# rusttest

**Repository Path**: mirrors_jmacd/rusttest

## Basic Information

- **Project Name**: rusttest
- **Description**: No description available
- **Primary Language**: Unknown
- **License**: Not specified
- **Default Branch**: main
- **Homepage**: None
- **GVP Project**: No

## Statistics

- **Stars**: 0
- **Forks**: 0
- **Created**: 2024-09-11
- **Last Updated**: 2026-01-18

## Categories & Tags

**Categories**: Uncategorized

**Tags**: None

## README

Rust learner asks for help!

## Problem

In [src/main.rs](./src/main.rs), see a mock-up of the problem I ran into (from https://github.com/jmacd/duckpond).

The types are renamed for clarity.

- *System* represents a file system, representing the global state and containing a root directory. The program is single-threaded and control-flow always has an associated `&mut System`.
- *Folder* is a trait for directories, real or dynamic, with a `subdir()` method
- *Realdir* is a real directory
- *Dynadir* is a dynamic directory.
- *WD* is a working directory a single handle to a pair of `&mut System` and `Rc<RefCell<dyn Folder>>` with a `lookup` method

The problem is that I do not understand how to return a `Rc<RefCell<dyn Folder>>` from `subdir()` with proper lifetime annotations (with or without `'_`, as suggested by the compiler, or a variety of other rearrangements I've tried).

The error is:

```rust
error: lifetime may not live long enough
  --> src/main.rs:43:9
   |
42 |     fn subdir(&mut self, sys: &mut System, _name: &str) -> Option<Rc<RefCell<dyn Folder>>> {
   |                               - let's call the lifetime of this reference `'1`
43 |         Some(sys.wd().lookup(Path::new("d/e/f")))
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'static`
   |
   = note: requirement occurs because of the type `RefCell<dyn Folder>`, which makes the generic argument `dyn Folder` invariant
   = note: the struct `RefCell<T>` is invariant over the parameter `T`
   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
help: to declare that the trait object captures data from argument `sys`, you can add an explicit `'_` lifetime bound
   |
42 |     fn subdir(&mut self, sys: &mut System, _name: &str) -> Option<Rc<RefCell<dyn Folder + '_>>> {
   |                                                                                         ++++

```


```
    fn lookup<'b>(&'b mut self, path: &'_ Path) -> Option<WD<'a>>
    where
        'a: 'b,
```

gives
```
error: lifetime may not live long enough
   --> src/main.rs:101:13
    |
82  | impl<'a> WD<'a> {
    |      -- lifetime `'a` defined here
...
86  |     fn lookup<'b>(&'b mut self, path: &'_ Path) -> Option<WD<'a>>
    |               -- lifetime `'b` defined here
...
101 |             Some(child)
    |             ^^^^^^^^^^^ method was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
    |
    = help: consider adding the following bound: `'b: 'a`
    = note: requirement occurs because of the type `WD<'_>`, which makes the generic argument `'_` invariant
    = note: the struct `WD<'a>` is invariant over the parameter `'a`
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance
```

```
    fn lookup<'b>(&'b mut self, path: &'_ Path) -> Option<WD<'a>>
    where
        'b: 'a,
```

gives e.g.,

```
error[E0515]: cannot return value referencing temporary value
  --> src/main.rs:46:9
   |
46 |         sys.wd().lookup(Path::new("d/e/f")).unwrap()
   |         --------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |         |
   |         returns a value referencing data owned by the current function
   |         temporary value created here

```

(and didn't seem right)

lastly

```
    fn lookup(&'_ mut self, path: &'_ Path) -> Option<WD<'a>> {
```

```
error: lifetime may not live long enough
  --> src/main.rs:95:13
   |
82 | impl<'a> WD<'a> {
   |      -- lifetime `'a` defined here
83 |     fn lookup(&'_ mut self, path: &'_ Path) -> Option<WD<'a>> {
   |               - let's call the lifetime of this reference `'1`
...
95 |             Some(child)
   |             ^^^^^^^^^^^ method was supposed to return data with lifetime `'a` but it is returning data with lifetime `'1`
   |
```