How do you debug @safe @nogc code? Can't figure out how to print.

[Stanislav Blinov]

On Friday, 16 November 2018 at 12:59:22 UTC, aliak wrote:

> Adding @trusted to declaration of opDispatch gets rid of @safe 
> error but I still get "Error: @nogc function D main cannot call 
> non- at nogc function". And going through the codebase and 
> figuring out where to add @trusted is *very* cumbersome.
>
> I can't figure out how to make this work. The ideal way would 
> be just a debug_print() function that works in nogc and safe 
> code. I.e. does not affect the attribute inference of templates.

As Zoadian points out, you can now (since 2.079: 
https://dlang.org/changelog/2.079.0.html#bugfix-list) shamelessly 
call @nogc inside debug blocks.

To get at where the problematic areas were in the code in 
question though, it's as follows:

> auto assumeNoGC(T)(T t) {  // point 1
>     import std.traits;
>     enum attrs = functionAttributes!T | FunctionAttribute.nogc;
>     // point 2:
>     return cast(SetFunctionAttributes!(T, functionLinkage!T, 
> attrs)) t;
> }

At 'point 1', you just take by value. If `t` ends up being a 
closure, D will allocate. That's where it breaks the @nogc. 
Solution:

auto assumeNoGC(T)(return scope T t) { /* ... */ }

Now you take (and return) a `scope` t, in that case when `t` is a 
closure D won't allocate it on the GC heap.

At 'point 2' you make an un- at safe cast, that's where it breaks 
@safe. Given that the whole deal is just a debugging hack, you 
could mark the whole function @trusted:

auto assumeNoGC(T)(return scope T t) @trusted { /* ... */ }