Auto merge of #117726 - eggyal:automatically_make_trivial_types_noop-traversable, r=<try>

Automatically make trivial types noop-traversable

This is a second reincarnation of #108214 (or at least, what that PR ultimately became), the previous reincarnation in #117620 having been closed accidentally.  It may be easier to review by commit, albeit there are quite a few of them.

## Terminology

I refer to:
* folds and visits as "traversals"
* the types that are folded or visited as "traversables"
* the folders and visitors as "traversers"
* traversables on which traversers can act as "interesting" (there are currently only five: binders, types, consts, regions and predicates) and all others as "trivial" (traversals of which are no-ops)
* the `TypeFoldable` and `TypeVisitable` traits (and only these traits) as the "traversable traits"

## Content

This PR:

* introduces a macro, `rustc_type_ir::noop_if_trivially_traversable` that uses [auto-deref specialisation](http://lukaskalbertodt.github.io/2019/12/05/generalized-autoref-based-specialization.html) to:
    * perform a no-op traversal on values of type `T` if the interner implements `rustc_type_ir::TriviallyTraverses<T>` (thereby obviating all need for most trivial types to implement the traversable traits—and indeed such implementations are removed) or
    * delegate to the relevant traversable trait implementation on `T` otherwise;
* introduces an auto-trait, `rustc_middle::ty::TriviallyTraversable`, that is then "unimplemented" on the interesting types (and thereby remains implemented only on, but on all, trivial types);
* implements `rustc_type_ir::TriviallyTraverses<T>` on the `TyCtxt<'tcx>` interner for all `T: rustc_middle::ty::TriviallyTraversable` (thus ensuring that, for that interner at least, trivial types do not require implementations of the traversable traits);
* updates the traversable traits' derive macros to:
    * skip fields that reference neither any generic type parameters nor, if present, the `'tcx` lifetime parameter
    * use the above specialisation macro when traversing fields
    * if the traversable is not parameterised by a `'tcx` lifetime, generate implementations that are generic over the interner;
* replaces those derive macros' distinct helper attributes (introduced in #108040) with a unified `#[skip_traversal]` helper attribute that requires a justifying reason to be provided (this does mean that the different types of traversal can no longer be independently skipped, but nowhere is that currently required or ever expected to be);
    * the derive macros by default refuse to be implemented on trivial types as specialisation usually negates any need—but this can be overridden with a `#[skip_traversal(impl_despite_trivial_because = "<reason>")]` attribute;
* uses those derive macros in place of remaining usages of the `TrivialTypeTraversal` macros and some explicit implementations; and
* a few other minor relevant refactorings.

## Commentary

One limitation of auto-deref specialisation is that it cannot work for unconstrained generic types.  Therefore, absent real specialisation, implementations of the traversable traits must constrain their generics:
* by default, the derive macros constrain every field type `T` that references one or more generic type parameters to implementors of the relevant traversable trait—however this can be modified on a field (or variant) basis with a `#[skip_traversal(because_trivial)]` attribute so that instead the interner is constrained to implement `TriviallyTraverses<T>` (and the field is not traversed)
    * the constraints are applied to the field types rather than the generic type parameters to achieve a "[perfect derive](https://smallcultfollowing.com/babysteps/blog/2022/04/12/implied-bounds-and-perfect-derive/)" that not only avoids having to propagate `#[skip_traversal]` annotations into wrapping types but also works with associated types and other esoteric situations—however this could result in trait solver cycles if interesting generic field types are one day involved in recursive type definitions;
    * for exceptionally rare cases where traversal of an item/variant/field should be a no-op despite it being (potentially, if generic) interesting, it can be annotated with the explicit and deliberately cumbersome `#[skip_traversal(despite_potential_miscompilation_because = "<reason>")]` (which produces a no-op without adding any constraint).
* (the few remaining) explicit implementations of the traversable traits over generic types are similarly constrained
    * indeed, for generic tuples *all element types* must implement the trait—however, since (most) trivial types no longer do, this unfortunately means that the implementations are not applicable to tuples with trivial elements and hence some such tuples have been replaced with more concrete newtypes that have appropriate constraints: c5e9447 is a particularly hefty example of this (where tuples containing `Span` are replaced with `Spanned<T>` instead).

