Save/load extern specs in metadata

Cargo.toml

@@ -25,6 +25,7 @@ flux-syntax = { path = "./crates/flux-syntax", version = "0.1.0" }
 tests = { path = "./tests", version = "0.1.0" }
 
 dashmap = { version = "5.5.3", features = ["raw-api"] }
+derive-where = "1.2.7"
 home = "0.5.9"
 itertools = "0.13.0"
 serde = { version = "1.0", features = ["derive"] }

crates/flux-driver/src/callbacks.rs

@@ -71,7 +71,7 @@ impl FluxCallbacks {
             let arena = fhir::Arena::new();
             GlobalEnv::enter(tcx, &sess, Box::new(cstore), &arena, providers, |genv| {
                 if check_crate(genv).is_ok() {
-                    save_metadata(&genv);
+                    save_metadata(genv);
                 }
             });
             sess.finish_diagnostics();
@@ -113,7 +113,7 @@ fn collect_specs(genv: GlobalEnv) -> Specs {
     }
 }
 
-fn save_metadata(genv: &GlobalEnv) {
+fn save_metadata(genv: GlobalEnv) {
     let tcx = genv.tcx();
     if tcx
         .sess

crates/flux-fixpoint/Cargo.toml

@@ -7,11 +7,12 @@ edition.workspace = true
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-derive-where = "1.2.7"
 flux-common.workspace = true
 flux-config.workspace = true
+
+derive-where.workspace = true
 itertools.workspace = true
-serde = { version = "1.0", features = ["derive"] }
+serde.workspace = true
 serde_json = "1.0"
 
 [package.metadata.rust-analyzer]

crates/flux-metadata/Cargo.toml

@@ -11,6 +11,8 @@ flux-common.workspace = true
 flux-errors.workspace = true
 flux-macros.workspace = true
 flux-middle.workspace = true
+
+derive-where.workspace = true
 itertools.workspace = true
 
 [package.metadata.rust-analyzer]

crates/flux-metadata/locales/en-US.ftl

@@ -1 +1,3 @@
 metadata_decode_file_error = "error when decoding flux metadata file {$path}: {$err}"
+
+metadata_duplicate_spec = "duplicate spec for {$def_name}"

crates/flux-metadata/src/encoder.rs

@@ -29,7 +29,7 @@ struct EncodeContext<'a, 'tcx> {
     symbol_table: FxHashMap<Symbol, usize>, // interpret_allocs: FxIndexSet<interpret::AllocId>,
 }
 
-pub fn encode_metadata(genv: &GlobalEnv, path: &std::path::Path) {
+pub fn encode_metadata(genv: GlobalEnv, path: &std::path::Path) {
     let mut encoder = opaque::FileEncoder::new(path).unwrap_or_else(|err| {
         genv.tcx()
             .sess