Add functions to ModuleEnvironment to support reserving buffers up front.

These default to doing nothing, but implementations can override them to
preallocate buffers.

Also, reorder the functions in `ModuleEnvironment` to more closely
reflect the sequence from the WebAssembly module layout.

lib/wasm/src/environ/spec.rs

@@ -245,9 +245,17 @@ pub trait ModuleEnvironment<'data> {
     /// Get the information needed to produce Cranelift IR for the current target.
     fn target_config(&self) -> TargetFrontendConfig;
 
+    /// Provides the number of signatures up front. By default this does nothing, but
+    /// implementations can use this to preallocate memory if desired.
+    fn reserve_signatures(&mut self, _num: u32) {}
+
     /// Declares a function signature to the environment.
     fn declare_signature(&mut self, sig: &ir::Signature);
 
+    /// Provides the number of imports up front. By default this does nothing, but
+    /// implementations can use this to preallocate memory if desired.
+    fn reserve_imports(&mut self, _num: u32) {}
+
     /// Declares a function import to the environment.
     fn declare_func_import(
         &mut self,
@@ -256,20 +264,71 @@ pub trait ModuleEnvironment<'data> {
         field: &'data str,
     );
 
-    /// Declares the type (signature) of a local function in the module.
-    fn declare_func_type(&mut self, sig_index: SignatureIndex);
+    /// Declares a table import to the environment.
+    fn declare_table_import(&mut self, table: Table, module: &'data str, field: &'data str);
 
-    /// Declares a global to the environment.
-    fn declare_global(&mut self, global: Global);
+    /// Declares a memory import to the environment.
+    fn declare_memory_import(&mut self, memory: Memory, module: &'data str, field: &'data str);
 
     /// Declares a global import to the environment.
     fn declare_global_import(&mut self, global: Global, module: &'data str, field: &'data str);
 
+    /// Notifies the implementation that all imports have been declared.
+    fn finish_imports(&mut self) {}
+
+    /// Provides the number of defined functions up front. By default this does nothing, but
+    /// implementations can use this to preallocate memory if desired.
+    fn reserve_func_types(&mut self, _num: u32) {}
+
+    /// Declares the type (signature) of a local function in the module.
+    fn declare_func_type(&mut self, sig_index: SignatureIndex);
+
+    /// Provides the number of defined tables up front. By default this does nothing, but
+    /// implementations can use this to preallocate memory if desired.
+    fn reserve_tables(&mut self, _num: u32) {}
+
     /// Declares a table to the environment.
     fn declare_table(&mut self, table: Table);
 
-    /// Declares a table import to the environment.
-    fn declare_table_import(&mut self, table: Table, module: &'data str, field: &'data str);
+    /// Provides the number of defined memories up front. By default this does nothing, but
+    /// implementations can use this to preallocate memory if desired.
+    fn reserve_memories(&mut self, _num: u32) {}
+
+    /// Declares a memory to the environment
+    fn declare_memory(&mut self, memory: Memory);
+
+    /// Provides the number of defined globals up front. By default this does nothing, but
+    /// implementations can use this to preallocate memory if desired.
+    fn reserve_globals(&mut self, _num: u32) {}
+
+    /// Declares a global to the environment.
+    fn declare_global(&mut self, global: Global);
+
+    /// Provides the number of exports up front. By default this does nothing, but
+    /// implementations can use this to preallocate memory if desired.
+    fn reserve_exports(&mut self, _num: u32) {}
+
+    /// Declares a function export to the environment.
+    fn declare_func_export(&mut self, func_index: FuncIndex, name: &'data str);
+
+    /// Declares a table export to the environment.
+    fn declare_table_export(&mut self, table_index: TableIndex, name: &'data str);
+
+    /// Declares a memory export to the environment.
+    fn declare_memory_export(&mut self, memory_index: MemoryIndex, name: &'data str);
+
+    /// Declares a global export to the environment.
+    fn declare_global_export(&mut self, global_index: GlobalIndex, name: &'data str);
+
+    /// Notifies the implementation that all exports have been declared.
+    fn finish_exports(&mut self) {}
+
+    /// Declares the optional start function.
+    fn declare_start_func(&mut self, index: FuncIndex);
+
+    /// Provides the number of element initializers up front. By default this does nothing, but
+    /// implementations can use this to preallocate memory if desired.
+    fn reserve_table_elements(&mut self, _num: u32) {}
 
     /// Fills a declared table with references to functions in the module.
     fn declare_table_elements(
@@ -280,11 +339,12 @@ pub trait ModuleEnvironment<'data> {
         elements: Vec<FuncIndex>,
     );
 
-    /// Declares a memory to the environment
-    fn declare_memory(&mut self, memory: Memory);
+    /// Provides the contents of a function body.
+    fn define_function_body(&mut self, body_bytes: &'data [u8]) -> WasmResult<()>;
 
-    /// Declares a memory import to the environment.
-    fn declare_memory_import(&mut self, memory: Memory, module: &'data str, field: &'data str);
+    /// Provides the number of data initializers up front. By default this does nothing, but
+    /// implementations can use this to preallocate memory if desired.
+    fn reserve_data_initializers(&mut self, _num: u32) {}
 
     /// Fills a declared memory with bytes at module instantiation.
     fn declare_data_initialization(
@@ -294,19 +354,4 @@ pub trait ModuleEnvironment<'data> {
         offset: usize,
         data: &'data [u8],
     );
-
-    /// Declares a function export to the environment.
-    fn declare_func_export(&mut self, func_index: FuncIndex, name: &'data str);
-    /// Declares a table export to the environment.
-    fn declare_table_export(&mut self, table_index: TableIndex, name: &'data str);
-    /// Declares a memory export to the environment.
-    fn declare_memory_export(&mut self, memory_index: MemoryIndex, name: &'data str);
-    /// Declares a global export to the environment.
-    fn declare_global_export(&mut self, global_index: GlobalIndex, name: &'data str);
-
-    /// Declares a start function.
-    fn declare_start_func(&mut self, index: FuncIndex);
-
-    /// Provides the contents of a function body.
-    fn define_function_body(&mut self, body_bytes: &'data [u8]) -> WasmResult<()>;
 }