Wrong error message for missed type inference

[leonardo-m]

Code that asks for some type inference:

fn main() {
    let n: u32 = 1;
    let mut d: u64 = 2;
    d = d % n.into();
}

It gives:

error[E0284]: type annotations needed for `u64`
 --> ...\test.rs:4:11
  |
3 |     let mut d: u64 = 2;
  |         ----- consider giving `d` a type
4 |     d = d % n.into();
  |           ^ cannot infer type for type `u64`
  |
  = note: cannot satisfy `<u64 as std::ops::Rem<_>>::Output == u64`

The error message "consider giving d a type" refers to a "d" that is actually typed as u64.
Using rustc 1.44.0-nightly (3360cc3 2020-04-24).

[cuviper]

This is similar to my #71538, although your example is much simpler.

[estebank]

CC #70082 #69683 #69455 #69214

[jonaskoelker]

I've seen the same(ish) bug on stable rustc 1.43.0 (4fb7144ed 2020-04-20), so the fault is apparently not a regression from my stable 1.43 to OP's nightly 1.44. I'm using this code:

fn main() {
    if true {
        let f = |number: usize| number;
    } else {
        let confusion: usize = 0usize + 0usize.into();
    }
}

fn main2() {
    let f = |number: usize| number;
    let confusion: usize = 0usize + 0usize.into();
}

fn main3() {
    let x: usize = 0usize + 0usize.into();
}

The errors are

-*- mode: compilation; default-directory: "~/src/rustc-misleading-error-message/src/" -*-
Compilation started at Wed Apr 29 00:02:23

cargo build --bin bug
   Compiling misleading v0.1.0 (/home/jonas/src/rustc-misleading-error-message)
error[E0284]: type annotations needed for `usize`
 --> src/main.rs:3:18
  |
3 |         let f = |number: usize| number;
  |                  ^^^^^^ consider giving this closure parameter a type
  |
  = note: cannot resolve `<usize as std::ops::Add<_>>::Output == usize`

error[E0284]: type annotations needed for `usize`
  --> src/main.rs:10:14
   |
10 |     let f = |number: usize| number;
   |              ^^^^^^ consider giving this closure parameter a type
   |
   = note: cannot resolve `<usize as std::ops::Add<_>>::Output == usize`

error[E0284]: type annotations needed for `usize`
  --> src/main.rs:15:27
   |
15 |     let x: usize = 0usize + 0usize.into();
   |         -                 ^ cannot infer type for type `usize`
   |         |
   |         consider giving `x` a type
   |
   = note: cannot resolve `<usize as std::ops::Add<_>>::Output == usize`

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0284`.
error: could not compile `misleading`.

To learn more, run the command again with --verbose.

Compilation exited abnormally with code 101 at Wed Apr 29 00:02:23

(copy-pasted from an emacs compilation buffer)

It amuses me that |x: t| x "needs" a t type annotation thanks to code inside a never-taken branch. One would think that deleting dead code is a behavior-preserving transformation, at least when no error messages are given about the dead code :)