stage2: basic inferred error set support

 * Inferred error sets are stored in the return Type of the function,
   owned by the Module.Fn. So it cleans up that memory in deinit().
 * Sema: update the inferred error set in zirRetErrValue
   - Update relevant code in wrapErrorUnion
 * C backend: improve some some instructions to take advantage of
   liveness analysis to avoid being emitted when unused.
 * C backend: when an error union has a payload type with no runtime
   bits, emit the error union as the same type as the error set.

1 files changed, 5 insertions, 2 deletions

diff --git a/src/type.zig b/src/type.zig
index f9385e90bc..e8f0998332 100644
--- a/src/type.zig
+++ b/src/type.zig
@@ -1041,7 +1041,7 @@ pub const Type = extern union {
                     return writer.writeAll(std.mem.spanZ(error_set.owner_decl.name));
                 },
                 .error_set_inferred => {
-                    const func = ty.castTag(.error_set_inferred).?.data;
+                    const func = ty.castTag(.error_set_inferred).?.data.func;
                     return writer.print("(inferred error set of {s})", .{func.owner_decl.name});
                 },
                 .error_set_single => {
@@ -3154,7 +3154,10 @@ pub const Type = extern union {
             pub const base_tag = Tag.error_set_inferred;
 
             base: Payload = Payload{ .tag = base_tag },
-            data: *Module.Fn,
+            data: struct {
+                func: *Module.Fn,
+                map: std.StringHashMapUnmanaged(void),
+            },
         };
 
         pub const Pointer = struct {