WIP: Extend cases where expressions are passed by move [skip ci]

[nordlow]

This enables more situations where variables and parameters are automatically passed by move. It works by detecting when all the references to a VarDeclaration can be passed by move. For those cases move is triggered by setting STC.rvalue of the VarDeclaration.

The first goal here is to enable a lazy range to be constructed with a non-copyable container type as its source ranges, typically, as

this(R)(R source)
{
    this.source = source; // `source` can be passed by move here
}

. This case can be activated by a simple special-casing in inferStatementRvalues without the current added book-keepings

• VarExp.vrefOrder
• VarDeclaration.vrefCount
• VarDeclaraiton.wasMovedBy

which are currently there for experimenting purposes.

There book-keepings are here because they enable more pass-by-move cases. The question is if we should try enabling these as well.

There's now also a possibility to enable better error messages by giving the location of the VarExp that already moved a VarDeclaration if more than one move occurs for the same VarDeclaration.

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[Geod24]

Taking a quick look, I'm not 100% certain I understand exactly how you plan to achieve this.

Bear in mind that anything that requires caller-callee coordination cannot be done "magically", because of separate compilation. Even for templates, the compiler will not emit templates that it knows are already instantiated in the root module that defines the template.

The first goal here is to enable a lazy range to be constructed with a non-copyable container type as its source ranges, typically, as [...]

Why not use ref here ? Or a pointer.

[nordlow]

Why not use ref here ? Or a pointer.

Pointer is not @safe. ref cannot be assignment to range source member. For reference, in Rust this feature is into_iter.

@andralex has approved this move-enhancement here dlang/phobos#4971.

[RazvanN7]

Currently, there is a move DIP in the DIP queue that is authored by @WalterBright . I suspect that part of this implementation is going to be part the implementation of the DFA that identifies where to insert calls to the move constructor.