Use #[derive(Default)] instead of manually implementing it

core/tests/hash/mod.rs

@@ -4,16 +4,11 @@ use std::hash::{BuildHasher, Hash, Hasher};
 use std::ptr;
 use std::rc::Rc;
 
+#[derive(Default)]
 struct MyHasher {
     hash: u64,
 }
 
-impl Default for MyHasher {
-    fn default() -> MyHasher {
-        MyHasher { hash: 0 }
-    }
-}
-
 impl Hasher for MyHasher {
     fn write(&mut self, buf: &[u8]) {
         for byte in buf {
@@ -107,6 +102,8 @@ fn test_writer_hasher() {
 struct Custom {
     hash: u64,
 }
+
+#[derive(Default)]
 struct CustomHasher {
     output: u64,
 }
@@ -123,12 +120,6 @@ impl Hasher for CustomHasher {
     }
 }
 
-impl Default for CustomHasher {
-    fn default() -> CustomHasher {
-        CustomHasher { output: 0 }
-    }
-}
-
 impl Hash for Custom {
     fn hash<H: Hasher>(&self, state: &mut H) {
         state.write_u64(self.hash);

proc_macro/src/bridge/fxhash.rs

@@ -22,6 +22,7 @@ pub type FxHashMap<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher>>;
 /// out-performs an FNV-based hash within rustc itself -- the collision rate is
 /// similar or slightly worse than FNV, but the speed of the hash function
 /// itself is much higher because it works on up to 8 bytes at a time.
+#[derive(Default)]
 pub struct FxHasher {
     hash: usize,
 }
@@ -31,13 +32,6 @@ const K: usize = 0x9e3779b9;
 #[cfg(target_pointer_width = "64")]
 const K: usize = 0x517cc1b727220a95;
 
-impl Default for FxHasher {
-    #[inline]
-    fn default() -> FxHasher {
-        FxHasher { hash: 0 }
-    }
-}
-
 impl FxHasher {
     #[inline]
     fn add_to_hash(&mut self, i: usize) {

std/src/panicking.rs

@@ -81,7 +81,9 @@ extern "C" fn __rust_foreign_exception() -> ! {
     rtabort!("Rust cannot catch foreign exceptions");
 }
 
+#[derive(Default)]
 enum Hook {
+    #[default]
     Default,
     Custom(Box<dyn Fn(&PanicHookInfo<'_>) + 'static + Sync + Send>),
 }
@@ -96,13 +98,6 @@ impl Hook {
     }
 }
 
-impl Default for Hook {
-    #[inline]
-    fn default() -> Hook {
-        Hook::Default
-    }
-}
-
 static HOOK: RwLock<Hook> = RwLock::new(Hook::Default);
 
 /// Registers a custom panic hook, replacing the previously registered hook.

std/src/sys_common/process.rs

@@ -8,19 +8,13 @@ use crate::sys::process::{EnvKey, ExitStatus, Process, StdioPipes};
 use crate::{env, fmt, io};
 
 // Stores a set of changes to an environment
-#[derive(Clone)]
+#[derive(Clone, Default)]
 pub struct CommandEnv {
     clear: bool,
     saw_path: bool,
     vars: BTreeMap<EnvKey, Option<OsString>>,
 }
 
-impl Default for CommandEnv {
-    fn default() -> Self {
-        CommandEnv { clear: false, saw_path: false, vars: Default::default() }
-    }
-}
-
 impl fmt::Debug for CommandEnv {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let mut debug_command_env = f.debug_struct("CommandEnv");