Fix type shortening writing to file

Make sure that we append to the file for long ty paths. Do not write the same type more than once. Shorten the calculated width a bit.

compiler/rustc_middle/src/ty/error.rs

@@ -1,5 +1,7 @@
 use std::borrow::Cow;
+use std::fs::File;
 use std::hash::{DefaultHasher, Hash, Hasher};
+use std::io::{Read, Write};
 use std::path::PathBuf;
 
 use rustc_errors::pluralize;
@@ -250,8 +252,8 @@ impl<'tcx> TyCtxt<'tcx> {
         }
 
         let width = self.sess.diagnostic_width();
-        let length_limit = width.saturating_sub(30);
-        if regular.len() <= width {
+        let length_limit = width / 2;
+        if regular.len() <= width * 2 / 3 {
             return regular;
         }
         let short = self.ty_string_with_limit(ty, length_limit);
@@ -265,7 +267,20 @@ impl<'tcx> TyCtxt<'tcx> {
         *path = Some(path.take().unwrap_or_else(|| {
             self.output_filenames(()).temp_path_ext(&format!("long-type-{hash}.txt"), None)
         }));
-        match std::fs::write(path.as_ref().unwrap(), &format!("{regular}\n")) {
+        let Ok(mut file) =
+            File::options().create(true).read(true).append(true).open(&path.as_ref().unwrap())
+        else {
+            return regular;
+        };
+
+        // Do not write the same type to the file multiple times.
+        let mut contents = String::new();
+        let _ = file.read_to_string(&mut contents);
+        if let Some(_) = contents.lines().find(|line| line == &regular) {
+            return short;
+        }
+
+        match write!(file, "{regular}\n") {
             Ok(_) => short,
             Err(_) => regular,
         }