Type inference(?) regression, rand fails to build on nightly 2016-01-04

[SimonSapin]

The rand crate (rust-random/rand@4aa8f5b) builds correctly with rustc 1.7.0-nightly (8f11a9e 2016-01-03), but fails with rustc 1.7.0-nightly (d5e2290 2016-01-04):

% cargo build
   Compiling winapi-build v0.1.1
   Compiling libc v0.2.4
   Compiling winapi v0.2.5
   Compiling advapi32-sys v0.1.2
   Compiling rand v0.3.12 (file:///home/simon/projects/servo/deps/rand)
src/isaac.rs:134:37: 134:48 error: the type of this value must be known in this context
src/isaac.rs:134             ($x:expr) => ( self.mem[($x >> 2).0 as usize & (RAND_SIZE_USIZE - 1)] )
                                                     ^~~~~~~~~~~
src/isaac.rs:147:29: 147:36 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:171:17: 171:38 note: in this expansion of rngstepp! (defined in src/isaac.rs)
src/isaac.rs:134:37: 134:48 error: the type of this value must be known in this context
src/isaac.rs:134             ($x:expr) => ( self.mem[($x >> 2).0 as usize & (RAND_SIZE_USIZE - 1)] )
                                                     ^~~~~~~~~~~
src/isaac.rs:150:25: 150:49 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:171:17: 171:38 note: in this expansion of rngstepp! (defined in src/isaac.rs)
src/isaac.rs:134:37: 134:48 error: the type of this value must be known in this context
src/isaac.rs:134             ($x:expr) => ( self.mem[($x >> 2).0 as usize & (RAND_SIZE_USIZE - 1)] )
                                                     ^~~~~~~~~~~
src/isaac.rs:162:29: 162:36 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:172:17: 172:37 note: in this expansion of rngstepn! (defined in src/isaac.rs)
src/isaac.rs:134:37: 134:48 error: the type of this value must be known in this context
src/isaac.rs:134             ($x:expr) => ( self.mem[($x >> 2).0 as usize & (RAND_SIZE_USIZE - 1)] )
                                                     ^~~~~~~~~~~
src/isaac.rs:165:25: 165:49 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:172:17: 172:37 note: in this expansion of rngstepn! (defined in src/isaac.rs)
src/isaac.rs:134:37: 134:48 error: the type of this value must be known in this context
src/isaac.rs:134             ($x:expr) => ( self.mem[($x >> 2).0 as usize & (RAND_SIZE_USIZE - 1)] )
                                                     ^~~~~~~~~~~
src/isaac.rs:147:29: 147:36 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:173:17: 173:37 note: in this expansion of rngstepp! (defined in src/isaac.rs)
src/isaac.rs:134:37: 134:48 error: the type of this value must be known in this context
src/isaac.rs:134             ($x:expr) => ( self.mem[($x >> 2).0 as usize & (RAND_SIZE_USIZE - 1)] )
                                                     ^~~~~~~~~~~
src/isaac.rs:150:25: 150:49 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:173:17: 173:37 note: in this expansion of rngstepp! (defined in src/isaac.rs)
src/isaac.rs:134:37: 134:48 error: the type of this value must be known in this context
src/isaac.rs:134             ($x:expr) => ( self.mem[($x >> 2).0 as usize & (RAND_SIZE_USIZE - 1)] )
                                                     ^~~~~~~~~~~
src/isaac.rs:162:29: 162:36 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:174:17: 174:38 note: in this expansion of rngstepn! (defined in src/isaac.rs)
src/isaac.rs:134:37: 134:48 error: the type of this value must be known in this context
src/isaac.rs:134             ($x:expr) => ( self.mem[($x >> 2).0 as usize & (RAND_SIZE_USIZE - 1)] )
                                                     ^~~~~~~~~~~
src/isaac.rs:165:25: 165:49 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:174:17: 174:38 note: in this expansion of rngstepn! (defined in src/isaac.rs)
src/isaac.rs:373:42: 373:53 error: the type of this value must be known in this context
src/isaac.rs:373                 *self.mem.get_unchecked((($x >> 3).0 as usize) & (RAND_SIZE_64 - 1))
                                                          ^~~~~~~~~~~
src/isaac.rs:389:37: 389:44 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:416:17: 416:34 note: in this expansion of rngstepp! (defined in src/isaac.rs)
src/isaac.rs:373:42: 373:53 error: the type of this value must be known in this context
src/isaac.rs:373                 *self.mem.get_unchecked((($x >> 3).0 as usize) & (RAND_SIZE_64 - 1))
                                                          ^~~~~~~~~~~
src/isaac.rs:392:33: 392:60 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:416:17: 416:34 note: in this expansion of rngstepp! (defined in src/isaac.rs)
src/isaac.rs:373:42: 373:53 error: the type of this value must be known in this context
src/isaac.rs:373                 *self.mem.get_unchecked((($x >> 3).0 as usize) & (RAND_SIZE_64 - 1))
                                                          ^~~~~~~~~~~
src/isaac.rs:407:37: 407:44 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:417:17: 417:33 note: in this expansion of rngstepn! (defined in src/isaac.rs)
src/isaac.rs:373:42: 373:53 error: the type of this value must be known in this context
src/isaac.rs:373                 *self.mem.get_unchecked((($x >> 3).0 as usize) & (RAND_SIZE_64 - 1))
                                                          ^~~~~~~~~~~
src/isaac.rs:410:33: 410:60 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:417:17: 417:33 note: in this expansion of rngstepn! (defined in src/isaac.rs)
src/isaac.rs:373:42: 373:53 error: the type of this value must be known in this context
src/isaac.rs:373                 *self.mem.get_unchecked((($x >> 3).0 as usize) & (RAND_SIZE_64 - 1))
                                                          ^~~~~~~~~~~
src/isaac.rs:389:37: 389:44 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:418:17: 418:34 note: in this expansion of rngstepp! (defined in src/isaac.rs)
src/isaac.rs:373:42: 373:53 error: the type of this value must be known in this context
src/isaac.rs:373                 *self.mem.get_unchecked((($x >> 3).0 as usize) & (RAND_SIZE_64 - 1))
                                                          ^~~~~~~~~~~
src/isaac.rs:392:33: 392:60 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:418:17: 418:34 note: in this expansion of rngstepp! (defined in src/isaac.rs)
src/isaac.rs:373:42: 373:53 error: the type of this value must be known in this context
src/isaac.rs:373                 *self.mem.get_unchecked((($x >> 3).0 as usize) & (RAND_SIZE_64 - 1))
                                                          ^~~~~~~~~~~
src/isaac.rs:407:37: 407:44 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:419:17: 419:34 note: in this expansion of rngstepn! (defined in src/isaac.rs)
src/isaac.rs:373:42: 373:53 error: the type of this value must be known in this context
src/isaac.rs:373                 *self.mem.get_unchecked((($x >> 3).0 as usize) & (RAND_SIZE_64 - 1))
                                                          ^~~~~~~~~~~
src/isaac.rs:410:33: 410:60 note: in this expansion of ind! (defined in src/isaac.rs)
src/isaac.rs:419:17: 419:34 note: in this expansion of rngstepn! (defined in src/isaac.rs)
error: aborting due to 16 previous errors
Could not compile `rand`.

