diff --git a/rust/rhg/src/commands/cat.rs b/rust/rhg/src/commands/cat.rs
--- a/rust/rhg/src/commands/cat.rs
+++ b/rust/rhg/src/commands/cat.rs
@@ -1,10 +1,9 @@
 use crate::error::CommandError;
+use crate::utils::path_utils::resolve_file_args;
 use clap::Arg;
 use format_bytes::format_bytes;
 use hg::operations::cat;
-use hg::utils::hg_path::HgPathBuf;
 use std::ffi::OsString;
-use std::os::unix::prelude::OsStrExt;
 
 pub const HELP_TEXT: &str = "
 Output the current or given revision of files
@@ -40,52 +39,15 @@ pub fn run(invocation: &crate::CliInvoca
         ));
     }
 
-    let rev = invocation.subcommand_args.get_one::<String>("rev");
-    let file_args =
-        match invocation.subcommand_args.get_many::<OsString>("files") {
-            Some(files) => files
-                .filter(|s| !s.is_empty())
-                .map(|s| s.as_os_str())
-                .collect(),
-            None => vec![],
-        };
-
     let repo = invocation.repo?;
-    let cwd = hg::utils::current_dir()?;
-    let working_directory = repo.working_directory_path();
-    let working_directory = cwd.join(working_directory); // Make it absolute
-
-    let mut files = vec![];
-    for file in file_args {
-        if file.as_bytes().starts_with(b"set:") {
-            let message = "fileset";
-            return Err(CommandError::unsupported(message));
-        }
 
-        let normalized = cwd.join(file);
-        // TODO: actually normalize `..` path segments etc?
-        let dotted = normalized.components().any(|c| c.as_os_str() == "..");
-        if file.as_bytes() == b"." || dotted {
-            let message = "`..` or `.` path segment";
-            return Err(CommandError::unsupported(message));
-        }
-        let relative_path = working_directory
-            .strip_prefix(&cwd)
-            .unwrap_or(&working_directory);
-        let stripped = normalized
-            .strip_prefix(&working_directory)
-            .map_err(|_| {
-                CommandError::abort(format!(
-                    "abort: {} not under root '{}'\n(consider using '--cwd {}')",
-                    String::from_utf8_lossy(file.as_bytes()),
-                    working_directory.display(),
-                    relative_path.display(),
-                ))
-            })?;
-        let hg_file = HgPathBuf::try_from(stripped.to_path_buf())
-            .map_err(|e| CommandError::abort(e.to_string()))?;
-        files.push(hg_file);
-    }
+    let rev = invocation.subcommand_args.get_one::<String>("rev");
+    let files = match invocation.subcommand_args.get_many::<OsString>("files")
+    {
+        None => vec![],
+        Some(files) => resolve_file_args(repo, files)?,
+    };
+
     let files = files.iter().map(|file| file.as_ref()).collect();
     // TODO probably move this to a util function like `repo.default_rev` or
     // something when it's used somewhere else
diff --git a/rust/rhg/src/utils/path_utils.rs b/rust/rhg/src/utils/path_utils.rs
--- a/rust/rhg/src/utils/path_utils.rs
+++ b/rust/rhg/src/utils/path_utils.rs
@@ -10,6 +10,9 @@ use hg::utils::files::{get_bytes_from_pa
 use hg::utils::hg_path::HgPath;
 use hg::utils::hg_path::HgPathBuf;
 use std::borrow::Cow;
+use std::ffi::OsString;
+
+use crate::error::CommandError;
 
 pub struct RelativizePaths {
     repo_root: HgPathBuf,
@@ -53,3 +56,41 @@ impl RelativizePaths {
         }
     }
 }
+
+/// Resolves `FILE ...` arguments to a list of paths in the repository.
+pub fn resolve_file_args<'a>(
+    repo: &Repo,
+    file_args: impl Iterator<Item = &'a OsString>,
+) -> Result<Vec<HgPathBuf>, CommandError> {
+    let cwd = hg::utils::current_dir()?;
+    let root = cwd.join(repo.working_directory_path());
+    let mut result = Vec::new();
+    for pattern in file_args {
+        // TODO: Support all the formats in `hg help patterns`.
+        if pattern.as_encoded_bytes().contains(&b':') {
+            return Err(CommandError::unsupported(
+                "rhg does not support file patterns",
+            ));
+        }
+        // TODO: use hg::utils::files::canonical_path (currently doesn't work).
+        let path = cwd.join(pattern);
+        let dotted = path.components().any(|c| c.as_os_str() == "..");
+        if pattern.as_encoded_bytes() == b"." || dotted {
+            let message = "`..` or `.` path segment";
+            return Err(CommandError::unsupported(message));
+        }
+        let relative_path = root.strip_prefix(&cwd).unwrap_or(&root);
+        let stripped = path.strip_prefix(&root).map_err(|_| {
+            CommandError::abort(format!(
+                "abort: {} not under root '{}'\n(consider using '--cwd {}')",
+                String::from_utf8_lossy(pattern.as_encoded_bytes()),
+                root.display(),
+                relative_path.display(),
+            ))
+        })?;
+        let hg_file = HgPathBuf::try_from(stripped.to_path_buf())
+            .map_err(|e| CommandError::abort(e.to_string()))?;
+        result.push(hg_file);
+    }
+    Ok(result)
+}
diff --git a/tests/test-rhg.t b/tests/test-rhg.t
--- a/tests/test-rhg.t
+++ b/tests/test-rhg.t
@@ -251,7 +251,7 @@ Fallback with shell path segments
 
 Fallback with filesets
   $ $NO_FALLBACK rhg cat "set:c or b"
-  unsupported feature: fileset
+  unsupported feature: rhg does not support file patterns
   [252]
 
 Fallback with generic hooks