Add within autodiff cmd

src/Enzyme.jl

@@ -1744,4 +1744,11 @@ macro import_rrule(args...)
     return _import_rrule(args...)
 end
 
+"""
+   within_autodiff()
+
+Returns true if within autodiff, otherwise false.
+"""
+within_autodiff() = false
+
 end # module

src/compiler/interpreter.jl

@@ -220,11 +220,23 @@ struct AutodiffCallInfo <: CallInfo
     info::CallInfo
 end
 
+always_true() = true
+
 function abstract_call_known(interp::EnzymeInterpreter, @nospecialize(f),
         arginfo::ArgInfo, si::StmtInfo, sv::AbsIntState,
         max_methods::Int = get_max_methods(interp, f, sv))
 
     (; fargs, argtypes) = arginfo
+
+    if f === Enzyme.within_autodiff
+          arginfo2 = ArgInfo(
+            fargs isa Nothing ? nothing : [:(Enzyme.Interpreter.always_true), fargs[2:end]...],
+            [Core.Const(Enzyme.Interpreter.always_true), argtypes[2:end]...]
+          )
+          return abstract_call_known(
+            interp, Enzyme.always_true, arginfo2,
+            si, sv, max_methods)
+    end
 
     if f === Enzyme.autodiff && length(argtypes) >= 4
         if widenconst(argtypes[2]) <: Enzyme.Mode && widenconst(argtypes[3]) <: Enzyme.Annotation && widenconst(argtypes[4]) <: Type{<:Enzyme.Annotation}

test/abi.jl

@@ -460,6 +460,11 @@ abssum(x) = sum(abs2, x);
 
 mulsin(x) = sin(x[1] * x[2])
 
+@testset "within_autodiff" begin
+    @test Enzyme.within_autodiff()
+    @test Enzyme.autodiff(ForwardWithPrimal, Enzyme.within_autodiff)[1]
+end
+
 @testset "Type inference" begin
     x = ones(10)
     @inferred autodiff(Enzyme.Reverse, abssum, Duplicated(x,x))