Assert that non-extended temporaries and super let bindings have scopes

[dianne]

This PR clarifies a point of confusion in the compiler: all bodies have an outer temporary drop scope, including static and const item bodies¹. Whenever a temporary should be dropped in its enclosing temporary scope, it should have a temporary scope to be dropped in so that its drop can be scheduled². As such, I've updated some relevant comments and made ScopeTree::default_temporary_scope and RvalueScopes::temporary_scope panic when an enclosing temporary scope isn't found instead of allowing potential bugs where potentially-drop-sensitive temporaries are effectively given static lifetimes.

Since non-extended super let bindings are dropped in their block's enclosing temporary scope, this applies to them as well: the enclosing temporary scope should exist.

Footnotes

1. See https://github.com/rust-lang/rust/blob/master/compiler/rustc_hir_analysis/src/check/region.rs#L773-L778 for non-fn/closure bodies. The this.cx.var_parent = None; enables lifetime extension to 'static lifetimes and the ScopeData::Destruction scope ensures non-extended temporaries are dropped in the body expression's scope. ↩

2. For certain borrowed temporaries, drops that don't require running destructors may later be removed by constant promotion. That is unrelated to this PR. ↩

[rustbot]

r? @nnethercote

rustbot has assigned @nnethercote.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[dianne]

I can't tell if the remarks on on Drop restrictions are out of date or just worded a bit confusingly. I'm also not sure whether this PR is the right place to update that.

[theemathas]

The only way a temporary should be extended to have a 'static lifetime is through lifetime extension

I'm not sure what this means. But I just wanted to note that constant-promotion in consts/statics can have a similar effect to lifetime extension, and has different behavior from constant-promotion outside of consts/statics. See #124328

[dianne]

I've clarified the PR description; thanks. Does it read better now? This is only about the temporary scopes assigned to expressions before PromoteTemps runs and is of most concern for non-promotable expressions (e.g. those that produce drop-sensitive temporaries).