Share Guard code across repositories

[antonfirsov]

Currently Guard and DebugGuard is being shared as an internal of SixLabors.Core. This could lead to compatibility issues.

I'm suggesting to share the code the code instead with the help of already existing "Standards" submodule. The classes are tested within the bounds of the SixLabors.Core repository.

Parallel PR-s to apply the change in across repositories

• Apply SharedInfrastructure, introduce shared guards Core#33
• Share Guard code across projects  Fonts#113
• Share Guard code across repositories Shapes#69
• TODO: open PR in SixLabors/ImageSharp when the rest has been merged

[JimBobSquarePants]

We should probably use Shared source packages for this.

Docs
https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/dependencies#nuget-shared-source-packages

Example
https://github.com/aspnet/Extensions/tree/master/src/Shared/src

[antonfirsov]

@JimBobSquarePants that would mean introducing yet another infrastructure element raising where/how +maintenance questions. Submodules are already in place for our stuff, and looking simpler to me (no need to publish on myget etc.).

[antonfirsov]

If we find it's worth the efforts, we can repackage it later anytime. For now I just want to get rid of those dangerous dependencies for the next beta.

[JimBobSquarePants]

Those projects don’t get deployed, only their source is shared to the consuming projects so package management is not a factor. We can also enforce standards rules on the shared code.

I’d much rather introduce a new submodule here anyway rather than pollute standards, or replace standards with something more aptly named if we are to use it as a general store.

[antonfirsov]

@JimBobSquarePants I've renamed the repository, and changed README.md according to it's new purpose.

Now it needs an extensive review together with (updated) SixLabors/Core#33, before so we can do the rest of the work. I'll change and finalize the remaining PR-s, as soon as this one is merged.

[JimBobSquarePants]

@antonfirsov I've just pushed an update that uses the most up-to-date .editorconfig (plus VS 16.2.2 fix) and .gitattributes so we can share them across our repos also.

Additionally I've reordered the SixLabors.ruleset config for readability and added a new SixLabors.Tests.ruleset that contains a slimmed down ruleset more suitable for our unit tests.

The readme has been updated to reflect those changes.

I still think we should use shared projects in the future over standalone .cs files but that can wait for future discussion.

[antonfirsov]

To me it looks like the "Shared project" is a concept predating the new SDK-style msbuild projects, and could be replaced by a simple <Import ... /> nowadays.

We should investigate these practices when we share .props / .targets across our repositories.