RangeIndex with datetime64 bounds cannot be printed (Unknown format code 'g'`) #11598

What happened?

A RangeIndex built from datetime64 start/stop and a timedelta64 step works: labels are computed correctly, and isel and sel(method="nearest") keep the index lazy. But its repr formats start, stop and step with :.3g, which only real numbers accept, so printing the index, or any DataArray/Dataset which holds it, raises.

A lazy time axis is the main reason we want RangeIndex: a long, evenly sampled acquisition has 1e8+ time labels and a handful of channels, so the time coordinate is the one worth not materializing.

What did you expect to happen?

The repr to print, as it does for a float range, e.g.

Indexes:
    time     RangeIndex (start=2023-01-01T00:00:00.000000000, stop=2023-01-01T00:00:10.000000000, step=1 seconds)

Minimal Complete Verifiable Example

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "xarray[complete]@git+https://github.com/pydata/xarray.git@main",
# ]
# ///
#
# This script automatically imports the development branch of xarray to check for issues.
# Please delete this header if you have _not_ tested this script with `uv run`!

import xarray as xr
import numpy as np
from xarray.indexes import RangeIndex
xr.show_versions()

t0 = np.datetime64("2023-01-01", "ns")
step = np.timedelta64(1, "s")
index = RangeIndex.arange(t0, t0 + 10 * step, step, dim="time")
da = xr.DataArray(np.zeros(10), dims="time", coords=xr.Coordinates.from_xindex(index))

print(da.time.values[:2])  # ['2023-01-01T00:00:00.000000000' '2023-01-01T00:00:01.000000000']
print(da)

Steps to reproduce

Just run it

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

Anything else we need to know?

No response

Environment