Sema: support pointer subtraction

test/behavior/pointers.zig

@@ -17,7 +17,7 @@ fn testDerefPtr() !void {
     try expect(x == 1235);
 }
 
-test "pointer arithmetic" {
+test "pointer-integer arithmetic" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
@@ -43,6 +43,62 @@ test "pointer arithmetic" {
     try expect(ptr[0] == 'a');
 }
 
+test "pointer subtraction" {
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_riscv64) return error.SkipZigTest;
+
+    {
+        const a: *u8 = @ptrFromInt(100);
+        const b: *u8 = @ptrFromInt(50);
+        try expect(a - b == 50);
+    }
+    {
+        var ptr: [*]const u8 = "abc";
+        try expect(&ptr[1] - &ptr[0] == 1);
+        try expect(&ptr[2] - &ptr[0] == 2);
+    }
+    {
+        const a: *[100]u16 = @ptrFromInt(100);
+        const b: *[100]u16 = @ptrFromInt(50);
+        try expect(a - b == 25);
+    }
+    {
+        var x: struct { a: u32, b: u32 } = undefined;
+        const a = &x.a;
+        const b = &x.b;
+        try expect(a - a == 0);
+        try expect(b - b == 0);
+        try expect(b - a == 1);
+    }
+    comptime {
+        var x: packed struct { a: u1, b: u1 } = undefined;
+        const a = &x.a;
+        const b = &x.b;
+        try expect(a - a == 0);
+        try expect(b - b == 0);
+        try expect(b - a == 0);
+    }
+    comptime {
+        var x: extern struct { a: u32, b: u32 } = undefined;
+        const a = &x.a;
+        const b = &x.b;
+        try expect(a - a == 0);
+        try expect(b - b == 0);
+        try expect(b - a == 1);
+    }
+    comptime {
+        const a: *const [3]u8 = "abc";
+        const b: [*]const u8 = @ptrCast(a);
+        try expect(&a[1] - &b[0] == 1);
+    }
+    comptime {
+        var x: [64][64]u8 = undefined;
+        const a = &x[0][12];
+        const b = &x[15][3];
+        try expect(b - a == 951);
+    }
+}
+
 test "double pointer parsing" {
     comptime assert(PtrOf(PtrOf(i32)) == **i32);
 }
@@ -382,7 +438,7 @@ test "pointer to array at fixed address" {
     try expect(@intFromPtr(&array[1]) == 0x14);
 }
 
-test "pointer arithmetic affects the alignment" {
+test "pointer-integer arithmetic affects the alignment" {
     {
         var ptr: [*]align(8) u32 = undefined;
         var x: usize = 1;