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### What it does
Checks for arguments that are only used in recursion with no side-effects.
### Why is this bad?
It could contain a useless calculation and can make function simpler.
The arguments can be involved in calculations and assignments but as long as
the calculations have no side-effects (function calls or mutating dereference)
and the assigned variables are also only in recursion, it is useless.
### Known problems
Too many code paths in the linting code are currently untested and prone to produce false
positives or are prone to have performance implications.
In some cases, this would not catch all useless arguments.
```
fn foo(a: usize, b: usize) -> usize {
let f = |x| x + 1;
if a == 0 {
1
} else {
foo(a - 1, f(b))
}
}
```
For example, the argument `b` is only used in recursion, but the lint would not catch it.
List of some examples that can not be caught:
- binary operation of non-primitive types
- closure usage
- some `break` relative operations
- struct pattern binding
Also, when you recurse the function name with path segments, it is not possible to detect.
### Example
```
fn f(a: usize, b: usize) -> usize {
if a == 0 {
1
} else {
f(a - 1, b + 1)
}
}
```
Use instead:
```
fn f(a: usize) -> usize {
if a == 0 {
1
} else {
f(a - 1)
}
}
```
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