Compilation fails with "nightly" feature due to packed-simd dependency

[isislovecruft]

Compiling rand 0.7.3 fails with rustc 1.46.0 (04488afe3 2020-08-24) and later with --features "nightly", due to the packed-simd 0.3.3 dependency failing to build.

error[E0432]: unresolved import `crate::arch::x86_64::__m64`                                                                                                                                                                                   
  --> /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/api/into_bits/arch_specific.rs:51:15                                                                                                                    
   |                                                                                                                                                                                                                                           
51 |               $($arch_ty),*                                                                                                                                                                                                               
   |                 ^^^^^^^^ no `__m64` in `arch::x86_64`                                                                                                                                                                                     
...                                                                                                                                                                                                                                            
86 | / impl_arch!(                                                                                                                                                                                                                             
87 | |     [x86["x86"]: __m64], [x86_64["x86_64"]: __m64],                                                                                                                                                                                     
88 | |     [arm["arm"]: int8x8_t, uint8x8_t, poly8x8_t, int16x4_t, uint16x4_t,                                                                                                                                                                 
89 | |      poly16x4_t, int32x2_t, uint32x2_t, float32x2_t, int64x1_t,                                                                                                                                                                         
...  |                                                                                                                                                                                                                                         
96 | |     test: test_v64                                                                                                                                                                                                                      
97 | | );                                                                                                                                                                                                                                      
   | |__- in this macro invocation                                                                                                                                                                                                             
   |                                                                                                                                                                                                                                           
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)                                                                                                                                     
                                                                                                                                                                                                                                               
error[E0432]: unresolved import `crate::arch::x86_64::_mm_movemask_pi8`                                                                                                                                                                        
  --> /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask/x86/sse.rs:47:21                                                                                                                
   |                                                                                                                                                                                                                                           
47 |                 use crate::arch::x86_64::_mm_movemask_pi8;                                                                                                                                                                                
   |                     ^^^^^^^^^^^^^^^^^^^^^----------------                                                                                                                                                                                 
   |                     |                    |                                                                                                                                                                                                
   |                     |                    help: a similar name exists in the module: `_mm_movemask_epi8`                                                                                                                                   
   |                     no `_mm_movemask_pi8` in `arch::x86_64`                                                                                                                                                                               
   |                                                                                                                                                                                                                                           
  ::: /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask.rs:41:1                                                                                                                         
   |                                                                                                                                                                                                                                           
41 | impl_mask_reductions!(m8x8);                                                                                                                                                                                                              
   | ---------------------------- in this macro invocation                                                                                                                                                                                     
   |                                                                                                                                                                                                                                           
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)                                                                                                                                     
                                                                                                                                                                                                                                               
error[E0432]: unresolved import `crate::arch::x86_64::_mm_movemask_pi8`                                                                                                                                                                        
  --> /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask/x86/sse.rs:62:21                                                                                                                
   |                                                                                                                                                                                                                                           
62 |                 use crate::arch::x86_64::_mm_movemask_pi8;                                                                                                                                                                                
   |                     ^^^^^^^^^^^^^^^^^^^^^----------------                                                                                                                                                                                 
   |                     |                    |                                                                                                                                                                                                
   |                     |                    help: a similar name exists in the module: `_mm_movemask_epi8`                                                                                                                                   
   |                     no `_mm_movemask_pi8` in `arch::x86_64`                                                                                                                                                                               
   |                                                                                                                                                                                                                                           
  ::: /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask.rs:41:1                                                                                                                         
   |
41 | impl_mask_reductions!(m8x8);
   | ---------------------------- in this macro invocation
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0432]: unresolved import `crate::arch::x86_64::_mm_movemask_pi8`
  --> /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask/x86/sse.rs:47:21                                                                                                               
   |
47 |                 use crate::arch::x86_64::_mm_movemask_pi8;
   |                     ^^^^^^^^^^^^^^^^^^^^^----------------
   |                     |                    |
   |                     |                    help: a similar name exists in the module: `_mm_movemask_epi8`
   |                     no `_mm_movemask_pi8` in `arch::x86_64`
   |
  ::: /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask.rs:47:1
   |
47 | impl_mask_reductions!(m16x4);
   | ----------------------------- in this macro invocation
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0432]: unresolved import `crate::arch::x86_64::_mm_movemask_pi8`
  --> /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask/x86/sse.rs:62:21
   |
62 |                 use crate::arch::x86_64::_mm_movemask_pi8;
   |                     ^^^^^^^^^^^^^^^^^^^^^----------------
   |                     |                    |
   |                     |                    help: a similar name exists in the module: `_mm_movemask_epi8`
   |                     no `_mm_movemask_pi8` in `arch::x86_64`
   |
  ::: /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask.rs:47:1
   |
47 | impl_mask_reductions!(m16x4);
   | ----------------------------- in this macro invocation
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0432]: unresolved import `crate::arch::x86_64::_mm_movemask_pi8`
  --> /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask/x86/sse.rs:47:21
   |
47 |                 use crate::arch::x86_64::_mm_movemask_pi8;
   |                     ^^^^^^^^^^^^^^^^^^^^^----------------
   |                     |                    |
   |                     |                    help: a similar name exists in the module: `_mm_movemask_epi8`
   |                     no `_mm_movemask_pi8` in `arch::x86_64`
   |
  ::: /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask.rs:52:1
   |
52 | impl_mask_reductions!(m32x2);
   | ----------------------------- in this macro invocation
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0432]: unresolved import `crate::arch::x86_64::_mm_movemask_pi8`
  --> /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask/x86/sse.rs:62:21
   |
62 |                 use crate::arch::x86_64::_mm_movemask_pi8;
   |                     ^^^^^^^^^^^^^^^^^^^^^----------------
   |                     |                    |
   |                     |                    help: a similar name exists in the module: `_mm_movemask_epi8`
   |                     no `_mm_movemask_pi8` in `arch::x86_64`
   |
  ::: /home/isis/.cargo/registry/src/github.com-1ecc6299db9ec823/packed_simd-0.3.3/src/codegen/reductions/mask.rs:52:1
   |
52 | impl_mask_reductions!(m32x2);
   | ----------------------------- in this macro invocation
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

It appears that packed-simd has been failing to build on nightlies for a while now. I've worked around this by not forwarding the "nightly" feature in https://github.com/dalek-cryptography/ed25519-dalek to our rand dependency.

[dhardy]

Luckily, there's not really any little reason to use nightly. All it does is enable other features which you can depend on directly.

@vks maybe we should remove the nightly feature?

[vks]

@dhardy This is not quite true, we also depend on the nightly feature for implementing TrustedLen and using slice_partition_at_index. These do not have their own features. Furthermore, sometimes nightly is required instead of or in addition to simd_support, but this should be fixed.

I don't think we can do much to avoid breakage like this, it's to be expected when using nightly which lacks the stability guarantees of stable Rust. An option would be to have a feature for each nightly functionality, so breakage can be isolated, but is this worth the complexity?

[dhardy]

Given the complex and unstable state of SIMD support I at least think this should not be implied by nightly.

[vks]

With Rand 0.8, you can consider forwarding the nightly feature again, as it will not longer enable simd_support with the associated stability issues.

[isislovecruft]

@dhardy @vks Thanks! I think it might be a while before we get to use rand 0.8 since it's part of the underlying curve25519-dalek dependency's public API, so the versions have to match, and to bump it we need to make a breaking change so we're tyring to batch up enough breakage all at once into each major version.