clarify promotion-in-consts

Author: RalfJung

promotion.md

@@ -137,24 +137,38 @@ const FOO: &'static i32 = {
 
 However, since this is in explicit const context, we are less strict about
 promotion in this situation: all function calls are promoted, not just
-`#[rustc_promotable]` functions.
-
-Promotion is *not* involved in something like this:
+`#[rustc_promotable]` functions:
 
 ```rust
-const EMPTY_BYTES: &Vec<u8> = &Vec::new();
+const fn bar() -> i32 { 42 }
+
+const FOO: &'static i32 = {
+    let x = &bar(); // this gets promoted
+    x
+};
+```
 
-const NESTED: &'static Vec<u8> = {
-    // This does not work when we have an inner scope:
+However, we still do not promote *everything*; e.g., drop-checking still applies:
+
+```rust
+const DROP: &'static Vec<u8> = {
     let x = &Vec::new(); //~ ERROR: temporary value dropped while borrowed
     x
 };
 ```
 
-In `EMPTY_BYTES`, the reference obtains the lifetime of the "enclosing scope",
-similar to how `let x = &mut x;` creates a reference whose lifetime lasts for
-the enclosing scope. This is decided during MIR building already, and does not
-involve promotion.
+Notice that some code involving `&` *looks* like it relies on promotion but
+actually does not:
+
+```rust
+const EMPTY_BYTES: &Vec<u8> = &Vec::new(); // Ok without promotion
+```
+
+As we have seen above, `Vec::new()` does not get promoted. And yet this
+compiles. Why that? The reason is that the reference obtains the lifetime of
+the "enclosing scope", similar to how `let x = &mut x;` creates a reference
+whose lifetime lasts for the enclosing scope. This is decided during MIR
+building already, and does not involve promotion.
 
 ## Open questions