More aggressive remapping (#2442)

Looking at our strings, I noticed two things:

- `/toolchain` remaps were always of the form
`/toolchain/lib/rustlib/src/rust/library/core/src`
- There were a bunch of strings of the form
`/hubris/target/thumbv7em-none-eabihf/release/build`, which are files
generated from `build.rs` scripts

This PR remaps those strings more aggressively, to `/rustlib` and
`/hubris/build` respectively.

This saves 17920 bytes of flash on the `cosmo-b-dev` image:

```
auxflash            15936   -> 15864
control_plane_agent 114488  -> 113928
cosmo_seq           43684   -> 38304 (!)
dump_agent          16860   -> 16712
fmc_demo            10312   -> 10236
hash_driver         6984    -> 6912
hf                  34704   -> 34552
hiffy               22568   -> 22416
host_sp_comms       68860   -> 68484
i2c_driver          12192   -> 12156
idle                88      -> 88
ignition_flash      7488    -> 7452
jefe                9184    -> 9184
kernel              32768   -> 32768
net                 74872   -> 74648
packrat             22868   -> 22800
power               34920   -> 34768
rng_driver          5800    -> 5724
sensor              8280    -> 8244
snitch              3340    -> 3340
spartan7_loader     12924   -> 12812
spd                 6788    -> 6712
spi2_driver         11020   -> 10948
spi3_driver         10544   -> 10472
sprot               45252   -> 45172
sys                 2720    -> 2720
thermal             21792   -> 21644
udpbroadcast        6472    -> 6400
udpecho             7008    -> 6972
udprpc              8208    -> 8172
update_server       9292    -> 9252
user_leds           1420    -> 1420
validate            7468    -> 7468
vpd                 5608    -> 5608
```

Author: mkeeter

build/xtask/src/dist.rs

@@ -140,7 +140,7 @@ impl PackageConfig {
             bail!("Failed to find {:?}", board_path);
         }
 
-        let remap_paths = Self::remap_paths(&sysroot)?;
+        let remap_paths = Self::remap_paths(&sysroot, &toml.target)?;
 
         Ok(Self {
             app_src_dir: app_src_dir.to_path_buf(),
@@ -167,7 +167,10 @@ impl PackageConfig {
         self.dist_dir.join(name)
     }
 
-    fn remap_paths(sysroot: &Path) -> Result<BTreeMap<PathBuf, &'static str>> {
+    fn remap_paths(
+        sysroot: &Path,
+        target: &str,
+    ) -> Result<BTreeMap<PathBuf, &'static str>> {
         // Panic messages in crates have a long prefix; we'll shorten it using
         // the --remap-path-prefix argument to reduce message size.  This is
         // good for both binary size and for reproducibility, since we don't
@@ -210,13 +213,36 @@ impl PackageConfig {
             remap_paths.insert(cargo_sparse_registry, "/crates.io");
         }
 
-        remap_paths.insert(sysroot.to_path_buf(), "/toolchain");
+        remap_paths.insert(
+            sysroot
+                .to_path_buf()
+                .join("lib")
+                .join("rustlib")
+                .join("src")
+                .join("rust")
+                .join("library")
+                .join("core")
+                .join("src"),
+            "/rustlib",
+        );
 
         if let Ok(dir) = std::env::var("CARGO_MANIFEST_DIR") {
             let mut hubris_dir = dunce::canonicalize(dir)?;
             hubris_dir.pop();
             hubris_dir.pop();
             remap_paths.insert(hubris_dir.to_path_buf(), "/hubris");
+            // Shorten the build directories from
+            // `hubris/target/thumbv7em-none-eabihf/release/build` to just
+            // `hubris/build`
+            remap_paths.insert(
+                hubris_dir
+                    .to_path_buf()
+                    .join("target")
+                    .join(target)
+                    .join("release")
+                    .join("build"),
+                "/hubris/build",
+            );
         }
         Ok(remap_paths)
     }