Allow diff rule function to be in an expression or QuoteNode

Julia 0.7 changes the macro argument parsing to consistently use QuoteNodes instead of
Expr(:quote...) objects (#23885).

This also fixes some swapped LHS/RHS uses in the docs/variables.

src/api.jl

@@ -12,7 +12,7 @@ struct DiffRule{M,f} end
 Define a new differentiation rule for the function `M.f` and the given arguments, which should
 be treated as bindings to Julia expressions.
 
-The RHS should be a function call with a non-splatted argument list, and the LHS should be
+The LHS should be a function call with a non-splatted argument list, and the RHS should be
 the derivative expression, or in the `n`-ary case, an `n`-tuple of expressions where the
 `i`th expression is the derivative of `f` w.r.t the `i`th argument. Arguments should be
 interpolated wherever they are used on the RHS.
@@ -27,17 +27,16 @@ Examples:
 
 """
 macro define_diffrule(def)
-    @assert isa(def, Expr) && def.head == :(=)
-    rhs = def.args[1]
-    lhs = def.args[2]
-    @assert isa(rhs, Expr) && rhs.head == :call
-    qualified_f = rhs.args[1]
-    @assert isa(qualified_f, Expr) && qualified_f.head == :(.)
+    @assert isa(def, Expr) && def.head == :(=) "Diff rule expression does not have a left and right side"
+    lhs = def.args[1]
+    rhs = def.args[2]
+    @assert isa(lhs, Expr) && lhs.head == :call "LHS is not a function call"
+    qualified_f = lhs.args[1]
+    @assert isa(qualified_f, Expr) && qualified_f.head == :(.) "Function is not qualified by module"
     M, quoted_f = qualified_f.args
-    @assert isa(quoted_f, Expr) && quoted_f.head == :quote
-    f = first(quoted_f.args)
-    args = rhs.args[2:end]
-    rhs.args[1] = :(::Type{$DiffRules.DiffRule{$(Expr(:quote, M)),$(Expr(:quote, f))}})
+    f = _get_quoted_symbol(quoted_f)
+    args = lhs.args[2:end]
+    lhs.args[1] = :(::Type{$DiffRules.DiffRule{$(Expr(:quote, M)),$(Expr(:quote, f))}})
     key = (M, f, length(args))
     in(DEFINED_DIFFRULES, key) || push!(DEFINED_DIFFRULES, key)
     return esc(def)
@@ -111,3 +110,17 @@ Examples:
 
 """
 diffrules() = DEFINED_DIFFRULES
+
+#For v0.6 and v0.7 compatibility, need to support having the diff rule function enter as a
+#`Expr(:quote...)` and a `QuoteNode`. When v0.6 support is dropped, the function will always enter
+#in a `QuoteNode` (#23885).
+function _get_quoted_symbol(ex::Expr)
+    @assert ex.head == :quote
+    @assert length(ex.args) == 1 && isa(ex.args[1], Symbol) "Function not a single symbol"
+    ex.args[1]
+end
+
+function _get_quoted_symbol(ex::QuoteNode)
+    @assert isa(ex.value, Symbol) "Function not a single symbol"
+    ex.value
+end