os: refactor err == IError(os.Eof{}) to err is os.Eof

vlib/os/file_test.v

@@ -144,7 +144,7 @@ fn test_read_eof_last_read_partial_buffer_fill() {
 		assert false
 	} else {
 		// Expected an error when received end-of-file.
-		assert err == IError(os.Eof{})
+		assert err is os.Eof
 	}
 	f.close()
 }
@@ -176,7 +176,7 @@ fn test_read_eof_last_read_full_buffer_fill() {
 		assert false
 	} else {
 		// Expect an error at EOF.
-		assert err == IError(os.Eof{})
+		assert err is os.Eof
 	}
 	f.close()
 }

vlib/os/os.c.v

@@ -89,7 +89,7 @@ fn slurp_file_in_builder(fp &C.FILE) !strings.Builder {
 	mut sb := strings.new_builder(os.buf_size)
 	for {
 		mut read_bytes := fread(&buf[0], 1, os.buf_size, fp) or {
-			if err == IError(Eof{}) {
+			if err is Eof {
 				break
 			}
 			unsafe { sb.free() }

vlib/os/os_test.v

@@ -194,7 +194,7 @@ fn test_write_and_read_bytes() {
 	// check that trying to read data from EOF doesn't error and returns 0
 	mut a := []u8{len: 5}
 	nread := file_read.read_bytes_into(5, mut a) or {
-		n := if err == IError(os.Eof{}) {
+		n := if err is os.Eof {
 			int(0)
 		} else {
 			eprintln(err)