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6ffa285fc3
This safety check was completely broken; it triggered unchecked illegal behavior *in order to implement the safety check*. You definitely can't do that! Instead, we must explicitly check the boundaries. This is a tiny bit fiddly, because we need to make sure we do floating-point rounding in the correct direction, and also handle the fact that the operation truncates so the boundary works differently for min vs max. Instead of implementing this safety check in Sema, there are now dedicated AIR instructions for safety-checked intfromfloat (two instructions; which one is used depends on the float mode). Currently, no backend directly implements them; instead, a `Legalize.Feature` is added which expands the safety check, and this feature is enabled for all backends we currently test, including the LLVM backend. The `u0` case is still handled in Sema, because Sema needs to check for that anyway due to the comptime-known result. The old safety check here was also completely broken and has therefore been rewritten. In that case, we just check for 'abs(input) < 1.0'. I've added a bunch of test coverage for the boundary cases of `@intFromFloat`, both for successes (in `test/behavior/cast.zig`) and failures (in `test/cases/safety/`). Resolves: #24161