Auto merge of #11197 - weihanglo:enum-default, r=epage

Use `#[default]` when possible

Just realize that we can derive `Default` for some enums with `#[default]` attribute. This makes default value closer to enum definition.

Ref: https://doc.rust-lang.org/std/default/trait.Default.html#enums

Author: bors

src/cargo/core/compiler/fingerprint.rs

@@ -559,10 +559,12 @@ pub struct Fingerprint {
 }
 
 /// Indication of the status on the filesystem for a particular unit.
+#[derive(Default)]
 enum FsStatus {
     /// This unit is to be considered stale, even if hash information all
     /// matches. The filesystem inputs have changed (or are missing) and the
     /// unit needs to subsequently be recompiled.
+    #[default]
     Stale,
 
     /// This unit is up-to-date. All outputs and their corresponding mtime are
@@ -579,12 +581,6 @@ impl FsStatus {
     }
 }
 
-impl Default for FsStatus {
-    fn default() -> FsStatus {
-        FsStatus::Stale
-    }
-}
-
 impl Serialize for DepFingerprint {
     fn serialize<S>(&self, ser: S) -> Result<S::Ok, S::Error>
     where

src/cargo/core/resolver/resolve.rs

@@ -48,13 +48,23 @@ pub struct Resolve {
 
 /// A version to indicate how a `Cargo.lock` should be serialized.
 ///
-/// When creating a new lockfile, the default version defined in
-/// [`ResolveVersion::default`] is used.
+/// When creating a new lockfile, the version with `#[default]` is used.
 /// If an old version of lockfile already exists, it will stay as-is.
 ///
+/// It's important that if a new version is added that this is not updated
+/// until *at least* the support for the version is in the stable release of Rust.
+///
+/// This resolve version will be used for all new lock files, for example
+/// those generated by `cargo update` (update everything) or building after
+/// a `cargo new` (where no lock file previously existed). This is also used
+/// for *updated* lock files such as when a dependency is added or when a
+/// version requirement changes. In this situation Cargo's updating the lock
+/// file anyway so it takes the opportunity to bump the lock file version
+/// forward.
+///
 /// It's theorized that we can add more here over time to track larger changes
 /// to the `Cargo.lock` format, but we've yet to see how that strategy pans out.
-#[derive(PartialEq, Eq, Clone, Copy, Debug, PartialOrd, Ord)]
+#[derive(Default, PartialEq, Eq, Clone, Copy, Debug, PartialOrd, Ord)]
 pub enum ResolveVersion {
     /// Historical baseline for when this abstraction was added.
     V1,
@@ -68,6 +78,7 @@ pub enum ResolveVersion {
     /// `branch = "master"` are no longer encoded the same way as those without
     /// branch specifiers. Introduced in 2020 in version 1.47. New lockfiles use
     /// V3 by default staring in 1.53.
+    #[default]
     V3,
 }
 
@@ -394,22 +405,3 @@ impl fmt::Debug for Resolve {
         write!(fmt, "}}")
     }
 }
-
-impl Default for ResolveVersion {
-    /// The default way to encode new or updated `Cargo.lock` files.
-    ///
-    /// It's important that if a new version of [`ResolveVersion`] is added that
-    /// this is not updated until *at least* the support for the version is in
-    /// the stable release of Rust.
-    ///
-    /// This resolve version will be used for all new lock files, for example
-    /// those generated by `cargo update` (update everything) or building after
-    /// a `cargo new` (where no lock file previously existed). This is also used
-    /// for *updated* lock files such as when a dependency is added or when a
-    /// version requirement changes. In this situation Cargo's updating the lock
-    /// file anyway so it takes the opportunity to bump the lock file version
-    /// forward.
-    fn default() -> ResolveVersion {
-        ResolveVersion::V3
-    }
-}

src/cargo/util/config/mod.rs

@@ -2127,9 +2127,10 @@ pub struct CargoFutureIncompatConfig {
     frequency: Option<CargoFutureIncompatFrequencyConfig>,
 }
 
-#[derive(Debug, Deserialize, PartialEq)]
+#[derive(Debug, Default, Deserialize, PartialEq)]
 #[serde(rename_all = "kebab-case")]
 pub enum CargoFutureIncompatFrequencyConfig {
+    #[default]
     Always,
     Never,
 }
@@ -2146,12 +2147,6 @@ impl CargoFutureIncompatConfig {
     }
 }
 
-impl Default for CargoFutureIncompatFrequencyConfig {
-    fn default() -> Self {
-        Self::Always
-    }
-}
-
 /// Configuration for `ssl-version` in `http` section
 /// There are two ways to configure:
 ///
@@ -2270,20 +2265,15 @@ pub struct ProgressConfig {
     pub width: Option<usize>,
 }
 
-#[derive(Debug, Deserialize)]
+#[derive(Debug, Default, Deserialize)]
 #[serde(rename_all = "lowercase")]
 pub enum ProgressWhen {
+    #[default]
     Auto,
     Never,
     Always,
 }
 
-impl Default for ProgressWhen {
-    fn default() -> ProgressWhen {
-        ProgressWhen::Auto
-    }
-}
-
 fn progress_or_string<'de, D>(deserializer: D) -> Result<Option<ProgressConfig>, D::Error>
 where
     D: serde::de::Deserializer<'de>,