r? `@lcnr`
cc `@oli-obk`

`@rustbot` label A-query-system T-compiler WG-trait-system-refactor

Author: bors

Cargo.lock

@@ -3918,6 +3918,7 @@ dependencies = [
  "rustc_serialize",
  "rustc_span",
  "rustc_target",
+ "rustc_type_ir",
  "smallvec",
  "tracing",
 ]
@@ -4032,6 +4033,7 @@ dependencies = [
  "rustc_serialize",
  "rustc_span",
  "rustc_target",
+ "rustc_type_ir",
  "smallvec",
  "tracing",
 ]
@@ -4531,6 +4533,7 @@ dependencies = [
  "rustc_index",
  "rustc_macros",
  "rustc_serialize",
+ "rustc_type_ir",
  "scoped-tls",
  "sha1",
  "sha2",
@@ -4570,6 +4573,7 @@ dependencies = [
  "rustc_macros",
  "rustc_serialize",
  "rustc_span",
+ "rustc_type_ir",
  "serde_json",
  "tracing",
 ]
@@ -4601,6 +4605,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "rustc_transmute",
+ "rustc_type_ir",
  "smallvec",
  "tracing",
 ]
@@ -4630,6 +4635,7 @@ dependencies = [
  "rustc_middle",
  "rustc_span",
  "rustc_target",
+ "rustc_type_ir",
  "tracing",
 ]
 

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -14,9 +14,9 @@ use rustc_middle::hir::nested_filter::OnlyBodies;
 use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::{
     self, AggregateKind, BindingForm, BorrowKind, CallSource, ClearCrossCrate, ConstraintCategory,
-    FakeReadCause, LocalDecl, LocalInfo, LocalKind, Location, MutBorrowKind, Operand, Place,
-    PlaceRef, ProjectionElem, Rvalue, Statement, StatementKind, Terminator, TerminatorKind,
-    VarBindingForm,
+    FakeReadCause, FakeReadCauseAndPlace, LocalDecl, LocalInfo, LocalKind, Location, MutBorrowKind,
+    Operand, Place, PlaceRef, ProjectionElem, Rvalue, Statement, StatementKind, Terminator,
+    TerminatorKind, VarBindingForm,
 };
 use rustc_middle::ty::{self, suggest_constraining_type_params, PredicateKind, Ty};
 use rustc_middle::util::CallKind;
@@ -671,8 +671,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         let tcx = self.infcx.tcx;
 
         // Find out if the predicates show that the type is a Fn or FnMut
-        let find_fn_kind_from_did = |(pred, _): (ty::Clause<'tcx>, _)| {
-            if let ty::ClauseKind::Trait(pred) = pred.kind().skip_binder()
+        let find_fn_kind_from_did = |pred: ty::Spanned<ty::Clause<'tcx>>| {
+            if let ty::ClauseKind::Trait(pred) = pred.node.kind().skip_binder()
                 && pred.self_ty() == ty
             {
                 if Some(pred.def_id()) == tcx.lang_items().fn_trait() {
@@ -698,7 +698,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => tcx
                 .explicit_item_bounds(def_id)
                 .iter_instantiated_copied(tcx, args)
-                .find_map(|(clause, span)| find_fn_kind_from_did((clause, span))),
+                .find_map(find_fn_kind_from_did),
             ty::Closure(_, args) => match args.as_closure().kind() {
                 ty::ClosureKind::Fn => Some(hir::Mutability::Not),
                 ty::ClosureKind::FnMut => Some(hir::Mutability::Mut),
@@ -3010,9 +3010,10 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         impl<'tcx> Visitor<'tcx> for FakeReadCauseFinder<'tcx> {
             fn visit_statement(&mut self, statement: &Statement<'tcx>, _: Location) {
                 match statement {
-                    Statement { kind: StatementKind::FakeRead(box (cause, place)), .. }
-                        if *place == self.place =>
-                    {
+                    Statement {
+                        kind: StatementKind::FakeRead(box FakeReadCauseAndPlace(cause, place)),
+                        ..
+                    } if *place == self.place => {
                         self.cause = Some(*cause);
                     }
                     _ => (),

