df: add support for --total option

Add support for the `--total` option to `df`, which displays the total
of each numeric column. For example,

    $ df --total
    Filesystem            1K-blocks     Used Available Use% Mounted on
    udev                    3858016        0   3858016   0% /dev
    ...
    /dev/loop14               63488    63488         0 100% /snap/core20/1361
    total                 258775268 98099712 148220200  40% -

src/uu/df/src/df.rs

@@ -106,6 +106,11 @@ impl From<&ArgMatches> for BlockSize {
     }
 }
 
+/// Parameters that control the behavior of `df`.
+///
+/// Most of these parameters control which rows and which columns are
+/// displayed. The `block_size` determines the units to use when
+/// displaying numbers of bytes or inodes.
 #[derive(Default)]
 struct Options {
     show_local_fs: bool,
@@ -115,6 +120,9 @@ struct Options {
     show_inode_instead: bool,
     block_size: BlockSize,
     fs_selector: FsSelector,
+
+    /// Whether to show a final row comprising the totals for each column.
+    show_total: bool,
 }
 
 impl Options {
@@ -128,6 +136,7 @@ impl Options {
             show_inode_instead: matches.is_present(OPT_INODES),
             block_size: BlockSize::from(matches),
             fs_selector: FsSelector::from(matches),
+            show_total: matches.is_present(OPT_TOTAL),
         }
     }
 }
@@ -307,9 +316,15 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         .filter(|fs| fs.usage.blocks != 0 || opt.show_all_fs || opt.show_listed_fs)
         .map(Into::into)
         .collect();
+
     println!("{}", Header::new(&opt));
+    let mut total = Row::new("total");
     for row in data {
         println!("{}", DisplayRow::new(&row, &opt));
+        total += row;
+    }
+    if opt.show_total {
+        println!("{}", DisplayRow::new(&total, &opt));
     }
 
     Ok(())

src/uu/df/src/table.rs

@@ -15,6 +15,7 @@ use crate::{BlockSize, Filesystem, Options};
 use uucore::fsext::{FsUsage, MountInfo};
 
 use std::fmt;
+use std::ops::AddAssign;
 
 /// A row in the filesystem usage data table.
 ///
@@ -67,6 +68,63 @@ pub(crate) struct Row {
     inodes_usage: Option<f64>,
 }
 
+impl Row {
+    pub(crate) fn new(source: &str) -> Self {
+        Self {
+            fs_device: source.into(),
+            fs_type: "-".into(),
+            fs_mount: "-".into(),
+            bytes: 0,
+            bytes_used: 0,
+            bytes_free: 0,
+            bytes_usage: None,
+            #[cfg(target_os = "macos")]
+            bytes_capacity: None,
+            inodes: 0,
+            inodes_used: 0,
+            inodes_free: 0,
+            inodes_usage: None,
+        }
+    }
+}
+
+impl AddAssign for Row {
+    /// Sum the numeric values of two rows.
+    ///
+    /// The `Row::fs_device` field is set to `"total"` and the
+    /// remaining `String` fields are set to `"-"`.
+    fn add_assign(&mut self, rhs: Self) {
+        let bytes = self.bytes + rhs.bytes;
+        let bytes_used = self.bytes_used + rhs.bytes_used;
+        let inodes = self.inodes + rhs.inodes;
+        let inodes_used = self.inodes_used + rhs.inodes_used;
+        *self = Self {
+            fs_device: "total".into(),
+            fs_type: "-".into(),
+            fs_mount: "-".into(),
+            bytes,
+            bytes_used,
+            bytes_free: self.bytes_free + rhs.bytes_free,
+            bytes_usage: if bytes == 0 {
+                None
+            } else {
+                Some(bytes_used as f64 / bytes as f64)
+            },
+            // TODO Figure out how to compute this.
+            #[cfg(target_os = "macos")]
+            bytes_capacity: None,
+            inodes,
+            inodes_used,
+            inodes_free: self.inodes_free + rhs.inodes_free,
+            inodes_usage: if inodes == 0 {
+                None
+            } else {
+                Some(inodes_used as f64 / inodes as f64)
+            },
+        }
+    }
+}
+
 impl From<Filesystem> for Row {
     fn from(fs: Filesystem) -> Self {
         let MountInfo {