Lint suggesting use of TryFrom for checked integer conversion

[sfackler]

Checked integer conversion code has historically looked something like this:

if foo <= i32::max_value() as usize {
    thing_a
} else {
    thing_b
}

But now that TryFrom is stable, it can be done in a more foolproof way like this:

match i32::try_from(foo) {
    Ok(foo) => thing_a,
    Err(_) => thing_b,
}

It'd be cool if there was a lint guiding users to migrate their code.

[pJunger]

So now that the simple check is merged, the next steps that I can see would be something like this:

1. Further transformations

   1. Transform simple casts
   2. Transform more involved cases?

2. Detect (probably) incorrect casts like (i < ITYPE::max_value()) && (i >= ITYPE::min_value()) ?

Regarding 1:

Simple transformations:

if (i <= ITYPE::max_value()) && (i >= ITYPE::min_value()) {
    ...(i as ITYPE)... i
}

or a transformation for the current rustfix solution

if ITYPE::try_from(i).is_ok()  {
    ...(i as ITYPE)... i
}

into

if let Some(i_as_ITYPE) = ITYPE::try_from(i) {
    ...(i_as_ITYPE)... i
}

Questions:

• Is there a way to get all usages of i that are in scope?
• How can you emit multiple related findings (that are resolved by rustfix in one step?)

Regarding 2:

Questions:

• Do we want a lint that will emit false positives?
• Or should code like this be transformed into something with more intent like:

match ITYPE::try_from(i) {
    Some(i_as_ITYPE) if i_as_ITYPE != ITYPE::max_value() => ...
    _ => {}

What do you think?