Code review feedback.

* Expand doc comment on `Engine::precompile_module`.
* Add FIXME comment regarding a future ISA flag compatibility check before
  doing a JIT from `Module::from_binary`.
* Remove no-longer-needed CLI groups from the `compile` command.

crates/wasmtime/src/engine.rs

@@ -89,6 +89,15 @@ impl Engine {
     ///   If this is supplied then the text format will be parsed before validation.
     ///   Note that the `wat` feature is enabled by default.
     ///
+    /// This method may be used to compile a module for use with a different target
+    /// host. The output of this method may be used with [`Module::new`](crate::Module::new)
+    /// on hosts compatible with the [`Config`] associated with this [`Engine`].
+    ///
+    /// The output of this method is safe to send to another host machine for later
+    /// execution. As the output is already a compiled module, translation and code
+    /// generation will be skipped and this will improve the performance of constructing
+    /// a [`Module`](crate::Module) from the output of this method.
+    ///
     /// [binary]: https://webassembly.github.io/spec/core/binary/index.html
     /// [text]: https://webassembly.github.io/spec/core/text/index.html
     pub fn precompile_module(&self, bytes: &[u8]) -> Result<Vec<u8>> {

crates/wasmtime/src/module.rs

@@ -279,6 +279,10 @@ impl Module {
             );
         }
 
+        // FIXME: we may want to validate that the ISA flags in the config match those that
+        // would be inferred for the host, otherwise the JIT might produce unrunnable code
+        // for the features the host's CPU actually has.
+
         const USE_PAGED_MEM_INIT: bool = cfg!(all(feature = "uffd", target_os = "linux"));
 
         cfg_if::cfg_if! {

src/commands/compile.rs

@@ -4,10 +4,7 @@ use crate::CommonOptions;
 use anyhow::{bail, Context, Result};
 use std::fs;
 use std::path::PathBuf;
-use structopt::{
+use structopt::{clap::AppSettings, StructOpt};
-    clap::{AppSettings, ArgGroup},
-    StructOpt,
-};
 use target_lexicon::Triple;
 use wasmtime::Engine;
 
@@ -42,10 +39,6 @@ lazy_static::lazy_static! {
     name = "compile",
     version = env!("CARGO_PKG_VERSION"),
     setting = AppSettings::ColoredHelp,
-    group = ArgGroup::with_name("x64").multiple(true),
-    group = ArgGroup::with_name("preset-x64"),
-    group = ArgGroup::with_name("aarch64").multiple(true).conflicts_with_all(&["x64", "preset-x64"]),
-    group = ArgGroup::with_name("preset-aarch64").conflicts_with_all(&["x64", "preset-x64"]),
     after_help = AFTER_HELP.as_str()
 )]
 pub struct CompileCommand {