StateMachineIter) -> Option<(&'static str)> {
 
 fn state_iter() -> StateMachineIter<'static> {
     StateMachineIter {
-        statefn: &(state1 as StateMachineFunc) //~ ERROR borrowed value does not live long enough
+        statefn: &id(state1 as StateMachineFunc)
+        //~^ ERROR borrowed value does not live long enough
     }
 }
 

src/test/compile-fail/regions-ret.rs

@@ -10,11 +10,13 @@
 
 #![feature(box_syntax)]
 
+fn id<T>(x: T) -> T { x }
+
 fn f<T:'static>(_: T) {}
 
 fn main() {
     let x: Box<_> = box 3;
     f(x);
-    let x = &3; //~ ERROR borrowed value does not live long enough
+    let x = &id(3); //~ ERROR borrowed value does not live long enough
     f(x);
 }

src/test/compile-fail/regions-var-type-out-of-scope.rs

@@ -8,8 +8,6 @@
 // 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;

src/test/compile-fail/static-reference-to-fn-2.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 fn f() {
-    let x = [1].iter();
+    let x = vec![1].iter();
 }
 
 fn main() {

src/test/compile-fail/static-region-bound.rs

@@ -1,14 +1,15 @@
 error[E0597]: borrowed value does not live long enough
-  --> $DIR/borrowck-let-suggestion.rs:12:23
+  --> $DIR/borrowck-let-suggestion.rs:12:27
    |
-12 |     let x = [1].iter();
-   |             ---       ^ temporary value dropped here while still borrowed
+12 |     let x = vec![1].iter();
+   |             -------       ^ temporary value dropped here while still borrowed
    |             |
    |             temporary value created here
 13 | }
    | - temporary value needs to live until here
    |
    = note: consider using a `let` binding to increase its lifetime
+   = note: this error originates in a macro outside of the current crate
 
 error: aborting due to previous error
 

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

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+fn id<T>(x: T) -> T { x }
+
 fn f() {
     let old = ['o'];         // statement 0
     let mut v1 = Vec::new(); // statement 1
@@ -21,7 +23,7 @@ fn f() {
 
     let mut v3 = Vec::new(); // statement 5
 
-    v3.push(&'x');           // statement 6
+    v3.push(&id('x'));           // statement 6
     //~^ ERROR borrowed value does not live long enough
     //~| NOTE temporary value created here
     //~| NOTE temporary value only lives until here
@@ -31,7 +33,7 @@ fn f() {
 
         let mut v4 = Vec::new(); // (sub) statement 0
 
-        v4.push(&'y');
+        v4.push(&id('y'));
         //~^ ERROR borrowed value does not live long enough
         //~| NOTE temporary value created here
         //~| NOTE temporary value only lives until here
@@ -42,7 +44,7 @@ fn f() {
 
     let mut v5 = Vec::new(); // statement 8
 
-    v5.push(&'z');
+    v5.push(&id('z'));
     //~^ ERROR borrowed value does not live long enough
     //~| NOTE temporary value created here
     //~| NOTE temporary value only lives until here

src/test/ui/lifetimes/borrowck-let-suggestion.rs

@@ -1,49 +1,49 @@
 error[E0597]: `young[..]` does not live long enough
-  --> $DIR/borrowck-let-suggestion-suffixes.rs:52:1
+  --> $DIR/borrowck-let-suggestion-suffixes.rs:54:1
    |
-19 |     v2.push(&young[0]);      // statement 4
+21 |     v2.push(&young[0]);      // statement 4
    |              -------- borrow occurs here
 ...
-52 | }
+54 | }
    | ^ `young[..]` dropped here while still borrowed
    |
    = note: values in a scope are dropped in the opposite order they are created
 
 error[E0597]: borrowed value does not live long enough
-  --> $DIR/borrowck-let-suggestion-suffixes.rs:24:18
+  --> $DIR/borrowck-let-suggestion-suffixes.rs:26:22
    |
-24 |     v3.push(&'x');           // statement 6
-   |              --- ^ temporary value dropped here while still borrowed
+26 |     v3.push(&id('x'));           // statement 6
+   |              ------- ^ temporary value dropped here while still borrowed
    |              |
    |              temporary value created here
 ...
-52 | }
+54 | }
    | - temporary value needs to live until here
    |
    = note: consider using a `let` binding to increase its lifetime
 
 error[E0597]: borrowed value does not live long enough
-  --> $DIR/borrowck-let-suggestion-suffixes.rs:34:22
+  --> $DIR/borrowck-let-suggestion-suffixes.rs:36:26
    |
-34 |         v4.push(&'y');
-   |                  --- ^ temporary value dropped here while still borrowed
+36 |         v4.push(&id('y'));
+   |                  ------- ^ temporary value dropped here while still borrowed
    |                  |
    |                  temporary value created here
 ...
-40 |     }                       // (statement 7)
+42 |     }                       // (statement 7)
    |     - temporary value needs to live until here
    |
    = note: consider using a `let` binding to increase its lifetime
 
 error[E0597]: borrowed value does not live long enough
-  --> $DIR/borrowck-let-suggestion-suffixes.rs:45:18
+  --> $DIR/borrowck-let-suggestion-suffixes.rs:47:22
    |
-45 |     v5.push(&'z');
-   |              --- ^ temporary value dropped here while still borrowed
+47 |     v5.push(&id('z'));
+   |              ------- ^ temporary value dropped here while still borrowed
    |              |
    |              temporary value created here
 ...
-52 | }
+54 | }
    | - temporary value needs to live until here
    |
    = note: consider using a `let` binding to increase its lifetime