Improve handling of strings for backtraces (#843)

* Improve handling of strings for backtraces

Largely avoid storing strings at all in the `wasmtime-*` internal
crates, and instead only store strings in a separate global cache
specific to the `wasmtime` crate itself. This global cache is inserted
and removed from dynamically as modules are created and deallocated, and
the global cache is consulted whenever a `Trap` is created to
symbolicate any wasm frames.

This also avoids the need to thread `module_name` through the jit crates
and back, and additionally removes the need for `ModuleSyncString`.

* Run rustfmt

crates/environ/src/module.rs

@@ -3,15 +3,16 @@
 use crate::module_environ::FunctionBodyData;
 use crate::tunables::Tunables;
 use cranelift_codegen::ir;
-use cranelift_entity::{EntityRef, PrimaryMap, SecondaryMap};
+use cranelift_entity::{EntityRef, PrimaryMap};
 use cranelift_wasm::{
     DefinedFuncIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, FuncIndex, Global,
     GlobalIndex, Memory, MemoryIndex, SignatureIndex, Table, TableIndex,
 };
 use indexmap::IndexMap;
 use more_asserts::assert_ge;
+use std::collections::HashMap;
 use std::hash::{Hash, Hasher};
-use std::sync::Arc;
+use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
 
 /// A WebAssembly table initializer.
 #[derive(Clone, Debug, Hash)]
@@ -129,33 +130,14 @@ impl TablePlan {
     }
 }
 
-/// Allows module strings to be cached as reused across
-/// multiple threads. Useful for debug/trace information.
-#[derive(Debug, Clone)]
-pub struct ModuleSyncString(Option<Arc<String>>);
-
-impl ModuleSyncString {
-    /// Gets optional string reference.
-    pub fn get(&self) -> Option<&str> {
-        self.0.as_deref().map(|s| s.as_str())
-    }
-    /// Constructs the string.
-    pub fn new(s: Option<&str>) -> Self {
-        ModuleSyncString(s.map(|s| Arc::new(s.to_string())))
-    }
-}
-
-impl Default for ModuleSyncString {
-    fn default() -> Self {
-        ModuleSyncString(None)
-    }
-}
-
 /// A translated WebAssembly module, excluding the function bodies and
 /// memory initializers.
 // WARNING: when modifying, make sure that `hash_for_cache` is still valid!
 #[derive(Debug)]
 pub struct Module {
+    /// A unique identifier (within this process) for this module.
+    pub id: usize,
+
     /// Unprocessed signatures exactly as provided by `declare_signature()`.
     pub signatures: PrimaryMap<SignatureIndex, ir::Signature>,
 
@@ -193,17 +175,17 @@ pub struct Module {
     /// WebAssembly table initializers.
     pub table_elements: Vec<TableElements>,
 
-    /// Module name.
-    pub name: ModuleSyncString,
-
-    /// Function names.
-    pub func_names: SecondaryMap<FuncIndex, ModuleSyncString>,
+    /// WebAssembly table initializers.
+    pub func_names: HashMap<FuncIndex, String>,
 }
 
 impl Module {
     /// Allocates the module data structures.
     pub fn new() -> Self {
+        static NEXT_ID: AtomicUsize = AtomicUsize::new(0);
+
         Self {
+            id: NEXT_ID.fetch_add(1, SeqCst),
             signatures: PrimaryMap::new(),
             imported_funcs: PrimaryMap::new(),
             imported_tables: PrimaryMap::new(),
@@ -216,8 +198,7 @@ impl Module {
             exports: IndexMap::new(),
             start_func: None,
             table_elements: Vec::new(),
-            name: Default::default(),
-            func_names: SecondaryMap::new(),
+            func_names: HashMap::new(),
         }
     }