Use derive_default_enum in the compiler

compiler/rustc_infer/src/infer/mod.rs

@@ -95,9 +95,10 @@ pub(crate) type UnificationTable<'a, 'tcx, T> = ut::UnificationTable<
 /// This is used so that the region values inferred by HIR region solving are
 /// not exposed, and so that we can avoid doing work in HIR typeck that MIR
 /// typeck will also do.
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, Default)]
 pub enum RegionckMode {
     /// The default mode: report region errors, don't erase regions.
+    #[default]
     Solve,
     /// Erase the results of region after solving.
     Erase {
@@ -108,12 +109,6 @@ pub enum RegionckMode {
     },
 }
 
-impl Default for RegionckMode {
-    fn default() -> Self {
-        RegionckMode::Solve
-    }
-}
-
 impl RegionckMode {
     /// Indicates that the MIR borrowck will repeat these region
     /// checks, so we should ignore errors if NLL is (unconditionally)

compiler/rustc_infer/src/lib.rs

@@ -15,6 +15,7 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
+#![feature(derive_default_enum)]
 #![feature(extend_one)]
 #![feature(iter_zip)]
 #![feature(let_else)]

compiler/rustc_middle/src/lib.rs

@@ -30,6 +30,7 @@
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(core_intrinsics)]
+#![feature(derive_default_enum)]
 #![feature(discriminant_kind)]
 #![feature(exhaustive_patterns)]
 #![feature(if_let_guard)]

compiler/rustc_middle/src/ty/sty.rs

@@ -2263,10 +2263,11 @@ impl<'tcx> TyS<'tcx> {
 /// a miscompilation or unsoundness.
 ///
 /// When in doubt, use `VarianceDiagInfo::default()`
-#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
 pub enum VarianceDiagInfo<'tcx> {
     /// No additional information - this is the default.
     /// We will not add any additional information to error messages.
+    #[default]
     None,
     /// We switched our variance because a type occurs inside
     /// the generic argument of a mutable reference or pointer
@@ -2301,9 +2302,3 @@ impl<'tcx> VarianceDiagInfo<'tcx> {
         }
     }
 }
-
-impl<'tcx> Default for VarianceDiagInfo<'tcx> {
-    fn default() -> Self {
-        Self::None
-    }
-}

compiler/rustc_session/src/config.rs

@@ -335,9 +335,10 @@ impl Default for ErrorOutputType {
 }
 
 /// Parameter to control path trimming.
-#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Hash)]
 pub enum TrimmedDefPaths {
     /// `try_print_trimmed_def_path` never prints a trimmed path and never calls the expensive query
+    #[default]
     Never,
     /// `try_print_trimmed_def_path` calls the expensive query, the query doesn't call `delay_good_path_bug`
     Always,
@@ -345,12 +346,6 @@ pub enum TrimmedDefPaths {
     GoodPath,
 }
 
-impl Default for TrimmedDefPaths {
-    fn default() -> Self {
-        Self::Never
-    }
-}
-
 /// Use tree-based collections to cheaply get a deterministic `Hash` implementation.
 /// *Do not* switch `BTreeMap` out for an unsorted container type! That would break
 /// dependency tracking for command-line arguments. Also only hash keys, since tracking

compiler/rustc_session/src/lib.rs

@@ -1,4 +1,5 @@
 #![feature(crate_visibility_modifier)]
+#![feature(derive_default_enum)]
 #![feature(min_specialization)]
 #![feature(once_cell)]
 #![recursion_limit = "256"]

compiler/rustc_trait_selection/src/lib.rs

@@ -14,6 +14,7 @@
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(drain_filter)]
+#![feature(derive_default_enum)]
 #![feature(hash_drain_filter)]
 #![feature(in_band_lifetimes)]
 #![feature(iter_zip)]

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -82,9 +82,14 @@ pub use self::chalk_fulfill::FulfillmentContext as ChalkFulfillmentContext;
 pub use rustc_infer::traits::*;
 
 /// Whether to skip the leak check, as part of a future compatibility warning step.
-#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+///
+/// The "default" for skip-leak-check corresponds to the current
+/// behavior (do not skip the leak check) -- not the behavior we are
+/// transitioning into.
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Default)]
 pub enum SkipLeakCheck {
     Yes,
+    #[default]
     No,
 }
 
@@ -94,15 +99,6 @@ impl SkipLeakCheck {
     }
 }
 
-/// The "default" for skip-leak-check corresponds to the current
-/// behavior (do not skip the leak check) -- not the behavior we are
-/// transitioning into.
-impl Default for SkipLeakCheck {
-    fn default() -> Self {
-        SkipLeakCheck::No
-    }
-}
-
 /// The mode that trait queries run in.
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 pub enum TraitQueryMode {

library/core/src/lib.rs

@@ -163,6 +163,7 @@
 #![cfg_attr(bootstrap, feature(const_raw_ptr_deref))]
 #![feature(const_refs_to_cell)]
 #![feature(decl_macro)]
+#![feature(derive_default_enum)]
 #![feature(doc_cfg)]
 #![feature(doc_notable_trait)]
 #![feature(doc_primitive)]