Make enable_in_core! safe for use in precompilation (#50)

Move the hacky code for overriding Core._parse from the sysimage
init module into JuliaSyntax.jl proper, to allow it to be reused as
necessary.

Author: c42f

src/hooks.jl

@@ -1,6 +1,26 @@
 # This file provides an adaptor to match the API expected by the Julia runtime
 # code in the binding Core._parse
 
+function _set_core_parse_hook(parser)
+    # HACK! Fool the runtime into allowing us to set Core._parse, even during
+    # incremental compilation. (Ideally we'd just arrange for Core._parse to be
+    # set to the JuliaSyntax parser. But how do we signal that to the dumping
+    # code outside of the initial creation of Core?)
+    i = findfirst(==(:incremental), fieldnames(Base.JLOptions))
+    ptr = convert(Ptr{fieldtype(Base.JLOptions, i)},
+                  cglobal(:jl_options, Base.JLOptions) + fieldoffset(Base.JLOptions, i))
+    incremental = unsafe_load(ptr)
+    if incremental != 0
+        unsafe_store!(ptr, 0)
+    end
+
+    Base.eval(Core, :(_parse = $parser))
+
+    if incremental != 0
+        unsafe_store!(ptr, incremental)
+    end
+end
+
 # Use caller's world age.
 const _caller_world = typemax(UInt)
 const _parser_world_age = Ref{UInt}(_caller_world)
@@ -152,8 +172,7 @@ function enable_in_core!(enable=true; freeze_world_age = true,
         close(_debug_log[])
         _debug_log[] = nothing
     end
-    parser = enable ? core_parser_hook : _default_parser
-    Base.eval(Core, :(_parse = $parser))
+    _set_core_parse_hook(enable ? core_parser_hook : _default_parser)
     nothing
 end
 

sysimage/JuliaSyntaxCore/src/JuliaSyntaxCore.jl

@@ -5,22 +5,8 @@ module JuliaSyntaxCore
 
 using JuliaSyntax
 
-import Base: JLOptions
-
 function __init__()
-    # HACK! Fool the runtime into allowing us to set Core._parse, even during
-    # incremental compilation. (Ideally we'd just arrange for Core._parse to be
-    # set to the JuliaSyntax parser. But how do we signal that to the dumping
-    # code outside of the initial creation of Core?)
-    i = findfirst(==(:incremental), fieldnames(JLOptions))
-    ptr = convert(Ptr{fieldtype(JLOptions, i)},
-                  cglobal(:jl_options, JLOptions) + fieldoffset(JLOptions, i))
-    incremental = unsafe_load(ptr)
-    incremental == 0 || unsafe_store!(ptr, 0)
-
     JuliaSyntax.enable_in_core!()
-
-    incremental == 0 || unsafe_store!(ptr, incremental)
 end
 
 end