rust-lang / rust-lang/rfcs

Should lossy conversions be compiler warning

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Description

Hi,
Should lossy conversions (i.e. let a = 300u16 as u8) be a compile time warning?
that way people can opt in with #![deny(cast_lossy)]

Otherwise maybe a way to make generic that have size constraints?

I'm working on a new library where I'm casting a bunch of stuff to usize and for example usize::from(0u32) isn't a thing. even though it should work on both 32bit and 64bit machines.

So right now I have a bunch of static asserts to check that size_of<usize> => size_of<u32>().

Edit: Related to #2715

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Research direction

Start by reading the discussion in this issue and the related #2715, focusing on the proposed lossy-conversion warning, opt-in lint, and generic size-constraint alternatives. Done requires a decided, scoped proposal that explains the intended behavior and resolves which direction an RFC should pursue.

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Assessment

Tech stack
rust
Domain
compilers
Issue type
Feature
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
25/100

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