Congratulations to Paul Backus. DIP 1038, "@mustUse" has been accepted after he implemented changes to address concerns from Walter.
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' 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.
@noDiscardis a negative, and though we have
@noGC, Walter has come to believe that negatives are suboptimal from a "human factors perspective". He noted that
must_usein Rust is the right idea, and suggested Paul change
@use. Paul opted to go with
@mustUseinstead on the grounds that
@useis too short and difficult to search for.
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.
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.
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.