diff --git a/tools/Cargo.lock b/tools/Cargo.lock
index 90aa3be7..274a23ad 100644
--- a/tools/Cargo.lock
+++ b/tools/Cargo.lock
@@ -624,8 +624,17 @@ dependencies = [
  "sha2",
  "syn 2.0.119",
  "tempfile",
- "toml",
  "twox-hash",
+ "xray-common",
+]
+
+[[package]]
+name = "xray-common"
+version = "0.1.0"
+dependencies = [
+ "serde",
+ "tempfile",
+ "toml",
  "walkdir",
 ]
 
diff --git a/tools/Cargo.toml b/tools/Cargo.toml
index e520828c..7f5779c2 100644
--- a/tools/Cargo.toml
+++ b/tools/Cargo.toml
@@ -9,7 +9,7 @@
 #     cargo clippy --manifest-path tools/Cargo.toml --all-targets -- -D warnings
 [workspace]
 resolver = "3"
-members = ["xray", "xray-clones", "xray-clusters"]
+members = ["xray", "xray-clones", "xray-clusters", "xray-common"]
 
 [workspace.package]
 version = "0.1.0"
diff --git a/tools/xray-clones/Cargo.toml b/tools/xray-clones/Cargo.toml
index 1ad5200c..2b8c4d3b 100644
--- a/tools/xray-clones/Cargo.toml
+++ b/tools/xray-clones/Cargo.toml
@@ -17,8 +17,7 @@ proc-macro2 = { version = "1", features = ["span-locations"] }
 clap = { version = "4", features = ["derive"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
-toml = "0.9"
-walkdir = "2"
+xray-common = { path = "../xray-common" }
 rayon = "1"
 # A fingerprint is written into allow.toml by hand, so it has to stay the same
 # across compiler and crate versions. sha2 and xxhash are both fixed by their
diff --git a/tools/xray-clones/src/collect.rs b/tools/xray-clones/src/collect.rs
index 6c3b18a1..1f63aa3c 100644
--- a/tools/xray-clones/src/collect.rs
+++ b/tools/xray-clones/src/collect.rs
@@ -71,7 +71,7 @@ impl Skipped {
 /// is machine-written or does not parse.
 pub fn collect_file(path: &Path, krate: &str, opts: &Options) -> Option<Collected> {
     let text = std::fs::read_to_string(path).ok()?;
-    if is_generated(&text) {
+    if xray_common::is_generated(&text) {
         return None;
     }
     let file = syn::parse_file(&text).ok()?;
@@ -90,12 +90,6 @@ pub fn collect_file(path: &Path, krate: &str, opts: &Options) -> Option<Collecte
     })
 }
 
-fn is_generated(text: &str) -> bool {
-    text.lines()
-        .take(5)
-        .any(|l| l.contains("@generated") || l.contains("DO NOT EDIT"))
-}
-
 struct Walker<'a> {
     krate: String,
     file: PathBuf,
diff --git a/tools/xray-clones/src/main.rs b/tools/xray-clones/src/main.rs
index 931eb6e8..b5df449b 100644
--- a/tools/xray-clones/src/main.rs
+++ b/tools/xray-clones/src/main.rs
@@ -2,7 +2,6 @@
 //!
 //! See `README.md` for what the measure catches and what it does not.
 
-mod allow;
 mod collect;
 mod normalize;
 mod similar;
@@ -12,6 +11,7 @@ use collect::{Item, Kind, Options, Skipped};
 use rayon::prelude::*;
 use std::collections::BTreeSet;
 use std::path::{Path, PathBuf};
+use xray_common::{discover, Allowlist};
 
 #[derive(Parser)]
 #[command(
@@ -111,10 +111,10 @@ fn main() -> std::process::ExitCode {
 
 fn run(args: &Args) -> Result<bool, String> {
     let allowlist = if args.no_allow {
-        allow::Allowlist::default()
+        Allowlist::default()
     } else {
         let path = args.allow.clone().unwrap_or_else(default_allow_path);
-        allow::Allowlist::load(&path)?
+        Allowlist::load(&path)?
     };
 
     let opts = Options {
@@ -230,47 +230,6 @@ fn default_allow_path() -> PathBuf {
     Path::new(env!("CARGO_MANIFEST_DIR")).join("allow.toml")
 }
 
