Themes: GAT & MultiTheme #57
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Fixes a borrow violation obscured by unsafe code: Manager includes TkWindow includes mut ref to kas_wgpu::shared; shared.theme is also borrowed via draw_handle.
Caveat: does not currently work with enums (rust#69184), hence MultiTheme is disabled
Unfortunately support for this feature within the compiler is insufficient (even if we put up with warning messages and accept another dependency on nightly Rust). Using this with stack_dst (see next commit) is problematic.
This was referenced Feb 18, 2020
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This PR uses Generic Associated Types to avoid the unsafety around
draw_handleandsize_handle, fixing a possible memory-safety violation in the process.Additionally, this uses the
stack_dstcrate to provideThemeDstandWindowDsttraits (parameterised only byDrawtype), which allowsMultiThemeto be made more generic and moved intokas. As part of this, we also depend on theunsizenightly feature (although this could be avoided if necessary).Unfortunately, the Rust's GAT implementation is too immature to enable by default, hence is gated behind the
gatfeature flag. As such, it doesn't play well with the usage ofstack_dst, hence that is behind another flag and the two are mutually incompatible.This PR therefore complicates testing significantly, as well as any
Themeimplementation (including thecustom-themeexample), however I feel it is justified since:unsafe(revealing a bug in the process), and are almost certainly the way forward (once the compiler properly supports this)stack_dstallows for a much improvedMultiThemeimplementation with negligible performance impact and most of theunsafepushed into a small, shared crate (even if currently it has zero dependencies and few downloads), though it does seem sensible to feature-gate this dependencyUnfortunately the two features do not combine well, not only because of bugs around GATs, but also because
stack_dst::ValueAis not parametrisable over lifetimes (probably a dedicatedRefAtype would be required).