Implement Safer D

[WalterBright]

DIP: https://github.com/WalterBright/documents/blob/38f0a846726b571f8108f6e63e5e217b91421c86/safer.md

This adds -preview=safer which enables safety checking on un-attributed functions. This just makes one check for starters.

Newsgroup:
https://www.digitalmars.com/d/archives/digitalmars/dip/development/Safer_D_first_draft_387.html

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

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dub run digger -- build "master + dmd#17044"

[WalterBright]

Finished the implementation. Turned out to be much less code than I anticipated.

[WalterBright]

It doesn't need a spec PR because it's behind a -preview switch.

[dkorpel]

I would prefer this checking a scope flag rather than a global variable

[WalterBright]

You are not wrong, but it would require more disruption as the flag isn't passed along. I suppose it would be alright for a future refactoring.

[rikkimax]

This will likely be limited to newer editions, so it'll have to check it at the module level. Which eliminates the global flag.

[vnayar]

This is a cool idea! It would also mean you can compile projects that depend on libraries that require @safe and make them work with other libraries that do not use an explicit @safe. That incompatibility creates a lot of friction, and the ability to automatically combine code that de-facto is safe would remove a lot of the hesitancy around using the @safe feature.