Rollup merge of #85200 - FabianWolff:issue-84647, r=nikomatsakis

Ignore derived Clone and Debug implementations during dead code analysis

This pull request fixes #84647. Derived implementations of `Clone` and `Debug` always trivially read all fields, so "field is never read" dead code warnings are never triggered. Arguably, though, a user most likely will only be interested in whether _their_ code ever reads those fields, which is the behavior I have implemented here.

Note that implementations of `Clone` and `Debug` are only ignored if they are `#[derive(...)]`d; a custom `impl Clone/Debug for ...` will still be analyzed normally (i.e. if a custom `Clone` implementation uses all fields of the struct, this will continue to suppress dead code warnings about unused fields); this seemed like the least intrusive change to me (although it would be easy to change — just drop the `&& [impl_]item.span.in_derive_expansion()` in the if conditions).

The only thing that I am slightly unsure about is that in #84647, `@matklad` said
> Doesn't seem easy to fix though :(

However, it _was_ pretty straightforward to fix, so did I perhaps overlook something obvious? `@matklad,` could you weigh in on this?

Author: Manishearth

compiler/rustc_feature/src/accepted.rs

@@ -16,7 +16,6 @@ macro_rules! declare_features {
                     since: $ver,
                     issue: to_nonzero($issue),
                     edition: None,
-                    description: concat!($($doc,)*),
                 }
             ),+
         ];

compiler/rustc_feature/src/active.rs

@@ -37,7 +37,6 @@ macro_rules! declare_features {
                     since: $ver,
                     issue: to_nonzero($issue),
                     edition: $edition,
-                    description: concat!($($doc,)*),
                 }
             ),+];
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -453,6 +453,9 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     // Enumerates "identity-like" conversion methods to suggest on type mismatch.
     rustc_attr!(rustc_conversion_suggestion, Normal, template!(Word), INTERNAL_UNSTABLE),
+    // Prevents field reads in the marked trait or method to be considered
+    // during dead code analysis.
+    rustc_attr!(rustc_trivial_field_reads, Normal, template!(Word), INTERNAL_UNSTABLE),
 
     // ==========================================================================
     // Internal attributes, Const related:

compiler/rustc_feature/src/lib.rs

@@ -51,7 +51,6 @@ pub struct Feature {
     pub since: &'static str,
     issue: Option<NonZeroU32>,
     pub edition: Option<Edition>,
-    description: &'static str,
 }
 
 #[derive(Copy, Clone, Debug)]

compiler/rustc_feature/src/removed.rs

@@ -16,7 +16,6 @@ macro_rules! declare_features {
                     since: $ver,
                     issue: to_nonzero($issue),
                     edition: None,
-                    description: concat!($($doc,)*),
                 }
             ),+
         ];
@@ -34,7 +33,6 @@ macro_rules! declare_features {
                     since: $ver,
                     issue: to_nonzero($issue),
                     edition: None,
-                    description: concat!($($doc,)*),
                 }
             ),+
         ];

compiler/rustc_middle/src/hir/map/collector.rs

@@ -62,13 +62,6 @@ fn hash_body(
     stable_hasher.finish()
 }
 
-/// Represents an entry and its parent `HirId`.
-#[derive(Copy, Clone, Debug)]
-pub struct Entry<'hir> {
-    parent: HirId,
-    node: Node<'hir>,
-}
-
 impl<'a, 'hir> NodeCollector<'a, 'hir> {
     pub(super) fn root(
         sess: &'a Session,

compiler/rustc_middle/src/ich/hcx.rs

@@ -28,7 +28,6 @@ fn compute_ignored_attr_names() -> FxHashSet<Symbol> {
 /// things (e.g., each `DefId`/`DefPath` is only hashed once).
 #[derive(Clone)]
 pub struct StableHashingContext<'a> {
-    sess: &'a Session,
     definitions: &'a Definitions,
     cstore: &'a dyn CrateStore,
     pub(super) body_resolver: BodyResolver<'a>,
@@ -78,7 +77,6 @@ impl<'a> StableHashingContext<'a> {
             !always_ignore_spans && !sess.opts.debugging_opts.incremental_ignore_spans;
 
         StableHashingContext {
-            sess,
             body_resolver: BodyResolver(krate),
             definitions,
             cstore,

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -900,10 +900,7 @@ fn traverse_candidate<'pat, 'tcx: 'pat, C, T, I>(
 struct Binding<'tcx> {
     span: Span,
     source: Place<'tcx>,
-    name: Symbol,
     var_id: HirId,
-    var_ty: Ty<'tcx>,
-    mutability: Mutability,
     binding_mode: BindingMode,
 }
 

compiler/rustc_mir_build/src/build/matches/simplify.rs

@@ -176,17 +176,22 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 Ok(())
             }
 
-            PatKind::Binding { name, mutability, mode, var, ty, ref subpattern, is_primary: _ } => {
+            PatKind::Binding {
+                name: _,
+                mutability: _,
+                mode,
+                var,
+                ty: _,
+                ref subpattern,
+                is_primary: _,
+            } => {
                 if let Ok(place_resolved) =
                     match_pair.place.clone().try_upvars_resolved(self.tcx, self.typeck_results)
                 {
                     candidate.bindings.push(Binding {
-                        name,
-                        mutability,
                         span: match_pair.pattern.span,
                         source: place_resolved.into_place(self.tcx, self.typeck_results),
                         var_id: var,
-                        var_ty: ty,
                         binding_mode: mode,
                     });
                 }

compiler/rustc_mir_build/src/build/scope.rs

@@ -118,9 +118,6 @@ struct Scope {
     /// the region span of this scope within source code.
     region_scope: region::Scope,
 
-    /// the span of that region_scope
-    region_scope_span: Span,
-
     /// set of places to drop when exiting this scope. This starts
     /// out empty but grows as variables are declared during the
     /// building process. This is a stack, so we always drop from the
@@ -420,7 +417,6 @@ impl<'tcx> Scopes<'tcx> {
         self.scopes.push(Scope {
             source_scope: vis_scope,
             region_scope: region_scope.0,
-            region_scope_span: region_scope.1.span,
             drops: vec![],
             moved_locals: vec![],
             cached_unwind_block: None,