fish_indent: fix accidentally quadratic perf regression due to chars().last()

Iterator::last() consumes the entire iterator, even for DoubleEndedIterator,
see https://github.com/rust-lang/rust/pull/28125#issuecomment-145070161

Because of this, "at_line_start()" took 90% of

    fish_indent share/completions/git.fish

making it take 1000ms instead of 30 ms. Fix that.

fish-rust/src/common.rs

@@ -545,7 +545,7 @@ fn unescape_string_internal(input: &wstr, flags: UnescapeFlags) -> Option<WStrin
                         // In general, this is ANY_STRING. But as a hack, if the last appended char
                         // is ANY_STRING, delete the last char and store ANY_STRING_RECURSIVE to
                         // reflect the fact that ** is the recursive wildcard.
-                        if result.chars().last() == Some(ANY_STRING) {
+                        if result.chars().next_back() == Some(ANY_STRING) {
                             assert!(!result.is_empty());
                             result.truncate(result.len() - 1);
                             to_append_or_none = Some(ANY_STRING_RECURSIVE);

fish-rust/src/fish_indent.rs

@@ -319,7 +319,7 @@ impl<'source, 'ast> PrettyPrinterState<'source, 'ast> {
 
     // \return whether we are at the start of a new line.
     fn at_line_start(&self) -> bool {
-        self.output.chars().last().is_none_or(|c| c == '\n')
+        self.output.chars().next_back().is_none_or(|c| c == '\n')
     }
 
     // \return whether we have a space before the output.

fish-rust/src/highlight.rs

@@ -782,7 +782,7 @@ pub fn is_potential_path(
             return false;
         }
         let mut abs_path = path_apply_working_directory(&clean_potential_path_fragment, wd);
-        let must_be_full_dir = abs_path.chars().last() == Some('/');
+        let must_be_full_dir = abs_path.chars().next_back() == Some('/');
         if flags.contains(PathFlags::PATH_FOR_CD) {
             abs_path = normalize_path(&abs_path, /*allow_leading_double_slashes=*/ true);
         }

fish-rust/src/path.rs

@@ -340,7 +340,7 @@ pub fn path_get_cdpath(dir: &wstr, wd: &wstr, vars: &dyn Environment) -> Option<
     if dir.is_empty() {
         return None;
     }
-    assert!(wd.chars().last() == Some('/'));
+    assert!(wd.chars().next_back() == Some('/'));
     let paths = path_apply_cdpath(dir, wd, vars);
 
     for a_dir in paths {

fish-rust/src/reader.rs

@@ -3798,7 +3798,7 @@ fn get_autosuggestion_performer(
         // Here we do something a little funny. If the line ends with a space, and the cursor is not
         // at the end, don't use completion autosuggestions. It ends up being pretty weird seeing
         // stuff get spammed on the right while you go back to edit a line
-        let last_char = search_string.chars().last().unwrap();
+        let last_char = search_string.chars().next_back().unwrap();
         let cursor_at_end = cursor_pos == search_string.len();
         if !cursor_at_end && last_char.is_whitespace() {
             return nothing;
@@ -4693,7 +4693,7 @@ impl ReaderData {
 
         // Historical behavior is to trim trailing spaces, unless escape (#7661).
         let mut text = self.command_line.text().to_owned();
-        while text.chars().last() == Some(' ')
+        while text.chars().next_back() == Some(' ')
             && count_preceding_backslashes(&text, text.len() - 1) % 2 == 0
         {
             text.pop();

fish-rust/src/wildcard.rs

@@ -632,7 +632,7 @@ mod expander {
                 // ANY_STRING_RECURSIVE character is present in both the head and the tail.
                 let head_any = wc_segment.slice_to(asr_idx + 1);
                 let any_tail = wc.slice_from(asr_idx);
-                assert!(head_any.chars().last().unwrap() == ANY_STRING_RECURSIVE);
+                assert!(head_any.chars().next_back().unwrap() == ANY_STRING_RECURSIVE);
                 assert!(any_tail.chars().next().unwrap() == ANY_STRING_RECURSIVE);
 
                 dir.rewind();