Count char width at most once in Formatter::pad

When both width and precision flags are specified, then the character
width is counted twice. Instead, record the character width when
truncating it to the precision, so it does not need to be recomputed.
Simplify control flow so the cases are more clear.

library/core/src/fmt/mod.rs

@@ -1693,49 +1693,41 @@ impl<'a> Formatter<'a> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn pad(&mut self, s: &str) -> Result {
-        // Make sure there's a fast path up front
+        // Make sure there's a fast path up front.
         if self.options.width.is_none() && self.options.precision.is_none() {
             return self.buf.write_str(s);
         }
-        // The `precision` field can be interpreted as a `max-width` for the
+
+        // The `precision` field can be interpreted as a maximum width for the
         // string being formatted.
-        let s = if let Some(max) = self.options.precision {
-            // If our string is longer that the precision, then we must have
-            // truncation. However other flags like `fill`, `width` and `align`
-            // must act as always.
-            if let Some((i, _)) = s.char_indices().nth(max) {
-                // LLVM here can't prove that `..i` won't panic `&s[..i]`, but
-                // we know that it can't panic. Use `get` + `unwrap_or` to avoid
-                // `unsafe` and otherwise don't emit any panic-related code
-                // here.
-                s.get(..i).unwrap_or(s)
-            } else {
-                &s
-            }
+        let (s, char_count) = if let Some(max_char_count) = self.options.precision {
+            let mut iter = s.char_indices();
+            let remaining = match iter.advance_by(max_char_count) {
+                Ok(()) => 0,
+                Err(remaining) => remaining.get(),
+            };
+            // SAFETY: The offset of `.char_indices()` is guaranteed to be
+            // in-bounds and between character boundaries.
+            let truncated = unsafe { s.get_unchecked(..iter.offset()) };
+            (truncated, max_char_count - remaining)
         } else {
-            &s
+            // Use the optimized char counting algorithm for the full string.
+            (s, s.chars().count())
         };
-        // The `width` field is more of a `min-width` parameter at this point.
-        match self.options.width {
-            // If we're under the maximum length, and there's no minimum length
-            // requirements, then we can just emit the string
-            None => self.buf.write_str(s),
-            Some(width) => {
-                let chars_count = s.chars().count();
-                // If we're under the maximum width, check if we're over the minimum
-                // width, if so it's as easy as just emitting the string.
-                if chars_count >= width {
-                    self.buf.write_str(s)
-                }
-                // If we're under both the maximum and the minimum width, then fill
-                // up the minimum width with the specified string + some alignment.
-                else {
-                    let align = Alignment::Left;
-                    let post_padding = self.padding(width - chars_count, align)?;
-                    self.buf.write_str(s)?;
-                    post_padding.write(self)
-                }
-            }
+
+        // The `width` field is more of a minimum width parameter at this point.
+        if let Some(width) = self.options.width
+            && char_count < width
+        {
+            // If we're under the minimum width, then fill up the minimum width
+            // with the specified string + some alignment.
+            let post_padding = self.padding(width - char_count, Alignment::Left)?;
+            self.buf.write_str(s)?;
+            post_padding.write(self)
+        } else {
+            // If we're over the minimum width or there is no minimum width, we
+            // can just emit the string.
+            self.buf.write_str(s)
         }
     }