Type can no longer be inferred in 1.49

[Skgland]

Code

I tried this code: Johni0702/rtp on the dtls-srtp Branch

I expected to see this happen: Compiles successfully as in 1.48

Instead, this happened: Error E0282 in src/rfc3711.rs

error[E0282]: type annotations needed
   --> src/rfc3711.rs:477:19
    |
476 |         let iv = iv ^ (BigUint::from(1_u8) << (context.session_encr_key.len() * 8));
    |             -- consider giving `iv` a type
477 |         let iv = &iv.to_bytes_be()[1..context.session_encr_key.len() + 1];
    |                   ^^ cannot infer type
    |
    = note: type must be known at this point

error[E0282]: type annotations needed
   --> src/rfc3711.rs:511:19
    |
510 |         let iv = iv ^ (BigUint::from(1_u8) << (context.session_encr_key.len() * 8));
    |             -- consider giving `iv` a type
511 |         let iv = &iv.to_bytes_be()[1..context.session_encr_key.len() + 1];
    |                   ^^ cannot infer type
    |
    = note: type must be known at this point

error: aborting due to 2 previous errors

Version it worked on

It most recently worked on: 1.48

Version with regression

rustc --version --verbose:

rustc 1.49.0 (e1884a8e3 2020-12-29)
binary: rustc
commit-hash: e1884a8e3c3e813aada8254edfa120e85bf5ffca
commit-date: 2020-12-29
host: x86_64-unknown-linux-gnu
release: 1.49.0

I tried to minimize the code necessary for reproduction and I got it down to:

//file: src/lib.rs
use num::BigUint; // Cargo.toml: num = "0.1"

pub trait Protocol {
    type PacketIndex: Into<u64> + Into<BigUint>;
}

fn from_nothing<T>() -> T {
    todo!()
}

pub fn decrypt_portion<P: Protocol>(index: P::PacketIndex) {
    let iv: BigUint = from_nothing();
    let len: usize = from_nothing();
    let iv = iv ^ (index.into() << 16);
    let iv = iv ^ (BigUint::from(1_u8) << (len * 8));
    let _iv: &[u8] = &iv.to_bytes_be()[1..len + 1];
}

[Skgland]

Based on that it worked on stable 1.48 and now doesn't on 1.49 I assume this is a stable to stable regression.

@rustbot modify labels: +regression-from-stable-to-stable-regression-untriaged

[camelid]

Minimized further:

use num::BigUint; // Cargo.toml: num = "0.1"

pub trait Protocol: Sized + Default {
    type PacketIndex: Into<u64> + Into<BigUint>;
}

pub fn decrypt_portion<P: Protocol>(index: P::PacketIndex) {
    let iv: BigUint = loop{};
    let len: usize = loop{};
    let iv = iv ^ (index.into() << 16);
    let iv = iv ^ (BigUint::from(1_u8) << (len * 8));
    let _iv: &[u8] = &iv.to_bytes_be()[1..len + 1];
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
warning: unreachable statement
 --> src/lib.rs:9:5
  |
8 |     let iv: BigUint = todo!();
  |                       ------- any code following this expression is unreachable
9 |     let len: usize = todo!();
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable statement
  |
  = note: `#[warn(unreachable_code)]` on by default

error[E0282]: type annotations needed
  --> src/lib.rs:12:23
   |
11 |     let iv = iv ^ (BigUint::from(1_u8) << (len * 8));
   |         -- consider giving `iv` a type
12 |     let _iv: &[u8] = &iv.to_bytes_be()[1..len + 1];
   |                       ^^ cannot infer type
   |
   = note: type must be known at this point

error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0282`.
error: could not compile `playground`

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

[Skgland]

