Yet another lifetime weirdness (data from `x` flows into `x` here)

help
1

I don't understand the error, is it just me?

these two types are declared with different lifetimes...

But only one is highlighted

...but data from `builder` flows into `builder` here

I don't see the problem, and how can the same variable have different lifetime?

error[E0623]: lifetime mismatch
  --> src/commands.rs:95:30
   |
51 |   builder: &mut FlatBufferBuilder
   |            ----------------------
   |            |
   |            these two types are declared with different lifetimes...
...
95 |           let rows = builder.create_vector(&[row]);
   |                              ^^^^^^^^^^^^^ ...but data from `builder` flows into `builder` here
2

Can you post the code this is from? It's hard to tell what's going on without knowing the context (like, where row is coming from.)

3

it's not easy to fully reproduce, there are a ton of dependencies, I've tried to best to extract

4

FlatBufferBuilder<'fbb> has an 'fbb lifetime, and the & borrow has another one. I think you'll be ok if you introduce a named lifetime 'a and link the lifetime of builder's borrow and and of the underlying FlatBufferBuilder: builder: &'a mut FlatBufferBuilder<'a>.

5

Now the error makes way more sense, thanks!
I should be able to solve the situation with a bit of patience :slight_smile:

error[E0499]: cannot borrow `*builder` as mutable more than once at a time
  --> src/commands.rs:91:33
   |
46 | pub fn exec<'a>(
   |             -- lifetime `'a` defined here
...
91 |         while iterator.has_next(builder) {
   |                                 ^^^^^^^ mutable borrow starts here in previous iteration of loop
...
95 |           let rows = builder.create_vector(&[row]);
   |                                            ------ cast requires that `*builder` is borrowed for `'a`
6

I don't have much time to look at this myself right now, but that's a rather surprising suggestion! Normally &'a mut Foo<'a> is a bad idea, because the invariance of &mut forces the value to be mutably borrowed for the rest of its existence.

7

I believe that when that happens rustc is way better at guiding the user in the right direction than in the original situation.