Add a structure hash to FeatureInspector

DEPENDENCIES.md

@@ -468,6 +468,7 @@ The following text applies to code linked from these dependencies:
 [core-foundation-sys](https://github.com/servo/core-foundation-rs),
 [core-foundation](https://github.com/servo/core-foundation-rs),
 [cpufeatures](https://github.com/RustCrypto/utils),
+[crypto-common](https://github.com/RustCrypto/traits),
 [digest](https://github.com/RustCrypto/traits),
 [dogear](https://github.com/mozilla/dogear),
 [either](https://github.com/bluss/either),
@@ -522,7 +523,6 @@ The following text applies to code linked from these dependencies:
 [num_cpus](https://github.com/seanmonstar/num_cpus),
 [ohttp](https://github.com/martinthomson/ohttp),
 [once_cell](https://github.com/matklad/once_cell),
-[opaque-debug](https://github.com/RustCrypto/utils),
 [openssl-macros](https://github.com/sfackler/rust-openssl),
 [openssl-probe](https://github.com/alexcrichton/openssl-probe),
 [openssl-src](https://github.com/alexcrichton/openssl-src-rs),

components/nimbus/Cargo.toml

@@ -30,7 +30,7 @@ url = "2.2"
 rkv = { version = "0.17", optional = true }
 jexl-eval = "0.2.2"
 uuid = { version = "0.8", features = ["serde", "v4"]}
-sha2 = "0.9"
+sha2 = "^0.10"
 hex = "0.4"
 once_cell = "1"
 uniffi = "0.24.1"

components/support/nimbus-fml/Cargo.toml

@@ -32,6 +32,7 @@ cfg-if = "1.0.0"
 console = "0.15.5"
 lazy_static = "1.4"
 email_address = { version = "0.2.4", features = ["serde"] }
+sha2 = "^0.10"
 
 [build-dependencies]
 uniffi = { version = "0.24.1", features = ["build"], optional = true }

components/support/nimbus-fml/src/client/inspector.rs

@@ -66,6 +66,12 @@ impl FmlFeatureInspector {
     pub fn get_errors(&self, string: String) -> Option<Vec<FmlEditorError>> {
         self.get_first_error(string).map(|e| vec![e])
     }
+
+    pub fn get_structure_hash(&self) -> String {
+        self.manifest
+            .get_structure_hash(&self.feature_id)
+            .unwrap_or_default()
+    }
 }
 
 impl FmlFeatureInspector {

components/support/nimbus-fml/src/fml.udl

@@ -103,6 +103,24 @@ interface FmlFeatureInspector {
     // Returns the default JSON for the feature for this channel.
     [Throws=FMLError]
     JsonObject get_default_json();
+
+    // Returns a 8 digit hex hash of the structure of the feature.
+    //
+    // This is suitable for tracking over time.
+    //
+    // This is a truncated SHA256 hash which takes in to consideration
+    // the name and types of all the variables in this feature, and nested
+    // objects, as well as the variants of enums.
+    //
+    // It does not take into consideration the default values for this
+    // feature, ordering of fields or variables or enum variants, or
+    // documentation, so as to remain stable despite superficial changes
+    // to the feature's configuration.
+    //
+    // If this hash changes for a given feature then it is almost
+    // certain that the code which uses this configuration will also have
+    // changed.
+    string get_structure_hash();
 };
 
 dictionary FmlEditorError {

components/support/nimbus-fml/src/intermediate_representation.rs

@@ -5,7 +5,7 @@ use crate::defaults::{DefaultsMerger, DefaultsValidator};
 use crate::error::FMLError::InvalidFeatureError;
 use crate::error::{FMLError, Result};
 use crate::frontend::{AboutBlock, FeatureMetadata};
-use crate::structure::StructureValidator;
+use crate::structure::{StructureHasher, StructureValidator};
 use crate::util::loaders::FilePath;
 use anyhow::{bail, Error, Result as AnyhowResult};
 use serde::{Deserialize, Serialize};
@@ -456,6 +456,22 @@ impl FeatureManifest {
 
         Ok(feature_def)
     }
+
+    pub fn get_structure_hash(&self, feature_name: &str) -> Result<String> {
+        let (manifest, feature_def) = self
+            .find_feature(feature_name)
+            .ok_or_else(|| InvalidFeatureError(feature_name.to_string()))?;
+
+        Ok(manifest.feature_structure_hash(feature_def))
+    }
+}
+
+impl FeatureManifest {
+    pub(crate) fn feature_structure_hash(&self, feature_def: &FeatureDef) -> String {
+        let hasher = StructureHasher::new(&self.enum_defs, &self.obj_defs);
+        let hash = hasher.hash(feature_def) & 0xffffffff;
+        format!("{hash:x}")
+    }
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq, Default)]