Improve error message for incorrect 'for' input

[martinlindhe]

Hello, with rustc 1.17.0-nightly (8c4f2c6 2017-03-22), the following code

fn main () {
    for i 0..2 {
        println!("{}", i);
    }
}

gives this error message:

error: expected one of `@` or `in`, found `0`
  --> src/disasm/mod.rs:27:15
   |
27 |         for i 0..2 {
   | 

An improved error message could add a hint : did you mean 'for i in 0..2'

[estebank]

Current output:

error: expected one of `@` or `in`, found `0`
 --> src/main.rs:2:11
  |
2 |     for i 0..2 {
  |          -^ unexpected token
  |          |
  |          expected one of `@` or `in` here

How would you want to improve upon this? Adding a suggestion?

error: expected one of `@` or `in`, found `0`
 --> src/main.rs:2:11
  |
2 |     for i 0..2 {
  |          -^ unexpected token
  |          |
  |          expected one of `@` or `in` here
  = note: did you mean to write `in` here?
2 |     for i in 0..2 {
  |           ^^

To do so, we could use the same strategy as in #42578: in parse_for_expr if the In keyword is not found, try to continue building the expression as if the In keyword had been found. If we find further errors, we revert the state of the compiler to right after trying to continue and return the error from expect_keyword. Otherwise we cancel that one and create a new custom diagnostic:

error: missing `in` in `for` loop
 --> src/main.rs:2:11
  |
2 |     for i 0..2 {
  |          ^ expected `in` here
  = note: did you mean to write `in` here?
2 |     for i in 0..2 {
  |           ^^

or just modify the existing error with the proposed span_suggestion (so it looks like the 2 above).

[martinlindhe]

Looks like current output already is an improvement over the reported one, but eithwr 2 or 3 would make it even more obvious i guess. No strong opinions here

[nikomatsakis]

Seems to me that the third error is the most clear ("error: missing in in for loop"); I suspect people using fancy pattern features will figure out what's going on.