Add a wasmtime settings command to print Cranelift settings.

This commit adds the `wasmtime settings` command to print out available
Cranelift settings for a target (defaults to the host).

The compile command has been updated to remove the Cranelift ISA options in
favor of encouraging users to use `wasmtime settings` to discover what settings
are available.  This will reduce the maintenance cost for syncing the compile
command with Cranelift ISA flags.

src/commands/settings.rs

@@ -0,0 +1,103 @@
+//! The module that implements the `wasmtime settings` command.
+
+use anyhow::{anyhow, Result};
+use std::str::FromStr;
+use structopt::StructOpt;
+use wasmtime_environ::settings::{self, Setting, SettingKind};
+use wasmtime_jit::native;
+
+/// Displays available Cranelift settings for a target.
+#[derive(StructOpt)]
+#[structopt(name = "run")]
+pub struct SettingsCommand {
+    /// The target triple to get the settings for; defaults to the host triple.
+    #[structopt(long, value_name = "TARGET")]
+    target: Option<String>,
+}
+
+impl SettingsCommand {
+    /// Executes the command.
+    pub fn execute(self) -> Result<()> {
+        let settings = match &self.target {
+            Some(target) => {
+                native::lookup(target_lexicon::Triple::from_str(target).map_err(|e| anyhow!(e))?)?
+            }
+            None => native::builder(),
+        };
+
+        let mut enums = (Vec::new(), 0);
+        let mut nums = (Vec::new(), 0);
+        let mut bools = (Vec::new(), 0);
+        let mut presets = (Vec::new(), 0);
+
+        for setting in settings.iter() {
+            let (collection, max) = match setting.kind {
+                SettingKind::Enum => &mut enums,
+                SettingKind::Num => &mut nums,
+                SettingKind::Bool => &mut bools,
+                SettingKind::Preset => &mut presets,
+            };
+
+            if setting.name.len() > *max {
+                *max = setting.name.len();
+            }
+
+            collection.push(setting);
+        }
+
+        if enums.0.is_empty() && nums.0.is_empty() && bools.0.is_empty() && presets.0.is_empty() {
+            println!("Target '{}' has no settings.", settings.triple());
+            return Ok(());
+        }
+
+        println!("Cranelift settings for target '{}':", settings.triple());
+
+        if !enums.0.is_empty() {
+            println!();
+            Self::print_settings("Enum settings:", enums.0, enums.1);
+        }
+
+        if !nums.0.is_empty() {
+            println!();
+            Self::print_settings("Numerical settings:", nums.0, nums.1);
+        }
+
+        if !bools.0.is_empty() {
+            println!();
+            Self::print_settings("Boolean settings:", bools.0, bools.1);
+        }
+
+        if !presets.0.is_empty() {
+            println!();
+            Self::print_settings("Presets:", presets.0, presets.1);
+        }
+
+        if self.target.is_none() {
+            let isa = settings.finish(settings::Flags::new(settings::builder()));
+            println!();
+            println!("Settings enabled for this host:");
+
+            for flag in isa.enabled_isa_flags() {
+                println!("  - {}", flag);
+            }
+        }
+
+        Ok(())
+    }
+
+    fn print_settings(header: &str, settings: impl IntoIterator<Item = Setting>, width: usize) {
+        println!("{}", header);
+        for setting in settings {
+            println!(
+                "  {:width$} {}{}",
+                setting.name,
+                setting.description,
+                setting
+                    .values
+                    .map(|v| format!(" Supported values: {}.", v.join(", ")))
+                    .unwrap_or("".to_string()),
+                width = width + 2
+            );
+        }
+    }
+}