Non-lexical borrow scopes and better treatment of nested method calls

[pnkfelix]

In general, it can be useful to have support for borrows that aren't tied to lexical scopes.

Related links:

• Nested method calls with &mut receivers result in borrowck errors (Nested method calls with &mut receivers result in borrowck errors rust#6268)
  • Internals thread: "Accepting nested method calls with an &mut self receiver"
• Borrow scopes should not always be lexical (borrow scopes should not always be lexical rust#6393)
• Single-entry, multiple-exit regions for borrows (RFC: Single-entry / multiple-exit regions for borrows #396)

UPDATE: The relevant RFCs have been accepted. See rust-lang/rust#43234 to track implementation progress here.

[barosl]

@mike-marcacci pointed out that the single-entry, multiple-exit regions issue also applies to the if let struct. While a code like

if let Some(val) = map.get_mut(&id) {
   /* ... */
} else {
   map.insert(id, new_val);
}

seems "innocent", it is actually just a syntax sugar for the match block, so the borrow lasts until the end of the else block. This particular case could be improved, or at least diagnostics could be, but I'm not sure fixing the specific point without introducing a general solution is a good idea.

[flavius]

What's the state of this feature? Any new plans and/or changes with this?

[steveklabnik]

@flavius basically, the HIR/MIR work needs to be completed first.