Compile the mount-table reader only where there is a mount table

`platform_volumes` has been gated to Linux since it was written, but the
eight items it is built from were not, so every Android build compiled a
`/proc/mounts` parser it could never call and then printed eight dead-code
warnings about it. Real warnings hide in that kind of noise.

Gated per item rather than moved into a module, because the file already
draws the line that way one function above and two patterns for one idea
is worse than a repeated attribute.

The tests go with them. They parse a mount table and assert on names like
`mmcblk0p1`, so they are as Linux-bound as the code they exercise, and
`Path` turns out to be too -- only the reader borrows one, where `Volume`
owns its own.

Android now builds dr-plat with no warnings at all.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

platform/dr-plat/src/volumes.rs

@@ -27,7 +27,10 @@
 //! directory is reported alongside and weighted more heavily than the flag:
 //! a volume with `DCIM` on it is a camera card whatever sysfs believes.
 
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
+// Only the mount-table reader below borrows a path; `Volume` owns one.
+#[cfg(target_os = "linux")]
+use std::path::Path;
 
 /// Somewhere a card might be.
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -136,6 +139,7 @@ fn platform_volumes() -> Vec<Volume> {
 }
 
 /// One line of the mount table.
+#[cfg(target_os = "linux")]
 #[derive(Debug, Clone, PartialEq, Eq)]
 struct Mount {
     device: String,
@@ -147,6 +151,7 @@ struct Mount {
 ///
 /// Split out from the reading so it can be tested against real tables without
 /// a card plugged in — which is the only way this file is testable at all.
+#[cfg(target_os = "linux")]
 fn parse_mounts(table: &str) -> Vec<Mount> {
     table
         .lines()
@@ -169,6 +174,7 @@ fn parse_mounts(table: &str) -> Vec<Mount> {
 /// A card is very often labelled with a space in it — `EOS DIGITAL` is what
 /// Canon writes — and mounted at `/run/media/duncan/EOS\040DIGITAL`. Without
 /// this the path does not exist and the card is invisible.
+#[cfg(target_os = "linux")]
 fn unescape(field: &str) -> String {
     let mut out = String::with_capacity(field.len());
     let bytes = field.as_bytes();
@@ -192,6 +198,7 @@ fn unescape(field: &str) -> String {
 ///
 /// Excludes the pseudo-filesystems, which is most of the table, and anything
 /// not backed by a block device. A card reader always is.
+#[cfg(target_os = "linux")]
 fn is_candidate(m: &Mount) -> bool {
     const FILESYSTEMS: &[&str] = &[
         // What cameras format cards as, in practice: FAT32 below 32 GB,
@@ -204,6 +211,7 @@ fn is_candidate(m: &Mount) -> bool {
 }
 
 /// Whether the desktop automounted this, which means it already decided.
+#[cfg(target_os = "linux")]
 fn under_media_dir(mount_point: &Path) -> bool {
     let s = mount_point.to_string_lossy();
     s.starts_with("/run/media/") || s.starts_with("/media/") || s.starts_with("/mnt/media/")
@@ -217,6 +225,7 @@ fn under_media_dir(mount_point: &Path) -> bool {
 /// lose only the `pN` suffix — strip naively and `mmcblk0p1` becomes
 /// `mmcblk`, which does not exist, and every SD card in an internal reader
 /// reports itself fixed.
+#[cfg(target_os = "linux")]
 fn device_is_removable(device: &str) -> Option<bool> {
     let name = device.strip_prefix("/dev/")?;
     let disk = whole_disk(name);
@@ -225,6 +234,7 @@ fn device_is_removable(device: &str) -> Option<bool> {
 }
 
 /// The whole-disk name a partition belongs to.
+#[cfg(target_os = "linux")]
 fn whole_disk(partition: &str) -> String {
     // `mmcblk0p1` -> `mmcblk0`, `nvme0n1p3` -> `nvme0n1`: these spell the
     // partition with a `p`, and the disk name legitimately ends in a digit.
@@ -243,6 +253,7 @@ fn whole_disk(partition: &str) -> String {
 }
 
 /// What to call a volume in a list.
+#[cfg(target_os = "linux")]
 fn label_for(mount_point: &Path) -> String {
     mount_point
         .file_name()
@@ -250,7 +261,9 @@ fn label_for(mount_point: &Path) -> String {
         .unwrap_or_else(|| mount_point.to_string_lossy().to_string())
 }
 
-#[cfg(test)]
+// Reading `/proc/mounts` and naming block devices is what these test, so
+// they are as Linux-bound as the code above.
+#[cfg(all(test, target_os = "linux"))]
 mod tests {
     use super::*;