new lint: unnecessary_fallible_conversions

[y21]

Closes #11577

A new lint that looks for calls such as i64::try_from(1i32) and suggests i64::from(1i32). See lint description (and linked issue) for more details for why.

There's a tiny bit of overlap with the useless_conversion lint, in that the other one warns T::try_from(T) (i.e., fallibly converting to the same type), so this lint ignores cases like i32::try_from(1i32) to avoid emitting two warnings for the same expression.

Also, funnily enough, with this one exception, this lint would warn on exactly every case in the useless_conversion_try ui test that useless_conversion didn't cover (but never two warnings at the same time), which is neat. I did add an #![allow] though since we don't want interleaved warnings from multiple lints in the same uitest.

changelog: new lint: unnecessary_fallible_conversions

[rustbot]

r? @Jarcho

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[sgued]

Thanks for the implementation of my suggested lint!

[bors]

☔ The latest upstream changes (presumably #11698) made this pull request unmergeable. Please resolve the merge conflicts.

[Jarcho]

Looks good overall. Just a couple of small nits. If you don't feel like rewording things it's workable the way it is.

[Jarcho]

Thanks for the small cleanup. @bors r+

[bors]

📌 Commit 69c3b9c has been approved by Jarcho

It is now in the queue for this repository.

[Jarcho]

@bors r-

[Jarcho]

@bors r+

[bors]

📌 Commit 69c3b9c has been approved by Jarcho

It is now in the queue for this repository.

[bors]

⌛ Testing commit 69c3b9c with merge 7d34406...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Jarcho
Pushing 7d34406 to master...

[alexanderkjeldaas]

Just a maybe unrelated comment on this, but would a lint that finds unnecessary TryFrom implementations that instead could be just From be implemented alongside this one?

impl TryFrom<X> for Y {
    type Error = <Foo as FromStr>::Err;
    fn try_from(x: X) -> Result<Self, Self::Error> {
        Ok(Self {
            height: x.height.0.as_u64(),
            hash: x.hash.0,
        })
    }
}

Easy to detect when the return value is always Ok?

[flip1995]

Looks like a possible lint. If you want this, please open an issue or if you want to get into Clippy development consider opening a PR for it :)