Regression: "temporary value does not live long enough" in a `static` item in Servo

[SimonSapin]

Reduced test case:

#![feature(const_fn)]
const fn require_static<T>(_: &'static T) {}
const fn foo() -> u32 { 0 }
pub static A: () = require_static(&foo());

rustc 1.31.0-nightly (5597ee8 2018-10-03): OK

rustc 1.31.0-nightly (8c4ad4e 2018-10-04):

error[E0597]: borrowed value does not live long enough
 --> a.rs:4:36
  |
4 | pub static A: () = require_static(&foo());
  |                                    ^^^^^- temporary value only lives until here
  |                                    |
  |                                    temporary value does not live long enough
  |
  = note: borrowed value must be valid for the static lifetime...

error: aborting due to previous error

Commit range: 5597ee8...8c4ad4e. #53851 seems relevant. @oli-obk, should we use #[rustc_promotable] in unstable code, despite the rustc_ prefix in its name?

[SimonSapin]

Hmm, this diff does not seem to have any effect on the 275 occurrences of this error in Servo, am I missing something:

diff --git a/components/script/dom/bindings/interface.rs b/components/script/dom/bindings/interface.rs
index 210e9ae557..7632d341b1 100644
--- a/components/script/dom/bindings/interface.rs
+++ b/components/script/dom/bindings/interface.rs
@@ -88,6 +88,7 @@ pub struct InterfaceConstructorBehavior(ClassOps);
 
 impl InterfaceConstructorBehavior {
     /// An interface constructor that unconditionally throws a type error.
+    #[rustc_promotable]
     pub const fn throw() -> Self {
         InterfaceConstructorBehavior(ClassOps {
             addProperty: None,
@@ -105,6 +106,7 @@ impl InterfaceConstructorBehavior {
     }
 
     /// An interface constructor that calls a native Rust function.
+    #[rustc_promotable]
     pub const fn call(hook: ConstructorClassHook) -> Self {
         InterfaceConstructorBehavior(ClassOps {
             addProperty: None,
diff --git a/components/script/lib.rs b/components/script/lib.rs
index 097f69d780..a5355a3937 100644
--- a/components/script/lib.rs
+++ b/components/script/lib.rs
@@ -7,6 +7,7 @@
 #![feature(const_fn)]
 #![feature(drain_filter)]
 #![feature(plugin)]
+#![feature(rustc_attrs)]
 #![feature(try_from)]
 #![deny(unsafe_code)]
 #![allow(non_snake_case)]

Sample error in non-reduced code:

error[E0597]: borrowed value does not live long enough
    --> /home/simon/servo2/target/debug/build/script-1105d98fbcf10485/out/Bindings/AnalyserNodeBinding.rs:1522:10
     |
1522 |         &InterfaceConstructorBehavior::call(_constructor),
     |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ temporary value does not live long enough
...
1525 |         2);
     |          - temporary value only lives until here
     |
     = note: borrowed value must be valid for the static lifetime...

[oli-obk]

hmm... I thought const_fn was overriding any rules. And... I just realized that's only true for nll ;) So add #![feature(nll)]

Or you can always use an intermediate constant.

#[rustc_promotable] is expected to stay unstable forever, but you can use it if you want. You're on your own with guarantees though.

[oli-obk]

I can hack the const_fn workaround into old borrowck if you really need it

[SimonSapin]

Or you can always use an intermediate constant.

I’d forgotten this was an option! It works for Servo here. However it’s not really discoverable…