`.try_into().unwrap()` is suggested even when `.into()` would work

[jonas-schievink]

fn returns_usize() -> usize {
    let x = 0u8;
    x
}

error[E0308]: mismatched types
 --> src/lib.rs:3:5
  |
1 | fn returns_usize() -> usize {
  |                       ----- expected `usize` because of return type
2 |     let x = 0u8;
3 |     x
  |     ^ expected `usize`, found `u8`
  |
help: you can convert an `u8` to `usize` and panic if the converted value wouldn't fit
  |
3 |     x.try_into().unwrap()
  |

It would be better to suggest x.into() instead, which is shorter, cannot fail, and doesn't require importing a trait.

This issue has been assigned to @ryr3 via this comment.

[estebank]

You need to change the following

rust/src/librustc_typeck/check/demand.rs

Lines 755 to 770 in 6dee5f1

	(&ty::Int(ref exp), &ty::Int(ref found)) => {
	let is_fallible = match (found.bit_width(), exp.bit_width()) {
	(Some(found), Some(exp)) if found > exp => true,
	(None, _) | (_, None) => true,
	_ => false,
	};
	suggest_to_change_suffix_or_into(err, is_fallible);
	true
	}
	(&ty::Uint(ref exp), &ty::Uint(ref found)) => {
	let is_fallible = match (found.bit_width(), exp.bit_width()) {
	(Some(found), Some(exp)) if found > exp => true,
	(None, _) | (_, None) => true,
	_ => false,
	};
	suggest_to_change_suffix_or_into(err, is_fallible);

so that when one of the bit_with is None and the other is 8 or 16 bits is_fallible is set to true.

[ryr3]

Hi, this seems like a good first issue. Can I take this up as my first contribution?

[estebank]

@ryr3 sure! Go ahead. Visit the rustc guide if this is your first time compiling rustc.

[Dylan-DPC-zz]

@rustbot assign @ryr3

[LeSeulArtichaut]

so that when one of the bit_with is None and the other is 8 or 16 bits is_fallible is set to true

@estebank Isn't it the opposite? If we try to convert a u8 to a usize, we want is_faillible to be false so we suggest .into() instead of .try_into().unwrap().

[tsandstr]

Yeah. It's the other way around.