Audit usages of PreserveNewest/Always metadata to ensure support for IfDifferent #51829
Description
Derived from: #51826 (comment) (by @baronfel)
Context
dotnet/msbuild#11052 introduced support of 'IfDifferent' for CopyToOutputDirectory metadata. This mostly helps avoid usages of 'Always' in cases where timestamp check is not sufficient (e.g. some version control systems reverts or build/publish outputs actually being used and altered during debugging).
Without complementing change in sdk, we would have confusing mix of the metadata being supported just in some cases. Also we cannot expose it in VS (as it possibly could lead to edits that wouldn't be fully supported down the road) - which limits disccoverability:
Why do we want IfDifferent?
To stear users from usages of 'Always' copy metdata - as that is one of the main factors preventing incrementality of builds
Alternatives?
Mentioned MSBuild PR introduced opt-in mechanism to flip copy-always behavior to if-different behavior - via setting $(SkipUnchangedFilesOnCopyAlways) property. So we can start start setting it for projects targetting newer TFMs. But again it'd be helpful to have complementing change in sdk as well.
What is suggested to be done
- Revisit all instances of logic that splits items based on CopyToOutputDirectory/CopyToPublishDirectory/Copy... metadata values - to account for the 'IfDifferent' value
- Revisit the usages of the logic where we set the value of metadata - if we should actualy replace instances of 'PreserveNewest' with 'IfDifferent'