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -6,8 +6,8 @@ use rustc_hir::intravisit::Visitor;
 use rustc_index::IndexSlice;
 use rustc_infer::infer::NllRegionVariableOrigin;
 use rustc_middle::mir::{
-    Body, CallSource, CastKind, ConstraintCategory, FakeReadCause, Local, LocalInfo, Location,
-    Operand, Place, Rvalue, Statement, StatementKind, TerminatorKind,
+    Body, CallSource, CastKind, ConstraintCategory, FakeReadCause, FakeReadCauseAndPlace, Local,
+    LocalInfo, Location, Operand, Place, Rvalue, Statement, StatementKind, TerminatorKind,
 };
 use rustc_middle::ty::adjustment::PointerCoercion;
 use rustc_middle::ty::{self, RegionVid, TyCtxt};
@@ -477,7 +477,11 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 let block = &self.body.basic_blocks[location.block];
 
                 let kind = if let Some(&Statement {
-                    kind: StatementKind::FakeRead(box (FakeReadCause::ForLet(_), place)),
+                    kind:
+                        StatementKind::FakeRead(box FakeReadCauseAndPlace(
+                            FakeReadCause::ForLet(_),
+                            place,
+                        )),
                     ..
                 }) = block.statements.get(location.statement_index)
                 {

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -13,9 +13,9 @@ use rustc_index::IndexSlice;
 use rustc_infer::infer::LateBoundRegionConversionTime;
 use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::{
-    AggregateKind, CallSource, ConstOperand, FakeReadCause, Local, LocalInfo, LocalKind, Location,
-    Operand, Place, PlaceRef, ProjectionElem, Rvalue, Statement, StatementKind, Terminator,
-    TerminatorKind,
+    AggregateKind, CallSource, ConstOperand, FakeReadCause, FakeReadCauseAndPlace, Local,
+    LocalInfo, LocalKind, Location, Operand, Place, PlaceRef, ProjectionElem, Rvalue, Statement,
+    StatementKind, Terminator, TerminatorKind,
 };
 use rustc_middle::ty::print::Print;
 use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
@@ -797,7 +797,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
         // StatementKind::FakeRead only contains a def_id if they are introduced as a result
         // of pattern matching within a closure.
-        if let StatementKind::FakeRead(box (cause, place)) = stmt.kind {
+        if let StatementKind::FakeRead(box FakeReadCauseAndPlace(cause, place)) = stmt.kind {
             match cause {
                 FakeReadCause::ForMatchedPlace(Some(closure_def_id))
                 | FakeReadCause::ForLet(Some(closure_def_id)) => {

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -130,7 +130,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 // opt_match_place is None for let [mut] x = ... statements,
                 // whether or not the right-hand side is a place expression
                 if let LocalInfo::User(BindingForm::Var(VarBindingForm {
-                    opt_match_place: Some((opt_match_place, match_span)),
+                    opt_match_place: Some(opt_match_place),
                     binding_mode: _,
                     opt_ty_info: _,
                     pat_span: _,
@@ -143,8 +143,8 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                         original_path,
                         *move_from,
                         local,
-                        opt_match_place,
-                        match_span,
+                        opt_match_place.node,
+                        opt_match_place.span,
                         stmt_source_info.span,
                     );
                     return;

compiler/rustc_borrowck/src/invalidation.rs

@@ -62,7 +62,7 @@ impl<'cx, 'tcx> Visitor<'tcx> for InvalidationGenerator<'cx, 'tcx> {
 
