Handle edge case of non-existant paths

Author: mtilda

src/bootstrap/src/core/builder.rs

@@ -442,17 +442,23 @@ impl StepDescription {
         let paths: Vec<_> = paths
             .iter()
             .map(|p| {
+                // If the path does not exist, it may represent the name of a Step, like `test tidy`
+                if !p.exists() {
+                    return p.clone();
+                }
                 // Get the canonical path, strip the prefix, and convert to a PathBuf.
                 match p.canonicalize() {
                     Ok(p) => p.strip_prefix(&builder.src).unwrap_or(&p).to_owned(),
-                    Err(_) => {
-                        panic!("failed to canonicalize path: {:?}", p);
+                    Err(e) => {
+                        eprintln!("ERROR: {:?}", e);
+                        panic!("Due to the above error, failed to canonicalize path: {:?}", p);
                     }
                 }
-            }).collect();
+            })
+            .collect();
 
         // Convert Vec<PathBuf> to `Vec<&Path>`
-        let mut paths:Vec<&Path> = paths.iter().map(|p| p.as_ref()).collect();
+        let mut paths: Vec<&Path> = paths.iter().map(|p| p.as_ref()).collect();
 
         remap_paths(&mut paths);