Reduced the code to not use external BigUint type:

use std::ops::{BitXor, Shl};

pub trait Protocol {
    type PacketIndex: Into<u64> + Into<BigUint>;
}

fn from_nothing<T>() -> T {
    todo!()
}

pub fn decrypt_portion<P: Protocol>(index: P::PacketIndex) {
    let iv: BigUint = from_nothing();
    let len: usize = from_nothing();
    let iv = iv ^ (index.into() << 16);
    let iv = iv ^ (BigUint::from(1_u8) << (len * 8));
    let _iv: &[u8] = &iv.to_bytes_be()[1..len + 1];
}

pub struct BigUint;

impl BigUint {
    fn to_bytes_be(&self) -> Vec<u8> {
        todo!()
    }
}

impl From<u8> for BigUint {
    fn from(_: u8) -> Self {
        unimplemented!()
    }
}

impl Shl<usize> for BigUint {
    type Output = BigUint;

    fn shl(self, _rhs: usize) -> Self::Output {
        unimplemented!()
    }
}

impl BitXor for BigUint {
    type Output = BigUint;

    fn bitxor(self, _rhs: Self) -> Self::Output {
        unimplemented!()
    }
}

impl<'a> BitXor<&'a BigUint> for BigUint {
    type Output = BigUint;

    fn bitxor(self, _rhs: &'a BigUint) -> Self::Output {
        unimplemented!()
    }
}

[ojeda]

pub struct S;

pub trait P {
    type I: Into<u64> + Into<S>;
}

trait A<B> {
    fn m(self, _: B) -> S;
}

impl A<S> for S {
    fn m(self, _: S) -> S {
        todo!()
    }
}

impl A<&'static S> for S {
    fn m(self, _: &'static S) -> S {
        todo!()
    }
}

pub fn f<T: P>(i: T::I) {
    S.m(i.into());
}

[Skgland]

Could this be related to #80816?

[Skgland]

searched nightlies: from nightly-2020-10-01 to nightly-2020-11-14
regressed nightly: nightly-2020-10-07
searched commits: from a1dfd24 to 98edd1f
regressed commit: 08e2d46 (#73905)

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --preserve --start=2020-10-01 --end=2020-11-14 -- check 

Using code:

pub struct S;

pub trait P {
    type I: Into<u64> + Into<S>;
}

trait A<B> {
    fn m(self, _: B) -> S;
}

impl A<S> for S {
    fn m(self, _: S) -> S {
        todo!()
    }
}

impl A<&'static S> for S {
    fn m(self, _: &'static S) -> S {
        todo!()
    }
}

pub fn f<T: P>(i: T::I) {
    let _ = S.m(i.into());
}

[mbartlett21]

An alternate reduction:

use core::ops::Shl;

struct A;

impl A {
    fn a(&self) {}
}

impl Shl<i8> for A {
    type Output = A;
    
    fn shl(self, _: i8) -> A {
        self
    }
}

impl Shl<i32> for A {
    type Output = A;
    
    fn shl(self, _: i32) -> A {
        self
    }
}

pub fn decrypt_portion_no_worky() {
    let iv = A << 16;
    iv.a();
}

pub fn decrypt_portion_works() {
    let iv = A << 16;
    // Calling no methods works?!
}

[Skgland]

@mbartlett21 that does not appear to be a valid reduction as it does not work on 1.48 for me

[Skgland]

use std::ops::BitXor;

pub struct S;

pub trait P {
    type I: Into<u64> + Into<S>;
}

pub fn decrypt_portion<T: P>(index: T::I) {
    let iv = S ^ index.into();
    &iv.to_bytes_be();
}

impl S {
    fn to_bytes_be(&self) -> &[u8] {
        unimplemented!()
    }
}

impl BitXor for S {
    type Output = S;

    fn bitxor(self, _rhs: Self) -> Self::Output {
        unimplemented!()
    }
}

impl<'a> BitXor<&'a S> for S {
    type Output = S;

    fn bitxor(self, _rhs: &'a S) -> Self::Output {
        unimplemented!()
    }
}

this reduction works for me and without changing the error message as with ojedas reduction

[apiraino]

Assigning P-high as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

[oli-obk]

duplicate of #80816 but keeping open so I make sure to test both repros

[pnkfelix]

Visiting for P-high review.

After some investigation, I do not think this is quite a duplicate of #80816. Or at least, the examples in #80816 are all now spitting out errors about mulitiple applicable impls. No such message is being emitted here.