-/// Every `.rs` file under `crates/*/src` and `crates/*/tests`, plus any
-/// directory named with `--scan`, paired with the crate it belongs to.
-fn discover(root: &Path, extra: &[PathBuf]) -> Vec<(PathBuf, String)> {
-    let mut out = Vec::new();
-    let crates = root.join("crates");
-    if let Ok(entries) = std::fs::read_dir(&crates) {
-        let mut dirs: Vec<_> = entries.flatten().map(|e| e.path()).collect();
-        dirs.sort();
-        for dir in dirs {
-            let Some(name) = dir.file_name().and_then(|n| n.to_str()) else {
-                continue;
-            };
-            let name = name.to_string();
-            for sub in ["src", "tests"] {
-                walk_rs(&dir.join(sub), &name, &mut out);
-            }
-        }
-    }
-    for dir in extra {
-        let name = dir
-            .file_name()
-            .and_then(|n| n.to_str())
-            .unwrap_or("scan")
-            .to_string();
-        walk_rs(dir, &name, &mut out);
-    }
-    out
-}
-
-fn walk_rs(dir: &Path, krate: &str, out: &mut Vec<(PathBuf, String)>) {
-    if !dir.is_dir() {
-        return;
-    }
-    for entry in walkdir::WalkDir::new(dir).into_iter().flatten() {
-        let p = entry.path();
-        if p.extension().is_some_and(|e| e == "rs") && p.is_file() {
-            out.push((p.to_path_buf(), krate.to_string()));
-        }
-    }
-}
-
 #[derive(serde::Serialize)]
 struct Summary {
     files: usize,
@@ -315,7 +274,7 @@ impl Report {
         g: similar::Group,
         items: &[Item],
         args: &Args,
-        allowlist: &allow::Allowlist,
+        allowlist: &Allowlist,
     ) -> Option<Self> {
         let mut members: Vec<&Item> = g.members.iter().map(|&i| &items[i]).collect();
         // An `impl` block and a method inside it overlap in the source, so
diff --git a/tools/xray-common/Cargo.toml b/tools/xray-common/Cargo.toml
new file mode 100644
index 00000000..9e96961c
--- /dev/null
+++ b/tools/xray-common/Cargo.toml
@@ -0,0 +1,17 @@
+[package]
+name = "xray-common"
+description = "What the xray tools share: which files to read, and which findings are already reviewed."
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+repository.workspace = true
+publish.workspace = true
+
+[dependencies]
+serde = { version = "1", features = ["derive"] }
+toml = "0.9"
+walkdir = "2"
+
+[dev-dependencies]
+tempfile = "3"
diff --git a/tools/xray-clones/src/allow.rs b/tools/xray-common/src/allow.rs
similarity index 83%
rename from tools/xray-clones/src/allow.rs
rename to tools/xray-common/src/allow.rs
index 01a25973..60919182 100644
--- a/tools/xray-clones/src/allow.rs
+++ b/tools/xray-common/src/allow.rs
@@ -1,8 +1,9 @@
-//! The reviewed-duplicate list.
+//! The reviewed-finding list.
 //!
-//! An entry silences one shape by its fingerprint and must say why. Silencing
-//! a reviewed duplicate one fingerprint at a time keeps the threshold honest;
-//! lowering the threshold to hide one pair hides every pair like it.
+//! An entry silences one finding by the fingerprint the tool prints beside it,
+//! and must say why. Silencing one fingerprint at a time keeps a tool's
+//! thresholds honest; loosening a threshold to hide one finding hides every
+//! finding like it.
 
 use std::collections::HashMap;
 use std::path::Path;
@@ -16,7 +17,7 @@ struct File {
 #[derive(serde::Deserialize)]
 struct Entry {
     fingerprint: String,
-    /// Why this duplicate is acceptable. Required: an entry without one is a
+    /// Why this finding is acceptable. Required: an entry without one is a
     /// parse error, not a silent allow.
     reason: String,
 }
@@ -41,7 +42,7 @@ impl Allowlist {
         ))
     }
 
-    /// The reason a group is allowed, if any of its fingerprints is listed.
+    /// The reason a finding is allowed, if any of its fingerprints is listed.
     pub fn reason<'a>(&'a self, fingerprints: impl Iterator<Item = &'a str>) -> Option<&'a str> {
         fingerprints
             .filter_map(|f| self.0.get(f))
@@ -52,6 +53,10 @@ impl Allowlist {
     pub fn len(&self) -> usize {
         self.0.len()
     }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
 }
 
 #[cfg(test)]
diff --git a/tools/xray-common/src/discover.rs b/tools/xray-common/src/discover.rs
new file mode 100644
index 00000000..c363b6b6
--- /dev/null
+++ b/tools/xray-common/src/discover.rs
@@ -0,0 +1,51 @@
+//! Which files an xray tool reads.
+
+use std::path::{Path, PathBuf};
+
+/// Every `.rs` file under `crates/*/src` and `crates/*/tests`, plus any
+/// directory named in `extra`, paired with the crate it belongs to.
+pub fn discover(root: &Path, extra: &[PathBuf]) -> Vec<(PathBuf, String)> {
+    let mut out = Vec::new();
+    let crates = root.join("crates");
+    if let Ok(entries) = std::fs::read_dir(&crates) {
+        let mut dirs: Vec<_> = entries.flatten().map(|e| e.path()).collect();
+        dirs.sort();
+        for dir in dirs {
+            let Some(name) = dir.file_name().and_then(|n| n.to_str()) else {
+                continue;
+            };
+            let name = name.to_string();
+            for sub in ["src", "tests"] {
+                walk_rs(&dir.join(sub), &name, &mut out);
+            }
+        }
+    }
+    for dir in extra {
+        let name = dir
+            .file_name()
+            .and_then(|n| n.to_str())
+            .unwrap_or("scan")
+            .to_string();
+        walk_rs(dir, &name, &mut out);
+    }
+    out
+}
+
+fn walk_rs(dir: &Path, krate: &str, out: &mut Vec<(PathBuf, String)>) {
+    if !dir.is_dir() {
+        return;
+    }
+    for entry in walkdir::WalkDir::new(dir).into_iter().flatten() {
+        let p = entry.path();
+        if p.extension().is_some_and(|e| e == "rs") && p.is_file() {
+            out.push((p.to_path_buf(), krate.to_string()));
+        }
+    }
+}
+
+/// True for a file whose opening lines say a machine wrote it.
+pub fn is_generated(text: &str) -> bool {
+    text.lines()
+        .take(5)
+        .any(|l| l.contains("@generated") || l.contains("DO NOT EDIT"))
+}
diff --git a/tools/xray-common/src/lib.rs b/tools/xray-common/src/lib.rs
new file mode 100644
index 00000000..a8a7b3f6
--- /dev/null
+++ b/tools/xray-common/src/lib.rs
@@ -0,0 +1,10 @@
+//! What the xray tools share.
+//!
+//! Both tools read the same files and both answer to a reviewed-finding list.
+//! A tool that reports duplication should not be built out of it.
+
+pub mod allow;
+pub mod discover;
+
+pub use allow::Allowlist;
+pub use discover::{discover, is_generated};