clippy::unnecessary_fallible_conversions: use of a fallible conversion when an infallible one could be used

What you see

warning: use of a fallible conversion when an infallible one could be used
 --> main.rs:3:30
  |
3 | let _: Result<i64, _> = 1i32.try_into();
  |                              ^^^^^^^^
  |
  = note: converting `i32` to `i64` cannot fail
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unnecessary_fallible_conversions
  = note: `#[warn(clippy::unnecessary_fallible_conversions)]` on by default
help: use
  |
3 - let _: Result<i64, _> = 1i32.try_into();
3 + let _: Result<i64, _> = 1i32.into();
  |

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

In those cases, the TryInto and TryFrom trait implementation is a blanket impl that forwards to Into or From, which always succeeds. The returned Result<_, Infallible> requires error handling to get the contained value even though the conversion can never fail.

Code that works but is not idiomatic Rust. Warns by default.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

let _: Result<i64, _> = 1i32.try_into();
let _: Result<i64, _> = <_>::try_from(1i32);

Write this instead

let _: i64 = 1i32.into();
let _: i64 = <_>::from(1i32);

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

#[allow(clippy::unnecessary_fallible_conversions)]
fn my_function() { /* ... */ }

[lints.clippy]
unnecessary_fallible_conversions = "allow"

cargo clippy -- -A clippy::unnecessary-fallible-conversions

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.