[Issue 17786] New: Define the FreeStanding subset of betterC

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=17786

          Issue ID: 17786
           Summary: Define the FreeStanding subset of betterC
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: petar.p.kirov at gmail.com

As it stands, `-betterC` is a subset of D under which features requiring
druntime are unavailable (and should probably be disallowed).

`-betterC` however still assumes that the C runtime / standard library is
available and the compiler freely emits references to it (such as to memset).

As of several releases, the compiler has a FreeStanding predefined version,
which according to the spec
(https://dlang.org/spec/version.html#predefined-versions) is defined as:

An environment without an operating system (such as Bare-metal targets)

We should:
* Add a compiler switch, so that code targeting bare-metal can actually use
this predefined version, and third party libraries could offer bare-metal
friendly implementations of their functionalities.
* Teach the compiler to not emit references to the C runtime.
* Define in the language specification all the features that are not available.

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