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);
+}
+```