Backport justinbarclay/parinfer-rust-emacs@67924eefef

This is redone by using `saturating_add_signed` instead of having an
extra helper function. The function is stable since Rust 1.66

Author: gagbo

src/parinfer.rs

@@ -1,6 +1,7 @@
 use super::std;
 use std::collections::HashMap;
 use std::borrow::Cow;
+use std::convert::TryInto;
 use unicode_segmentation::UnicodeSegmentation;
 use unicode_width::UnicodeWidthStr;
 use types::*;
@@ -1556,7 +1557,7 @@ fn finish_new_paren_trail<'a>(result: &mut State<'a>) {
 
 fn add_indent<'a>(result: &mut State<'a>, delta: Delta) {
     let orig_indent = result.x;
-    let new_indent = (orig_indent as Delta + delta) as Column;
+    let new_indent = orig_indent.saturating_add_signed(delta.try_into().expect("Delta (i64) has values that fit in a usize"));
     let indent_str = repeat_string(BLANK_SPACE, new_indent);
     let line_no = result.line_no;
     replace_within_line(result, line_no, 0, orig_indent, &indent_str);