diff --git a/src/doc/man/rustc.1 b/src/doc/man/rustc.1
index 534af3f85bb96..0cd2ff9899cce 100644
--- a/src/doc/man/rustc.1
+++ b/src/doc/man/rustc.1
@@ -62,7 +62,7 @@ Comma separated list of compiler information to print on stdout.
 Equivalent to \fI\-C\ debuginfo=2\fR.
 .TP
 \fB\-O\fR
-Equivalent to \fI\-C\ opt\-level=2\fR.
+Equivalent to \fI\-C\ opt\-level=3\fR.
 .TP
 \fB\-o\fR \fIFILENAME\fR
 Write output to \fIFILENAME\fR. Ignored if multiple \fI\-\-emit\fR outputs are specified which
diff --git a/src/tools/update-rustc-man-opt-level.py b/src/tools/update-rustc-man-opt-level.py
new file mode 100755
index 0000000000000..14f51003c8472
--- /dev/null
+++ b/src/tools/update-rustc-man-opt-level.py
@@ -0,0 +1,282 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: MIT OR Apache-2.0
+r"""
+Update the rustc manpage "-O" description to match `rustc --help`.
+
+Usage (dry-run by default):
+  ./src/tools/update-rustc-man-opt-level.py \
+      --man-file src/doc/man/rustc.1
+
+Apply changes (creates a timestamped backup):
+  ./src/tools/update-rustc-man-opt-level.py \
+      --man-file src/doc/man/rustc.1 --apply
+
+Force a level instead of querying rustc:
+  ./src/tools/update-rustc-man-opt-level.py \
+      --man-file ... --expected-level 3 --apply
+"""
+
+from __future__ import annotations
+
+import argparse
+import datetime
+import difflib
+import shutil
+import subprocess
+import sys
+import re
+from pathlib import Path
+
+DEFAULT_RUSTC = "rustc"
+
+# ANSI color codes
+_CLR = {
+    "reset": "\033[0m",
+    "red": "\033[31m",
+    "green": "\033[32m",
+    "yellow": "\033[33m",
+    "bold": "\033[1m",
+}
+
+
+def colorize(line: str, color: str, enabled: bool) -> str:
+    if not enabled or color not in _CLR:
+        return line
+    return f"{_CLR[color]}{line}{_CLR['reset']}"
+
+
+def get_rustc_opt_level(rustc_cmd: str = DEFAULT_RUSTC) -> int:
+    """Query `rustc --help` and parse the opt-level mapped to -O."""
+    try:
+        proc = subprocess.run(
+            [rustc_cmd, "--help"], capture_output=True, text=True, check=True
+        )
+    except FileNotFoundError:
+        raise RuntimeError(f"rustc not found at '{rustc_cmd}'")
+    except subprocess.CalledProcessError as e:
+        stderr = (e.stderr or "").strip()
+        raise RuntimeError(f"rustc --help failed: {stderr or e}") from e
+
+    help_text = (proc.stdout or "") + "\n" + (proc.stderr or "")
+    # Fixed: removed unnecessary \\ escape before the hyphen
+    m = re.search(r"-O[^\n]*opt(?:-)?level\s*=\s*(\d+)", help_text, flags=re.IGNORECASE)
+    if not m:
+        m2 = re.search(
+            r"Equivalent to\s+-C\s+opt(?:-)?level\s*=\s*(\d+)",
+            help_text,
+            flags=re.IGNORECASE,
+        )
+        if not m2:
+            raise RuntimeError(
+                "Could not find '-O' opt-level mapping in rustc --help output"
+            )
+        return int(m2.group(1))
+    return int(m.group(1))
+
+
+def find_and_replace_manpage_content(
+    content: str, new_level: int
+) -> tuple[str, int, bool]:
+    r"""
+    Replace opt-level numbers in 'Equivalent to ... opt-level=N.'
+    sentences tied to -O.
+
+    Conservative heuristic:
+      - Locate sentences starting with 'Equivalent to' up to the
+        next period.
+      - Ensure the sentence mentions opt-level (accepting escaped
+        '\-').
+      - Confirm a -O header appears within a lookback window before
+        the sentence.
