Rust `if let` 语句中的双重可变借用

Question

Why does the following code coed in Rust yell a double mutable borrow error? I would expect the first scope to be the first block since the returned values can't outlive this block, and the else if won't be called if the first if returned Some(arr) .

                if let Some(arr) = v.as_array_mut(){
                    ...

                } else if let Some(obj) = v.as_object_mut(){
                    ...
                } else {
                    ...
                }

    |
178 |                 if let Some(arr) = v.as_array_mut(){
    |                                    -------------------------- first mutable borrow occurs here
...
195 |                 } else if let Some(obj) = v.as_object_mut(){
    |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                                           |
    |                                           second mutable borrow occurs here
    |                                           first borrow later used here

BTW, breaking each call to two calls, which provides an exact similar behavior obviously doesn't yell an error:

                if v.is_array(){
                    
                    let arr = v.as_array_mut().unwrap();

                    ...

                } else if v.is_object(){
                    let obj = v.as_object_mut().unwrap();

                    ...
                } else {
                    ...
                }

Answer 1

This has to do with non-lexical lifetimes .

Extend Rust's borrow system to support non-lexical lifetimes -- these are lifetimes that are based on the control-flow graph, rather than lexical scopes. The RFC describes in detail how to infer these new, more flexible regions, and also describes how to adjust our error messages. The RFC also describes a few other extensions to the borrow checker, the total effect of which is to eliminate many common cases where small, function-local code modifications would be required to pass the borrow check. (The appendix describes some of the remaining borrow-checker limitations that are not addressed by this RFC.)

1.36 has NLL turned on for the 2015 edition. The 2018 edition enables them by default.

So in your case, I guess you are on a version not supporting NLL.