Don't generate ModuleInfo if it's not declared in druntime

[JinShil]

In response to http://forum.dlang.org/post/onn8vi$tgs$1@digitalmars.com and http://forum.dlang.org/post/p0280p$29t$1@digitalmars.com

Rationale

The compiler shouldn't require ModuleInfo to exist if the platform doesn't support it or the program author wishes to provide an alternate druntime implementation that doesn't support it.

Platform support is provided by druntime. ModuleInfo is declared in object.d. Therefore, the compiler can see, at compile-time, whether or not the platform engineer has provided support for ModuleInfo and generate object code accordingly. (i.e. Design by Introspection)

There are also use cases where, although the platform could support ModuleInfo, the program author may choose to provide an alternate implementation of druntime features just for their program. That alternate implementation may not support ModuleInfo.

The current compiler implementation will emit errors if ModuleInfo is not declared. It is an unnecessary restriction to require the author to workaround this constraint when they don't wish to support ModuleInfo.

Current workarounds are to define a stub ModuleInfo and have it discarded through LTO, --gc-sections, or a custom linker script.

Benefits

• Eliminate the need for hackish workarounds due to arbitrary constraints imposed by the compiler
• Reduce friction for platform engineers wishing to incrementally port D to new platforms
• A step in the right direction for using D in a more pay-as-you-go fashion.
• Broaden D's relevance in new, interesting, and niche use cases currently dominated by minimal runtime languages such as Rust and C.

[dlang-bot]

Thanks for your pull request, @JinShil!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

[WalterBright]

Something of this significance needs a changelog entry.

[WalterBright]

In fact, much of what you wrote in the explanation above is ideal for the changelog entry.

[JinShil]

Something of this significance needs a changelog entry.

Changelog entry added.

[WalterBright]

It's a good idea. Wish I'd thought of it.