Move most latestworlds to linearization

From the docs:
```
The following statements raise the current world age:
    1. An explicit invocation of Core.@latestworld
    2. The start of every top-level statement
    3. The start of every REPL prompt
    4. Any type or struct definition
    5. Any method definition
    6. Any constant declaration
    7. Any global variable declaration (but not a global variable assignment)
    8. Any using, import, export or public statement
    9. Certain other macros like eval (depends on the macro implementation)
```

This commit handles each case as follows:

```
    1. = 9
    2. I'm not sure this actually happens (or needs to happen, unless we're
       being defensive? Doing it after each world-changing operation should
       suffice).  But if we need it, this would just be emitting once at the
       beginning of every lowered output.
    3. = 2
    4. = 6
    5. Emit seeing `method` in linearize
    6. Emit seeing `constdecl` in linearize
    7. Emit seeing `global` or `globaldecl` in linearize
    8. We just defer to `eval`, but should probably go in desugaring later
       - using/import recently became builtin calls, and I haven't
         updated JL to use them yet.  Base._import_using has an expr-based
         API that may change, and our importpath destructuring is worth keeping.
       - export and public (special forms) are handled in toplevel.c
    9. Done for us
```

Other quirks:

- `JuliaLowering.eval_closure_type` calls eval to assign a const, so we still
    need to deal with that in closure conversion.

- The `include` hack isn't mentioned in the docs, but can stay in desugaring.
      I'm not certain why we don't do the same for non-macro `eval`.

Author: mlechu

src/desugaring.jl

@@ -1174,7 +1174,6 @@ function expand_unionall_def(ctx, srcref, lhs, rhs, is_const=true)
         @ast ctx srcref [K"block"
             rr := [K"where" rhs lhs[2:end]...]
             [is_const ? K"constdecl" : K"assign_or_constdecl_if_global" name rr]
-            [K"latestworld_if_toplevel"]
             [K"removable" rr]
         ]
     )
@@ -1229,7 +1228,6 @@ function expand_assignment(ctx, ex, is_const=false)
             @ast ctx ex [K"block"
                 rr := expand_forms_2(ctx, rhs)
                 [K"constdecl" lhs rr]
-                [K"latestworld"]
                 [K"removable" rr]
             ]
         else
@@ -2293,7 +2291,6 @@ function method_def_expr(ctx, srcref, callex_srcref, method_table,
                 ret_var  # might be `nothing` and hence removed
             ]
         ]
-        [K"latestworld"]
         [K"removable" method_metadata]
     ]
 end
@@ -2422,12 +2419,10 @@ function expand_function_generator(ctx, srcref, callex_srcref, func_name, func_n
     # Code generator definition
     gen_func_method_defs = @ast ctx srcref [K"block"
         [K"function_decl" gen_name]
-        [K"latestworld_if_toplevel"]
         [K"scope_block"(scope_type=:hard)
             [K"method_defs"
                 gen_name
                 [K"block"
-                    [K"latestworld_if_toplevel"]
                     method_def_expr(ctx, srcref, callex_srcref, nothing, SyntaxList(ctx),
                                     gen_arg_names, gen_arg_types, gen_body, nothing)
                 ]
@@ -2761,7 +2756,6 @@ function keyword_function_defs(ctx, srcref, callex_srcref, name_str, typevar_nam
 
     kw_func_method_defs = @ast ctx srcref [K"block"
         [K"function_decl" body_func_name]
-        [K"latestworld"]
         [K"scope_block"(scope_type=:hard)
             [K"method_defs"
                 body_func_name
@@ -2831,7 +2825,6 @@ function expand_function_def(ctx, ex, docs, rewrite_call=identity, rewrite_body=
         end
         return @ast ctx ex [K"block"
             [K"function_decl" name]
-            [K"latestworld"]
             name
         ]
     end
@@ -3068,7 +3061,6 @@ function expand_function_def(ctx, ex, docs, rewrite_call=identity, rewrite_body=
         end
         gen_func_method_defs
         kw_func_method_defs
-        [K"latestworld_if_toplevel"]
         [K"scope_block"(scope_type=:hard)
             [K"method_defs"
                 isnothing(bare_func_name) ? "nothing"::K"core" : bare_func_name
@@ -3386,7 +3378,6 @@ function expand_abstract_or_primitive_type(ctx, ex)
             nothing_(ctx, ex)
             [K"constdecl" name newtype_var]
         ]
-        [K"latestworld"]
         nothing_(ctx, ex)
     ]
 end
@@ -3961,7 +3952,6 @@ function expand_struct_def(ctx, ex, docs)
                     global_struct_name
                     newdef
                  ]
-                [K"latestworld"]
                 # Default constructors
                 if isempty(inner_defs)
                     default_inner_constructors(ctx, ex, global_struct_name,

src/linear_ir.jl

@@ -657,7 +657,7 @@ function compile(ctx::LinearIRContext, ex, needs_value, in_tail_pos)
         end
     elseif k == K"=" || k == K"constdecl"
         lhs = ex[1]
-        if kind(lhs) == K"Placeholder"
+        res = if kind(lhs) == K"Placeholder"
             compile(ctx, ex[2], needs_value, in_tail_pos)
         else
             rhs = compile(ctx, ex[2], true, false)
@@ -674,6 +674,8 @@ function compile(ctx::LinearIRContext, ex, needs_value, in_tail_pos)
                 emit_assignment(ctx, ex, lhs, rhs, k)
             end
         end
+        k == K"constdecl" && emit_latestworld(ctx, ex)
+        res
     elseif k == K"block" || k == K"scope_block"
         nc = numchildren(ex)
         if nc == 0
@@ -782,7 +784,7 @@ function compile(ctx::LinearIRContext, ex, needs_value, in_tail_pos)
         # TODO
         # throw(LoweringError(ex,
         #     "Global method definition needs to be placed at the top level, or use `eval`"))
-        if numchildren(ex) == 1
+        res = if numchildren(ex) == 1
             if in_tail_pos
                 emit_return(ctx, ex)
             elseif needs_value
@@ -807,6 +809,8 @@ function compile(ctx::LinearIRContext, ex, needs_value, in_tail_pos)
             @assert !needs_value && !in_tail_pos
             nothing
         end
+        emit_latestworld(ctx, ex)
+        res
     elseif k == K"opaque_closure_method"
         @ast ctx ex [K"opaque_closure_method"
             ex[1]

test/assignments_ir.jl

@@ -161,8 +161,7 @@ end
 7   (call core.apply_type %₄ %₅ %₆)
 8   (call core.UnionAll %₃ %₇)
 9   (= slot₁/X %₈)
-10  latestworld
-11  (return %₈)
+10  (return %₈)
 
 ########################################
 # Error: Invalid lhs in `=`