Add hook for testing JuliaLowering in core

Author: mlechu

src/JuliaLowering.jl

@@ -33,6 +33,7 @@ _include("syntax_macros.jl")
 
 _include("eval.jl")
 _include("compat.jl")
+_include("hooks.jl")
 
 function __init__()
     _register_kinds()

src/hooks.jl

@@ -0,0 +1,46 @@
+"""
+Becomes `Core._lower()` upon activating JuliaLowering.
+
+Returns an svec with the lowered code (usually expr) as its first element, and
+(until integration is less experimental) whatever we want after it
+"""
+function core_lowering_hook(@nospecialize(code), mod::Module,
+                            file="none", line=0, world=typemax(Csize_t), warn=false)
+    if !(code isa SyntaxTree || code isa Expr)
+        # e.g. LineNumberNode, integer...
+        return Core.svec(code)
+    end
+    st0 = code isa Expr ? expr_to_syntaxtree(code) : code
+    try
+        ctx1, st1 = expand_forms_1(  mod,  st0)
+        ctx2, st2 = expand_forms_2(  ctx1, st1)
+        ctx3, st3 = resolve_scopes(  ctx2, st2)
+        ctx4, st4 = convert_closures(ctx3, st3)
+        ctx5, st5 = linearize_ir(    ctx4, st4)
+        ex = to_lowered_expr(mod, st5)
+        return Core.svec(ex, st5, ctx5)
+    catch exc
+        @error("JuliaLowering failed — falling back to flisp!",
+               exception=(exc,catch_backtrace()),
+               code=code, file=file, line=line, mod=mod)
+        return Base.fl_lower(code, mod, file, line, world, warn)
+    end
+end
+
+# TODO: Write a parser hook here.  The input to `core_lowering_hook` should
+# eventually be a (convertible to) SyntaxTree, but we need to make updates to
+# the parsing API to include a parameter for AST type.
+
+const _has_v1_13_hooks = isdefined(Core, :_lower)
+
+function activate!(enable=true)
+    if !_has_v1_13_hooks
+        error("Cannot use JuliaLowering without `Core._lower` binding or in $VERSION < 1.13")
+    end
+
+    if enable
+        Core._setlowerer!(core_lowering_hook)
+    else
+        Core._setlowerer!(Base.fl_lower)
+    end
+end