Conditional compilation

[SkymanOne]

Closes #1231

Summary

This PR introduces a native support for conditional compilation in ink! smart contracts

High level overview

It is now possible to attribute ink! elements with #[cfg(...)] macro attribute. You can use platform specific conditions or custom features specified in Cargo.toml file. (See integration-tests/condtional-compilation for example).

Important notice: Attributing event fields is not allowed.

Low level overview

IR structures have stayed the same, no significant changes there: just added a helper function to retrieve a list of all cfg attributes from the element.

Codegen has undergone some changes. Structures ContractAmountDispatchables, ContractDispatchableMessages and ContractDispatchableConstructors have been removed due to tight dependance on syntax analysis to count the number of elements which do not work with conditional compilation.

Main changes are based around DispatchableConstructorInfo and DispatchableMessageInfo code generation. Earlier, the logic was based around retrieving dispatchable IDs from the constant array. As mentioned in the #1231, this logic doesn't work when some ink items are omitted at the macro expansion. Therefore, it was decided to explicitly pass dispatchable id in the codegen instead of retrieving it from the array.

Other changes involve just putting bunch of cfg attributes around the codegen to omit associated structures and type definitions at the macro expansion stage.

The feature flag only works with the release of cargo-contract which includes: use-ink/cargo-contract#1004

Checklist

• cargo-contract feature handling - Pass feature flags to metadata-gen template cargo-contract#1004
• Inherent messages
• Trait messages
• Constructors
• Events, but not topics
• Disallow event fields to be attributed with cfg
• Docs & Cleanup
• Add CI run to build a contract with features

[codecov-commenter]

Codecov Report

Merging #1707 (2f54585) into master (c1111b5) will decrease coverage by 0.08%.
The diff coverage is 85.91%.

❗ Current head 2f54585 differs from pull request most recent head 07ba757. Consider uploading reports for the commit 07ba757 to get more accurate results

@@            Coverage Diff             @@
##           master    #1707      +/-   ##
==========================================
- Coverage   70.60%   70.53%   -0.08%     
==========================================
  Files         205      205              
  Lines        6434     6483      +49     
==========================================
+ Hits         4543     4573      +30     
- Misses       1891     1910      +19     

Impacted Files	Coverage Δ
crates/ink/ir/src/ir/item_impl/mod.rs	86.29% <0.00%> (-4.39%)	⬇️
crates/ink/src/reflect/dispatch.rs	0.00% <ø> (ø)
...ink/tests/ui/contract/pass/constructor-selector.rs	46.15% <ø> (ø)
...tes/ink/tests/ui/contract/pass/message-selector.rs	33.33% <ø> (ø)
crates/ink/ir/src/ir/item_impl/constructor.rs	89.70% <66.66%> (-2.23%)	⬇️
crates/ink/ir/src/ir/item_impl/message.rs	91.08% <66.66%> (-1.55%)	⬇️
crates/ink/ir/src/ir/trait_def/item/trait_item.rs	89.70% <66.66%> (-2.23%)	⬇️
crates/ink/ir/src/ir/item/event.rs	85.89% <80.00%> (-0.87%)	⬇️
crates/ink/codegen/src/generator/dispatch.rs	92.95% <93.33%> (-1.25%)	⬇️
...odegen/src/generator/as_dependency/call_builder.rs	100.00% <100.00%> (ø)
... and 11 more

... and 3 files with indirect coverage changes

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[ascjones]

LGTM

[ascjones]

We will need to document the breaking change that you can no longer access constructor/message info in this way by the index. But I doubt too many people are depending on that...

[SkymanOne]

Can you please open a separate issue for this? I guess this needs to be documented in release notes