C gen: don't tag closure objects

Similar to boxed products, closures are not analyzed in pattern matching
based on a tag, so the tag word can be removed.

When we implement the support for closures in variants we'll need to
assign them tags based on their function types.

Also remove an old comment in lowering saying that integers don't have
tags. They do now (used in variants), they're just not tagged by
default.

Author: osa1

src/lowering.rs

@@ -368,7 +368,6 @@ pub enum BuiltinConDecl {
         t: mono::Type,
     },
 
-    // Integers are value types (not boxed), so they don't have tags.
     I8,
     U8,
     I32,

src/to_c.rs

@@ -70,13 +70,10 @@ pub(crate) fn to_c(pgm: &LoweredPgm, main: &str) -> String {
 
     // Generate the CLOSURE type before other types. CLOSURE is a special built-in that doesn't have
     // a TypeDecl entry, so it's not part of the dependency-sorted types.
-    let closure_tag = CLOSURE_CON_IDX.0;
-    wln!(p, "#define CLOSURE_TAG {closure_tag}");
     writedoc!(
         p,
         "
         typedef struct {{
-            uint64_t tag;
             void (*fun)(void);
             // captures here ...
         }} CLOSURE;
@@ -328,7 +325,7 @@ pub(crate) fn to_c(pgm: &LoweredPgm, main: &str) -> String {
 
                 w!(
                     p,
-                    "static CLOSURE _con_closure_{tag}_data = {{ .tag = CLOSURE_TAG, .fun = (void(*)(void))_con_closure_{tag}_fun }};",
+                    "static CLOSURE _con_closure_{tag}_data = {{ .fun = (void(*)(void))_con_closure_{tag}_fun }};",
                 );
                 p.nl();
 
@@ -379,7 +376,7 @@ pub(crate) fn to_c(pgm: &LoweredPgm, main: &str) -> String {
 
         w!(
             p,
-            "static CLOSURE _fun_closure_{i}_data = {{ .tag = CLOSURE_TAG, .fun = (void(*)(void))_fun_closure_{i}_fun }};",
+            "static CLOSURE _fun_closure_{i}_data = {{ .fun = (void(*)(void))_fun_closure_{i}_fun }};",
         );
         p.nl();
 
@@ -484,7 +481,6 @@ fn gen_closure_struct(closure: &Closure, idx: usize, pgm: &LoweredPgm, p: &mut P
     w!(p, "typedef struct {{");
     p.indent();
     p.nl();
-    wln!(p, "uint64_t tag;");
     wln!(p, "void (*fun)(void);");
     for (i, fv) in closure.fvs.iter().enumerate() {
         if i != 0 {
@@ -2098,15 +2094,13 @@ fn expr_to_c(expr: &Expr, loc: &Loc, locals: &[LocalInfo], cg: &mut Cg, p: &mut
             p.nl();
             if closure.fvs.is_empty() {
                 wln!(p, "CLOSURE* _clos = (CLOSURE*)malloc(sizeof(CLOSURE));");
-                wln!(p, "_clos->tag = CLOSURE_TAG;");
                 w!(p, "_clos->fun = (void(*)(void))_closure_{};", idx);
                 p.nl();
             } else {
                 wln!(
                     p,
                     "_Closure_{idx}* _clos = (_Closure_{idx}*)malloc(sizeof(_Closure_{idx}));"
                 );
-                wln!(p, "_clos->tag = CLOSURE_TAG;");
                 w!(p, "_clos->fun = (void(*)(void))_closure_{};", idx);
                 p.nl();
                 for (i, fv) in closure.fvs.iter().enumerate() {