Add no_std support

[snjax]

Add no_std support for borsh.

Polyfill is implemented at borsh::lib::* and borsh::error (same as in serde).

For no_std borsh HashMap is imported from hashbrown and Error and Write imported from bare_io.

[MaksymZavershynskyi]

@ailisp , @willemneal could you take a look?

[codecov-io]

Codecov Report

Merging #108 (3768ab8) into master (4c2f1a0) will decrease coverage by 0.21%.
The diff coverage is 89.07%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
- Coverage   93.43%   93.21%   -0.22%     
==========================================
  Files          21       21              
  Lines        1219     1224       +5     
==========================================
+ Hits         1139     1141       +2     
- Misses         80       83       +3     

Impacted Files	Coverage Δ
borsh-rs/borsh/src/schema.rs	100.00% <ø> (ø)
borsh-rs/borsh/tests/test_schema_enums.rs	86.55% <ø> (ø)
borsh-rs/borsh/tests/test_schema_nested.rs	100.00% <ø> (ø)
borsh-rs/borsh/tests/test_schema_structs.rs	100.00% <ø> (ø)
borsh-rs/borsh/tests/test_simple_structs.rs	92.63% <ø> (ø)
borsh-rs/borsh/src/schema_helpers.rs	84.61% <50.00%> (ø)
borsh-rs/borsh/src/de/mod.rs	90.96% <81.03%> (-0.29%)	⬇️
borsh-rs/borsh-derive-internal/src/struct_ser.rs	83.67% <100.00%> (ø)
...rs/borsh-schema-derive-internal/src/enum_schema.rs	95.28% <100.00%> (ø)
...rsh-rs/borsh-schema-derive-internal/src/helpers.rs	92.00% <100.00%> (ø)
... and 6 more

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[frol]

Re: "Replace bare-io with semi-custom Error and Write". If the only reason for this is the support for the String error messages, I vote for dropping format! and using a static string message (&'static str). I don't want us to vendor the fork of std library.

[snjax]

@frol We updated the PR

[snjax]

Re: "Replace bare-io with semi-custom Error and Write". If the only reason for this is the support for the String error messages, I vote for dropping format! and using a static string message (&'static str). I don't want us to vendor the fork of std library.

bare-io is incomplete: there are no hashmaps and hashsets. That's why anyway we should use std polyfill (like serde).

[frol]

It seems that the test suite is not running under no-std mode. I have just tested cargo test -Zpackage-features --package borsh --no-default-features and it failed to compile for obvious reasons of using std contexts. Please, add the relevant cargo test commands to .travis.yml.

P.S. cargo test -Zpackage-features --workspace --no-default-features is not working properly rust-lang/cargo#7160 See next comment

[frol]

Oh, well, cargo test -Zpackage-features --workspace --no-default-features works fine if you edit benchmarks/Cargo.toml to disable the default features:

borsh = { path = "../borsh", default-features = false }

[frol]

In order to test no-std, we need to have #![cfg_attr(not(feature = "std"), no_std)] in the test files. Once I add this line to our tests, if fails to compile now. Please, address the compilation errors and make sure that no-std is indeed tested.

P.S. There seems to be a conflict in .travis.yml

P.S.2. Ideally, we need to test that the data structures from no-std are deserializable in std context and wise-versa, but we can work on this under a separate issue.

[frol]

On the other hand, it would fail due to unsupported std types anyway, so it is fine as is, I guess.

@snjax @voidxnull Great work! It is good to be merged. Please, resolve the merge conflict on .travis.yml (I don't have permissions to push to your PR branch to fix it myself)

[snjax]

Please, resolve the merge conflict on .travis.yml (I don't have permissions to push to your PR branch to fix it myself)

@frol Done.

[frol]

I am sorry to get back and forth here, but CI is completely broken now.

1. stable and beta Rust complain about the -Zpackage-features flag being unstable. Let's replace the command with cd borsh && cargo test --no-default-features
2. nightly-2019-05-22 is too old, replace it with nightly-2020-05-15, which we use for nearcore

[snjax]

@frol done

[frol]

Thank you!