Rollup merge of #56127 - rust-lang:oli-obk-patch-1, r=nikomatsakis

Update an outdated comment in mir building

r? @eddyb

1 files changed, 9 insertions, 3 deletions

diff --git a/src/librustc_mir/build/expr/as_temp.rs b/src/librustc_mir/build/expr/as_temp.rs
index 8f50a1e9a21..2db9fb9cb99 100644
--- a/src/librustc_mir/build/expr/as_temp.rs
+++ b/src/librustc_mir/build/expr/as_temp.rs
@@ -85,9 +85,15 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
 
         unpack!(block = this.into(&Place::Local(temp), block, expr));
 
-        // In constants, temp_lifetime is None. We should not need to drop
-        // anything because no values with a destructor can be created in
-        // a constant at this time, even if the type may need dropping.
+        // In constants, temp_lifetime is None for temporaries that live for the
+        // 'static lifetime. Thus we do not drop these temporaries and simply leak them.
+        // This is equivalent to what `let x = &foo();` does in functions. The temporary
+        // is lifted to their surrounding scope. In a function that means the temporary lives
+        // until just before the function returns. In constants that means it outlives the
+        // constant's initialization value computation. Anything outliving a constant
+        // must have the `'static` lifetime and live forever.
+        // Anything with a shorter lifetime (e.g the `&foo()` in `bar(&foo())` or anything
+        // within a block will keep the regular drops just like runtime code.
         if let Some(temp_lifetime) = temp_lifetime {
             this.schedule_drop_storage_and_value(
                 expr_span,