+
+    Returns:
+        tuple of (new_content, replacements_made, found_any_patterns)
+    """
+    replacements = 0
+    found_patterns = False
+    out_parts = []
+    last_index = 0
+
+    # More conservative limit of 200 chars instead of 800
+    sentence_pattern = re.compile(
+        r"Equivalent to([^\n\.]{0,200}?)\.", flags=re.IGNORECASE
+    )
+
+    for m in sentence_pattern.finditer(content):
+        start, end = m.span()
+        sentence = m.group(0)
+
+        if not re.search(r"opt(?:\\-)?level", sentence, flags=re.IGNORECASE):
+            continue
+
+        num_match = re.search(r"(\d+)", sentence)
+        if not num_match:
+            continue
+        old_level = int(num_match.group(1))
+
+        window_start = max(0, start - 1200)
+        window = content[window_start:start]
+
+        # Use any() for better readability
+        if not any(
+            [
+                re.search(r"(^|\n)\s*-O\b", window),
+                re.search(r"\\fB\\-?O\\fR", window),
+                re.search(r"\\-O\b", window),
+                re.search(r"\.B\s+-O\b", window),
+                re.search(r"\\fB-?O\\fP", window),
+            ]
+        ):
+            continue
+
+        # We found at least one -O entry with opt-level
+        found_patterns = True
+
+        if old_level == new_level:
+            continue
+
+        # More robust: replace only the number after opt-level=
+        new_sentence = re.sub(
+            r"(opt(?:\\-)?level\s*=\s*)\d+",
+            rf"\g<1>{new_level}",
+            sentence,
+            count=1,
+        )
+        out_parts.append(content[last_index:start])
+        out_parts.append(new_sentence)
+        last_index = end
+        replacements += 1
+
+    out_parts.append(content[last_index:])
+    return "".join(out_parts), replacements, found_patterns
+
+
+def show_colored_diff(old: str, new: str, filename: str, color: bool) -> None:
+    # Use keepends=False and lineterm="\n" for consistency
+    old_lines = old.splitlines(keepends=False)
+    new_lines = new.splitlines(keepends=False)
+    diff_iter = difflib.unified_diff(
+        old_lines,
+        new_lines,
+        fromfile=filename,
+        tofile=filename + " (updated)",
+        lineterm="\n",
+    )
+    for line in diff_iter:
+        if line.startswith("---") or line.startswith("+++"):
+            print(colorize(line, "bold", color))
+        elif line.startswith("@@"):
+            print(colorize(line, "yellow", color))
+        elif line.startswith("+"):
+            print(colorize(line, "green", color))
+        elif line.startswith("-"):
+            print(colorize(line, "red", color))
+        else:
+            print(line)
+
+
+def backup_file(path: Path) -> Path:
+    # Added microseconds to avoid collision if run multiple times
+    # per second
+    ts = datetime.datetime.now().strftime("%Y%m%dT%H%M%S%f")
+    backup = path.with_name(path.name + f".bak.{ts}")
+    shutil.copy2(path, backup)
+    return backup
+
+
+def parse_args() -> argparse.Namespace:
+    p = argparse.ArgumentParser(
+        description=("Update rustc man page -O opt-level to match rustc --help")
+    )
+    p.add_argument(
+        "--man-file",
+        "-m",
+        required=True,
+        help=("Path to rustc man page file to update (e.g. src/doc/man/rustc.1)"),
+    )
+    p.add_argument(
+        "--rustc-cmd",
+        default=DEFAULT_RUSTC,
+        help="rustc binary to query (default: rustc)",
+    )
+    p.add_argument(
+        "--expected-level",
+        "-e",
+        type=int,
+        help="Use this level instead of querying rustc",
+    )
+    p.add_argument(
+        "--apply",
+        action="store_true",
+        help=(
+            "Write changes to the man file (creates a backup). "
+            "Without this flag runs in dry-run mode and prints "
+            "a diff."
+        ),
+    )
+    p.add_argument("--no-color", action="store_true", help="Disable colored output")
+    return p.parse_args()
+
+
+def main() -> int:
+    args = parse_args()
+    color = (not args.no_color) and sys.stdout.isatty()
+
+    man_path = Path(args.man_file)
+    if not man_path.exists():
+        print(f"Error: man file not found: {man_path}", file=sys.stderr)
+        return 2
+
+    try:
+        new_level = (
+            args.expected_level
+            if args.expected_level is not None
+            else get_rustc_opt_level(args.rustc_cmd)
+        )
+    except RuntimeError as e:
+        print(f"Error determining rustc opt-level: {e}", file=sys.stderr)
+        return 3
+
+    try:
+        content = man_path.read_text(encoding="utf-8")
+    except Exception as e:
+        print(f"Error reading man file {man_path}: {e}", file=sys.stderr)
+        return 4
+
+    new_content, replacements, found_patterns = find_and_replace_manpage_content(
+        content, new_level
+    )
+
+    if replacements == 0:
+        if found_patterns:
+            print(f"✓ Manpage is already up-to-date (opt-level={new_level}).")
+        else:
+            print(
+                "Warning: Could not find -O entry with "
+                "'Equivalent to -C opt-level=' pattern in manpage.",
+                file=sys.stderr,
+            )
+            return 6
+        return 0
+
+    header = f"Found {replacements} replacement(s). Proposed changes:"
+    print(colorize(header, "bold", color))
+    show_colored_diff(content, new_content, str(man_path), color)
+
+    if args.apply:
+        try:
+            backup = backup_file(man_path)
+            man_path.write_text(new_content, encoding="utf-8")
+            msg = f"\nApplied changes to {man_path}. Backup saved to {backup}"
+            print(colorize(msg, "green", color))
+        except Exception as e:
+            print(f"Error writing updated man file: {e}", file=sys.stderr)
+            return 5
+    else:
+        print("\nDry-run only. Use --apply to write changes to disk.")
+
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())