[SimonSapin]

Also builds fine on 1.5.0 and 1.6.0-beta.1

[SimonSapin]

402259d from #30523 is suspicious.

[SimonSapin]

@eddyb pointed out on IRC that the real culprit is the new impls at https://github.com/rust-lang/rust/pull/30523/files#diff-0cfd5773528ef167f36c81b535a97a3dR127 which should be commented again until #23545 is fixed.

[solson]

There's a fix in #30733.

[alexcrichton]

Nominating for some discussion, but if there already has been discussion and I missed out please feel free to un-nominate :)

[eddyb]

@alexcrichton We're currently waiting for a new nightly.

[SimonSapin]

@alexcrichton this was discussed informally on IRC:

Previously, Wrapping(1_u32) << 2 would be unambiguous since Shl<usize> was the only Shl<_> trait implemented for Wrapping<u32>. #30523 added more impls such as Shl<u8>, so that expression became ambiguous and causes a type error.

#23545 is about resolving the ambiguity somehow (maybe defaulting to u32?) to avoid the compiler error. Until that’s done, #30733 removes (comments out) all but one impl again. #30733 has already landed in master, but has not reach the nightly channel yet. (multirust update today on Jan 11 gives me rustc 1.7.0-nightly (d5e2290 2016-01-04))

[alexcrichton]

Thanks for the info @eddyb and @SimonSapin! I'd personally prefer to keep those impls (e.g. not have anything commented out), but may be worth discussing and perhaps holding off that change until #23545 is handled.

[alexcrichton]

The libs team discussed this in triage yesterday and the conclusion was that the current state is where we'd like to be. We'll hold off on adding the relevant impls until #23545 is dealt with, and otherwise this has been fixed.