ability to cast explicitly from int to enum

This commit also fixes a bug where pure functions are marked with
the read-only attribute in debug mode. This resulted in incorrect
codegen because calls to read-only functions with unused values
were not generated.

For example, a call to assert() would not be generated if assert
is marked with read-only. Which it *is* marked with in release
mode.

4 files changed, 22 insertions, 1 deletions

diff --git a/src/all_types.hpp b/src/all_types.hpp
index 7e0655eacf..329bbb4249 100644
--- a/src/all_types.hpp
+++ b/src/all_types.hpp
@@ -383,6 +383,7 @@ enum CastOp {
     CastOpFloatToInt,
     CastOpBoolToInt,
     CastOpResizeSlice,
+    CastOpIntToEnum,
 };
 
 struct AstNodeFnCallExpr {
diff --git a/src/analyze.cpp b/src/analyze.cpp
index 26c85ea68f..8e3ce507f8 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -4428,6 +4428,14 @@ static TypeTableEntry *analyze_cast_expr(CodeGen *g, ImportTableEntry *import, B
         }
     }
 
+    // explicit cast from integer to enum type with no payload
+    if (actual_type->id == TypeTableEntryIdInt &&
+        wanted_type->id == TypeTableEntryIdEnum &&
+        wanted_type->data.enumeration.gen_field_count == 0)
+    {
+        return resolve_cast(g, context, node, expr_node, wanted_type, CastOpIntToEnum, false);
+    }
+
     add_node_error(g, node,
         buf_sprintf("invalid cast from type '%s' to '%s'",
             buf_ptr(&actual_type->name),
diff --git a/src/codegen.cpp b/src/codegen.cpp
index b25a3b6869..43fc41bed9 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -1032,6 +1032,8 @@ static LLVMValueRef gen_cast_expr(CodeGen *g, AstNode *node) {
             set_debug_source_node(g, node);
             return LLVMBuildZExt(g->builder, expr_val, wanted_type->type_ref, "");
 
+        case CastOpIntToEnum:
+            return gen_widen_or_shorten(g, node, actual_type, wanted_type->data.enumeration.tag_type, expr_val);
     }
     zig_unreachable();
 }
@@ -3913,7 +3915,7 @@ static void do_code_gen(CodeGen *g) {
             LLVMZigAddNonNullAttr(fn_table_entry->fn_value, 1);
             is_sret = true;
         }
-        if (fn_table_entry->is_pure && !is_sret) {
+        if (fn_table_entry->is_pure && !is_sret && g->is_release_build) {
             LLVMAddFunctionAttr(fn_table_entry->fn_value, LLVMReadOnlyAttribute);
         }
 
diff --git a/src/eval.cpp b/src/eval.cpp
index ca53388386..f7ef68467a 100644
--- a/src/eval.cpp
+++ b/src/eval.cpp
@@ -657,6 +657,16 @@ void eval_const_expr_implicit_cast(CastOp cast_op,
             bignum_init_unsigned(&const_val->data.x_bignum, other_val->data.x_bool ? 1 : 0);
             const_val->ok = true;
             break;
+        case CastOpIntToEnum:
+            {
+                uint64_t value = other_val->data.x_bignum.data.x_uint;
+                assert(new_type->id == TypeTableEntryIdEnum);
+                assert(value < new_type->data.enumeration.field_count);
+                const_val->data.x_enum.tag = value;
+                const_val->data.x_enum.payload = NULL;
+                const_val->ok = true;
+                break;
+            }
     }
 }