Ensure f32 deserialized from f64 and vice versa preserve NaN sign #2637
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dtolnay
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Oct 26, 2023
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| #[cfg(any(no_float_copysign, not(feature = "std")))] | ||
| { | ||
| Ok(v as Self::Value) | ||
| } |
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This produces nondeterministic wrong values if serde is built with "std" feature disabled. rust-lang/rust#50145
I'm not sure whether we can manually implement a correct copysign without going through the LLVM copysign intrinsics. If that were possible, I presume libcore would already have done it?
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f32 as f64andf64 as f32, if the input is NaN, produces an output NaN with nondeterministic sign. This leads to bugs like TOML+nanand-nanand YAML.nanbeing deserialized to f32 with the wrong sign nondeterministically because they're manipulated as f64 internally by the library's Value type.See rust-lang/miri#3139.