Allow caller to specify whether to eval an unlowerable thunk (#398)

While I'm not aware of any problems caused by the former setting,
when frames are being created for the purpose of analysis rather than
execution it seems worth being able to control the evaluation.

src/construct.jl

@@ -353,7 +353,7 @@ end
 Prepare `expr` for evaluation in `mod`. `expr` should be a "straightforward" expression,
 one that does not require special top-level handling (see [`JuliaInterpreter.split_expressions`](@ref)).
 """
-function prepare_thunk(mod::Module, thunk::Expr, recursive::Bool=false)
+function prepare_thunk(mod::Module, thunk::Expr, recursive::Bool=false; eval::Bool=true)
     if isexpr(thunk, :thunk)
         framecode = FrameCode(mod, thunk.args[1])
     elseif isexpr(thunk, :error) || isexpr(thunk, :incomplete)
@@ -362,13 +362,13 @@ function prepare_thunk(mod::Module, thunk::Expr, recursive::Bool=false)
         thunk = Meta.lower(mod, thunk)
         if isa(thunk, Expr)
             # If on 2nd attempt to lower it's still an Expr, just evaluate it
-            Core.eval(mod, thunk)
+            eval && Core.eval(mod, thunk)
             return nothing
         end
         framecode = FrameCode(mod, thunk.args[1])
     else
         lwr = Meta.lower(mod, thunk)
-        isa(lwr, Expr) && return prepare_thunk(mod, lwr, true)
+        isa(lwr, Expr) && return prepare_thunk(mod, lwr, true; eval=eval)
         return nothing
     end
     return Frame(framecode, prepare_framedata(framecode, []))

test/toplevel.jl

@@ -44,6 +44,8 @@ end
 
     @test JuliaInterpreter.prepare_thunk(Main, :(export foo)) === nothing
     @test JuliaInterpreter.prepare_thunk(Base.Threads, :(global Condition)) === nothing
+    @test_throws ArgumentError JuliaInterpreter.prepare_thunk(Main, :(using NoPkgOfThisName))
+    @test JuliaInterpreter.prepare_thunk(Main, :(using NoPkgOfThisName); eval=false) === nothing
 
     @test !isdefined(Main, :JIInvisible)
     JuliaInterpreter.split_expressions(JIVisible, :(module JIInvisible f() = 1 end))