Suggest try_into when possible

[estebank]

CC #47168

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[oli-obk]

I wonder if we should change the mismathced types root message to expected i16, found i8 so the suggestion can be inlined properly

[estebank]

It took me a second to understand but now I see what you mean... That's probably something worthy of doing in a separate PR. The actual code change should be smallish, but the effect in stderr files will probably be extensive.

[estebank]

r? @oli-obk

[oli-obk]

for now ok, but we should try to not suggest code that is known to panic

[estebank]

This is indeed a bad corner case :-/

[estebank]

Filed #60384

[oli-obk]

how does this code interact with constants? Do we have some tests for that? For now we should not be suggesting that const FOO: u32 = bar(); should use try_from, or from irrelevant of the return type of bar

[estebank]

how does this code interact with constants?

I'm checking but, I think we have any guards against that at the moment, given this current example where rustc gives suggestions in expr context, but not const context.

---

It does the right thing:

const C: usize = 32;
const D: i32 = 1usize;
const E: i32 = C;
const G: i8 = 1usize;
const H: i8 = C;

fn main() {
    let c: i32 = 1i8;
    let x: u8 = C;
    let x: u8 = D;
    let x: u8 = E;
}

error[E0308]: mismatched types
 --> file.rs:2:16
  |
2 | const D: i32 = 1usize;
  |                ^^^^^^ expected i32, found usize

error[E0308]: mismatched types
 --> file.rs:3:16
  |
3 | const E: i32 = C;
  |                ^ expected i32, found usize

error[E0308]: mismatched types
 --> file.rs:4:15
  |
4 | const G: i8 = 1usize;
  |               ^^^^^^ expected i8, found usize

error[E0308]: mismatched types
 --> file.rs:5:15
  |
5 | const H: i8 = C;
  |               ^ expected i8, found usize

error[E0308]: mismatched types
 --> file.rs:8:18
  |
8 |     let c: i32 = 1i8;
  |                  ^^^ expected i32, found i8
help: change the type of the numeric literal from `i8` to `i32`
  |
8 |     let c: i32 = 1i32;
  |                  ^^^^

error[E0308]: mismatched types
 --> file.rs:9:17
  |
9 |     let x: u8 = C;
  |                 ^ expected u8, found usize
help: you can convert an `usize` to `u8` or panic if it the converted value wouldn't fit
  |
9 |     let x: u8 = C.try_into().unwrap();
  |                 ^^^^^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
  --> file.rs:10:17
   |
10 |     let x: u8 = D;
   |                 ^ expected u8, found i32
help: you can convert an `i32` to `u8` or panic if it the converted value wouldn't fit
   |
10 |     let x: u8 = D.try_into().unwrap();
   |                 ^^^^^^^^^^^^^^^^^^^^^

error[E0308]: mismatched types
  --> file.rs:11:17
   |
11 |     let x: u8 = E;
   |                 ^ expected u8, found i32
help: you can convert an `i32` to `u8` or panic if it the converted value wouldn't fit
   |
11 |     let x: u8 = E.try_into().unwrap();
   |                 ^^^^^^^^^^^^^^^^^^^^^

[oli-obk]

You'll also need to extend that check to know about const fn: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=03af30e0c23632bd5b03bcde9385b116

[estebank]

@oli-obk oh :(

[estebank]

@oli-obk const fns are now accounted for.

[oli-obk]

@bors r+

[bors]

📌 Commit 31eb5cc has been approved by oli-obk

[bors]

⌛ Testing commit 31eb5cc with merge 862a878...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: oli-obk
Pushing 862a878 to master...