Add FuncEnvironment hooks to generate prologue and epilogue code.

In some cases, it is useful to do some work at entry to or exit from a
Cranelift function translated from WebAssembly. This PR adds two
optional methods to the `FuncEnvironment` trait to do just this,
analogous to the pre/post-hooks on operators that already exist.

This PR also includes a drive-by compilation fix due to the latest
nightly wherein `.is_empty()` on a `Range` ambiguously refers to either
the `Range` impl or the `ExactSizeIterator` impl and can't resolve.

cranelift/codegen/src/isa/aarch64/inst/unwind.rs

@@ -104,7 +104,7 @@ impl UnwindInfoGenerator<Inst> for AArch64UnwindInfo {
 
         // TODO epilogues
 
-        let prologue_size = if context.prologue.is_empty() {
+        let prologue_size = if context.prologue.len() == 0 {
             0
         } else {
             context.insts_layout[context.prologue.end as usize - 1]

cranelift/wasm/src/environ/spec.rs

@@ -624,6 +624,26 @@ pub trait FuncEnvironment: TargetEnvironment {
     ) -> WasmResult<()> {
         Ok(())
     }
+
+    /// Optional callback for the `FunctionEnvironment` performing this translation to perform work
+    /// before the function body is translated.
+    fn before_translate_function(
+        &mut self,
+        _builder: &mut FunctionBuilder,
+        _state: &FuncTranslationState,
+    ) -> WasmResult<()> {
+        Ok(())
+    }
+
+    /// Optional callback for the `FunctionEnvironment` performing this translation to perform work
+    /// after the function body is translated.
+    fn after_translate_function(
+        &mut self,
+        _builder: &mut FunctionBuilder,
+        _state: &FuncTranslationState,
+    ) -> WasmResult<()> {
+        Ok(())
+    }
 }
 
 /// An object satisfying the `ModuleEnvironment` trait can be passed as argument to the

cranelift/wasm/src/func_translator.rs

@@ -225,6 +225,7 @@ fn parse_function_body<FE: FuncEnvironment + ?Sized>(
     // The control stack is initialized with a single block representing the whole function.
     debug_assert_eq!(state.control_stack.len(), 1, "State not initialized");
 
+    environ.before_translate_function(builder, state)?;
     while !reader.eof() {
         let pos = reader.original_position();
         builder.set_srcloc(cur_srcloc(&reader));
@@ -234,6 +235,7 @@ fn parse_function_body<FE: FuncEnvironment + ?Sized>(
         translate_operator(validator, &op, builder, state, environ)?;
         environ.after_translate_operator(&op, builder, state)?;
     }
+    environ.after_translate_function(builder, state)?;
     let pos = reader.original_position();
     validator.finish(pos)?;
 
@@ -243,7 +245,6 @@ fn parse_function_body<FE: FuncEnvironment + ?Sized>(
     // If the exit block is unreachable, it may not have the correct arguments, so we would
     // generate a return instruction that doesn't match the signature.
     if state.reachable {
-        debug_assert!(builder.is_pristine());
         if !builder.is_unreachable() {
             match environ.return_mode() {
                 ReturnMode::NormalReturns => {