Annotate pure functions with return/scope

[dkorpel]

This PR is similar to dlang/phobos#8076, but for druntime and for dlang/dmd#12010 instead of dlang/dmd#10924
It's probably too big to merge in one go, this is a proof of concept that can later be split up into smaller chunks for easier review.

The offending functions were found with something I cobbled together in dmd: dkorpel/dmd@7d1f0c5
It outputs this when running druntime unittests:

src/core/internal/dassert.d(221): parameter `a` of `idup` is cheating `scope`
src/object.d(3410): parameter `value` of `getElement` is cheating `scope`
src/core/internal/dassert.d(164): parameter `val` of `atomicLoad` is cheating `scope`
src/core/atomic.d(1161): parameter `val` of `atomicFetchAdd` is cheating `scope`
src/core/atomic.d(1208): parameter `val` of `atomicFetchSub` is cheating `scope`
src/core/lifetime.d(1881): parameter `source` of `moveImpl` is cheating `scope`
src/core/memory.d(737): parameter `p` of `gc_query` is cheating `scope`
src/core/memory.d(1453): parameter `p` of `realloc` is cheating `scope`
src/core/internal/convert.d(479): parameter `a` of `dup` is cheating `scope`
src/core/internal/array/appending.d-mixin-42(50): parameter `px` of `_d_arrayappendcTX` is cheating `scope`
src/core/internal/array/casting.d(101): parameter `from` of `__ArrayCast` is cheating `scope`
src/core/internal/array/concatenation.d(41): parameter `arrs` of `_d_arraycatnTX` is cheating `scope`
src/core/internal/backtrace/dwarf.d(153): parameter `dst` of `demangle` is cheating `scope`
src/core/internal/hash.d(213): parameter `arr` of `toUbyte` is cheating `scope`
src/rt/aaA.d(802): parameter `cti` of `unqualify` is cheating `scope`

One of the harder ones is:

src/core/lifetime.d(1881): parameter `source` of `moveImpl` is cheating `scope`

It's really tricky, since it carefully uses @trusted blocks to nudge the @safe inference in the right direction, which now breaks. Maybe someone can help out with that.

[aG0aep6G]

The offending functions were found with something I cobbled together in dmd: dkorpel/dmd@7d1f0c5

Awesome!

One of the harder ones is:

src/core/lifetime.d(1881): parameter `source` of `moveImpl` is cheating `scope`

I think that boils down to moveEmplaceImpl assigning its first parameter to its second parameter. For DIP 1000 it must be other way around. No, really, it's in the spec.

1. https://dlang.org/spec/function.html#return-ref-parameters: "If the function returns void, and the first parameter is ref or out, then all subsequent return ref parameters are considered as being assigned to the first parameter for lifetime checking.".
2. https://dlang.org/spec/function.html#param-storage: "[A return parameter] may be returned or copied to the first parameter, but otherwise does not escape from the function."

For internal functions like moveEmplaceImpl we can just reverse the parameters. But move(source, target) might simply not be compatible with DIP 1000.

[RazvanN7]

Looks good to me. Why is this still a draft?

[dkorpel]

The remaining errors are with dup and move. dup should be fixed in #3474, move is still a blocker since the @safe unittests move rely on the pure bug. I've made a forum post about it.

This could be merged in the meanwhile, with a follow up PR for move eventually.

[dkorpel]

Wait a second, this is not right. ref should not be removed.

[dkorpel]

I remember ref return not compiling, I think it should be ref return scope. I removed this change, will look at it later today (or in a follow-up PR).

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + druntime#3480"