use the preference system for nan safe mode

Project.toml

@@ -8,6 +8,7 @@ DiffResults = "163ba53b-c6d8-5494-b064-1a9d43ac40c5"
 DiffRules = "b552c78f-8df3-52c6-915a-8e097449b14b"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 NaNMath = "77ba4419-2d1f-58cd-9bb1-8ffee604a2e3"
+Preferences = "21216c6a-2e73-6563-6e65-726566657250"
 Printf = "de0858da-6303-5e67-8744-51eddeeeb8d7"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
 SpecialFunctions = "276daf66-3868-5448-9aa4-cd146d93841b"
@@ -20,6 +21,7 @@ DiffResults = "0.0.1, 0.0.2, 0.0.3, 0.0.4, 1.0.1"
 DiffRules = "1.2.1"
 DiffTests = "0.0.1, 0.1"
 NaNMath = "0.2.2, 0.3"
+Preferences = "1"
 SpecialFunctions = "0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.10, 1.0"
 StaticArrays = "0.8.3, 0.9, 0.10, 0.11, 0.12, 1.0"
 julia = "1"

docs/src/user/advanced.md

@@ -138,8 +138,15 @@ decrease performance (~5%-10% on our benchmarks).
 
 In order to preserve performance in the majority of use cases, ForwardDiff disables this
 check by default. If your code is affected by this `NaN` behavior, you can enable
-ForwardDiff's `NaN`-safe mode by setting the `NANSAFE_MODE_ENABLED` constant to `true` in
-ForwardDiff's source. The constant is located in `src\prelude.jl`.
+ForwardDiff's `NaN`-safe mode by using the
+[Preferences.jl](https://github.com/JuliaPackaging/Preferences.jl) API to set
+the `nansafe_mode` preference to true, for example via:
+
+```julia
+julia> using ForwardDiff, Preferences
+
+julia> set_preferences!(ForwardDiff, "nansafe_mode" => true)
+```
 
 In the future, we plan on allowing users and downstream library authors to dynamically
 enable [NaN`-safe mode via the `AbstractConfig`

src/ForwardDiff.jl

@@ -3,6 +3,9 @@ module ForwardDiff
 using DiffRules, DiffResults
 using DiffResults: DiffResult, MutableDiffResult, ImmutableDiffResult
 using StaticArrays
+if VERSION >= v"1.6"
+    using Preferences
+end
 using Random
 using LinearAlgebra
 

src/prelude.jl

@@ -1,4 +1,8 @@
-const NANSAFE_MODE_ENABLED = false
+@static if VERSION >= v"1.6"
+    const NANSAFE_MODE_ENABLED = @load_preference("nansafe_mode", false)
+else
+    const NANSAFE_MODE_ENABLED = false
+end
 
 const AMBIGUOUS_TYPES = (AbstractFloat, Irrational, Integer, Rational, Real, RoundingMode)