Congratulations to Paul Backus. DIP 1038, "@mustUse" has been
accepted after he implemented changes to address concerns from
Walter.
https://github.com/dlang/DIPs/blob/master/DIPs/accepted/DIP1038.md
During the Formal Assessment, Walter and Paul discussed a few
aspects of the original proposal that Walter felt needed to be
addressed. I will summarize them here. Please refer to the
section of the DIP titled '[Design Goals and Possible
Alternatives](https://github.com/dlang/DIPs/blob/master/DIPs/DIP1038.md#design-goals-and-possible-alternatives)' for the rationale behind Paul's decisions regarding points 2 and 3 below.
The TL;DR: the attribute is now called `@mustUse` and it only
applies to structs and unions.
1. `@noDiscard` is a negative, and though we have `nothrow` and
`@noGC`, Walter has come to believe that negatives are suboptimal
from a "human factors perspective". He noted that `must_use` in
Rust is the right idea, and suggested Paul change `@noDiscard` to
`@use`. Paul opted to go with `@mustUse` instead on the grounds
that `@use` is too short and difficult to search for.
2. The original proposal allowed the attribute to apply to
classes and interfaces. Walter offered examples of where it can
cause problems when applied to classes and requested that Paul
address these cases in the DIP. Ultimately, Paul opted to
disallow the attribute on classes.
3. The original proposal allowed the attribute to apply to
functions. Walter requested that Paul develop rules for
covariance and contravariance, with would apply to function
overloading, function overriding, function parameters, and
implicit conversions. Ultimately, Paul opted to disallow the
attribute on functions.
I want to reiterate that the above is only a summary. Paul and
Walter exchanged multiple emails in discussion of these issues,
with both proposing ideas to solve them. Paul does an excellent
job describing his rationale for only allowing the attribute on
structs in the [section I noted
above](https://github.com/dlang/DIPs/blob/master/DIPs/DIP1038.md#design-goals-and-possible-alternatives).
The final form of this DIP does not preclude future expansion of
the feature to allow it on classes and functions. Paul has noted
elsewhere that Rust took the similar approach in that `must_use`
was only allowed on types initially and expanded to include
functions later.
Anyone interested in writing a DIP for such expansion of the
feature should certainly consult with Walter beforehand for a
more thorough overview on what must be accounted for.
