add no_std feature

[vitiral]

this adds support for no_std applications/libraries (like microcontrollers) by using conditional features

• Features cannot enable/disable features of dependencies rust-lang/cargo#633

On nightly with no_std:

#  rustup run nightly cargo test --features no_std                                                                                                                                           <<<
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running target/debug/deps/crc-c3e90d0dfab413be

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

     Running target/debug/deps/crc-9d7b680262789a74

running 10 tests
test crc32::checksum_castagnoli ... ok
test crc32::checksum_ieee ... ok
test crc32::digest_castagnoli ... ok
test crc32::checksum_koopman ... ok
test crc32::digest_ieee ... ok
test crc64::checksum_ecma ... ok
test crc32::digest_koopman ... ok
test crc64::digest_iso ... ok
test crc64::checksum_iso ... ok
test crc64::digest_ecma ... ok

test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured

     Running target/debug/deps/hash-bb44b103cb920ad0

running 2 tests
test hasher::checksum_hashcrc32 ... ok
test hasher::checksum_hashcrc64 ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured

   Doc-tests crc

running 2 tests
test src/lib.rs -  (line 6) ... ok
test src/lib.rs -  (line 27) ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured

on stable with std:

# rustup run stable cargo test                                                                                                                                                                 
   Compiling crc v1.4.0 (file:///home/garrett/projects/ruststuff/crc-rs)                                                                                                                                          
    Finished dev [unoptimized + debuginfo] target(s) in 1.64 secs                                                                                                                                                 
     Running target/debug/deps/crc-57a43f894e443797                                                                                                                                                               
                                                                                                                                                                                                                  
running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

     Running target/debug/deps/crc-79b2fcd023832a0c                                                                                                                                                               
                                                                                                                                                                                                                  
running 10 tests
test crc32::checksum_castagnoli ... ok                                                                                                                                                                            
test crc32::checksum_ieee ... ok                                                                                                                                                                                  
test crc32::checksum_koopman ... ok                                                                                                                                                                               
test crc32::digest_castagnoli ... ok                                                                                                                                                                              
test crc32::digest_ieee ... ok                                                                                                                                                                                    
test crc64::checksum_iso ... ok                                                                                                                                                                                   
test crc64::checksum_ecma ... ok                                                                                                                                                                                  
test crc64::digest_ecma ... ok                                                                                                                                                                                    
test crc32::digest_koopman ... ok                                                                                                                                                                                 
test crc64::digest_iso ... ok                                                                                                                                                                                     
                                                                                                                                                                                                                  
test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured

     Running target/debug/deps/hash-da1e5eadb26ad3bb                                                                                                                                                              
                                                                                                                                                                                                                  
running 2 tests
test hasher::checksum_hashcrc32 ... ok
test hasher::checksum_hashcrc64 ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured

   Doc-tests crc                                                                                                                                                                                                  
                                                                                                                                                                                                                  
running 2 tests
test src/lib.rs -  (line 27) ... ok                                                                                                                                                                               
test src/lib.rs -  (line 6) ... ok                                                                                                                                                                                
                                                                                                                                                                                                                  
test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured
#

[whitequark]

As a developer who deploys to embedded devices: I would not use a library that employs lazy_static. Not only there is no reason whatsoever to calculate CRC tables at runtime when you can simply include them in the source code--it's not like they ever change--or at least do it at compile-time, but on a real-time system this would easily cause a missed deadline by whichever code path calls the corresponding CRC function first.

[vitiral]

Huh, I wasn't thinking about that. Two questions :

• why is it not a table already? Can I open a PR for that?
• it seems to me like rust should be able to have a build_static crate that can do const evaluations like this at build time (using build.rs). Do you have thoughts on that?

[whitequark]

it seems to me like rust should be able to have a build_static crate that can do const evaluations like this at build time (using build.rs). Do you have thoughts on that?

I feel like this is unnecessary complexity and a simple accessory binary included in the source code for posterity and verification but never normally built is more than enough.

[vitiral]

I created a crate for this purpose. I could definitely also make a constructor that can be used for "scripts", which could be used to create a separate crate. https://github.com/vitiral/build_const

[vitiral]

just finished using build_const in the build.rs. I would say it is pretty simple -- but then again I am the developer of build_const.

If you would like I could put it in a script instead so the constant files could be generated whenever was convienient (build_const has helpers to make this transition seamless)

[vitiral]

Fixes #11

Can this be merged? There is literally no benefit to lazy_static for this crate anymore.

[oyvindln]

Looks like the crate author hasn't been active on github or twitter for a few months now. Maybe creating a new crate with the improvements in the open PRs could be considered.

[vitiral]

@mrhooray if you could give me rights on cargo I would gladly take over this repo! I totally understand that sometimes life gets hard 😄

[oyvindln]

@vitiral Would you be interested in making a fork of this crate and put it on crates.io to use until the author shows up? I can help out with it if needed.

[mrhooray]

@vitiral @oyvindln sorry for the late response and thanks @vitiral for integrating with his module to have build time constants

Not very familiar with build process, would this be a breaking change or it'd be fully compatible with v1?

[mrhooray]

does it make sense to move these two make table fns to a util file/module so that we don't have to repeat them in crc32.rs and crc64.rs

[vitiral]

We could move them into an internal "crate" which both build.rs and the individual modules depend on. I can do that if you would like.

I also didn't like copy/pasting them.

[mrhooray]

moving make_table out of crc32/crc64 is a breaking change since they're public
i'll see weather we could reference them in build.rs after merging.

[vitiral]

I was thinking we could have an internal crate which build.rs used and some of the functions were re-exported in crc32 and crc64 -- making it non-breaking.

[mrhooray]

good point. let me try that

[mrhooray]

#16 was created to address this

[vitiral]

@mrhooray it is not a breaking change, it just removes a dependency (and adds one) and allows for use in no_std libraries.