Change how flags are stored in serialized modules.

This commit changes how both the shared flags and ISA flags are stored in the
serialized module to detect incompatibilities when a serialized module is
instantiated.

It improves the error reporting when a compiled module has mismatched shared
flags.

cranelift/codegen/src/settings.rs

@@ -72,6 +72,50 @@ pub struct Setting {
     pub values: Option<&'static [&'static str]>,
 }
 
+/// Represents a setting value.
+///
+/// This is used for iterating values in `Flags`.
+pub struct Value {
+    /// The name of the setting associated with this value.
+    pub name: &'static str,
+    pub(crate) detail: detail::Detail,
+    pub(crate) values: Option<&'static [&'static str]>,
+    pub(crate) value: u8,
+}
+
+impl Value {
+    /// Gets the kind of setting.
+    pub fn kind(&self) -> SettingKind {
+        match &self.detail {
+            detail::Detail::Enum { .. } => SettingKind::Enum,
+            detail::Detail::Num => SettingKind::Num,
+            detail::Detail::Bool { .. } => SettingKind::Bool,
+            detail::Detail::Preset => unreachable!(),
+        }
+    }
+
+    /// Gets the enum value if the value is from an enum setting.
+    pub fn as_enum(&self) -> Option<&'static str> {
+        self.values.map(|v| v[self.value as usize])
+    }
+
+    /// Gets the numerical value if the value is from a num setting.
+    pub fn as_num(&self) -> Option<u8> {
+        match &self.detail {
+            detail::Detail::Num => Some(self.value),
+            _ => None,
+        }
+    }
+
+    /// Gets the boolean value if the value is from a boolean setting.
+    pub fn as_bool(&self) -> Option<bool> {
+        match &self.detail {
+            detail::Detail::Bool { bit } => Some(self.value & (1 << bit) != 0),
+            _ => None,
+        }
+    }
+}
+
 /// Collect settings values based on a template.
 #[derive(Clone, Hash)]
 pub struct Builder {