stage2: implement union coercion to its own tag

 * AIR: add `get_union_tag` instruction
   - implement in LLVM backend
 * Sema: implement == and != for union and enum literal
   - Also implement coercion from union to its own tag type
 * Value: implement hashing for union values

The motivating example is this snippet:

    comptime assert(@typeInfo(T) == .Float);

This was the next blocker for stage2 building compiler-rt.
Now it is switch at compile-time on an integer.

1 files changed, 14 insertions, 1 deletions

diff --git a/src/value.zig b/src/value.zig
index cb5d211b1e..69f8945e01 100644
--- a/src/value.zig
+++ b/src/value.zig
@@ -1275,7 +1275,12 @@ pub const Value = extern union {
                 }
             },
             .Union => {
-                @panic("TODO implement hashing union values");
+                const union_obj = val.castTag(.@"union").?.data;
+                if (ty.unionTagType()) |tag_ty| {
+                    union_obj.tag.hash(tag_ty, hasher);
+                }
+                const active_field_ty = ty.unionFieldType(union_obj.tag);
+                union_obj.val.hash(active_field_ty, hasher);
             },
             .Fn => {
                 @panic("TODO implement hashing function values");
@@ -1431,6 +1436,14 @@ pub const Value = extern union {
         }
     }
 
+    pub fn unionTag(val: Value) Value {
+        switch (val.tag()) {
+            .undef => return val,
+            .@"union" => return val.castTag(.@"union").?.data.tag,
+            else => unreachable,
+        }
+    }
+
     /// Returns a pointer to the element value at the index.
     pub fn elemPtr(self: Value, allocator: *Allocator, index: usize) !Value {
         if (self.castTag(.elem_ptr)) |elem_ptr| {