                 self.mutate_place(location, *lhs, Shallow(None));
             }
-            StatementKind::FakeRead(box (_, _)) => {
+            StatementKind::FakeRead(_) => {
                 // Only relevant for initialized/liveness/safety checks.
             }
             StatementKind::Intrinsic(box NonDivergingIntrinsic::Assume(op)) => {

compiler/rustc_borrowck/src/lib.rs

@@ -629,7 +629,7 @@ impl<'cx, 'tcx, R> rustc_mir_dataflow::ResultsVisitor<'cx, 'tcx, R> for MirBorro
 
                 self.mutate_place(location, (*lhs, span), Shallow(None), flow_state);
             }
-            StatementKind::FakeRead(box (_, place)) => {
+            StatementKind::FakeRead(box FakeReadCauseAndPlace(_, place)) => {
                 // Read for match doesn't access any memory and is used to
                 // assert that a place is safe and live. So we don't have to
                 // do any checks here.

compiler/rustc_borrowck/src/type_check/canonical.rs

@@ -99,10 +99,10 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         instantiated_predicates: ty::InstantiatedPredicates<'tcx>,
         locations: Locations,
     ) {
-        for (predicate, span) in instantiated_predicates {
-            debug!(?predicate);
-            let category = ConstraintCategory::Predicate(span);
-            let predicate = self.normalize_with_category(predicate, locations, category);
+        for predicate in instantiated_predicates {
+            debug!(?predicate.node);
+            let category = ConstraintCategory::Predicate(predicate.span);
+            let predicate = self.normalize_with_category(predicate.node, locations, category);
             self.prove_predicate(predicate, locations, category);
         }
     }

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1297,7 +1297,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                     );
                 }
             }
-            StatementKind::AscribeUserType(box (place, projection), variance) => {
+            StatementKind::AscribeUserType(box AscribeUserType(place, projection), variance) => {
                 let place_ty = place.ty(body, tcx).ty;
                 if let Err(terr) = self.relate_type_and_user_type(
                     place_ty,

compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs

@@ -90,13 +90,13 @@ fn make_mir_scope<'ll, 'tcx>(
     let file_metadata = file_metadata(cx, &loc.file);
 
     let parent_dbg_scope = match scope_data.inlined {
-        Some((callee, _)) => {
+        Some(callee) => {
             // FIXME(eddyb) this would be `self.monomorphize(&callee)`
             // if this is moved to `rustc_codegen_ssa::mir::debuginfo`.
             let callee = cx.tcx.instantiate_and_normalize_erasing_regions(
                 instance.args,
                 ty::ParamEnv::reveal_all(),
-                ty::EarlyBinder::bind(callee),
+                ty::EarlyBinder::bind(callee.node),
             );
             debug_context.inlined_function_scopes.entry(callee).or_insert_with(|| {
                 let callee_fn_abi = cx.fn_abi_of_instance(callee, ty::List::empty());
@@ -116,11 +116,11 @@ fn make_mir_scope<'ll, 'tcx>(
         )
     };
 
-    let inlined_at = scope_data.inlined.map(|(_, callsite_span)| {
+    let inlined_at = scope_data.inlined.map(|callee| {
         // FIXME(eddyb) this doesn't account for the macro-related
         // `Span` fixups that `rustc_codegen_ssa::mir::debuginfo` does.
-        let callsite_scope = parent_scope.adjust_dbg_scope_for_span(cx, callsite_span);
-        cx.dbg_loc(callsite_scope, parent_scope.inlined_at, callsite_span)
+        let callsite_scope = parent_scope.adjust_dbg_scope_for_span(cx, callee.span);
+        cx.dbg_loc(callsite_scope, parent_scope.inlined_at, callee.span)
     });
 
     debug_context.scopes[scope] = DebugScope {

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -1165,9 +1165,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     // If the stacktrace passes through MIR-inlined source scopes, add them.
                     let mir::SourceInfo { mut span, scope } = *frame.body.source_info(loc);
                     let mut scope_data = &frame.body.source_scopes[scope];
-                    while let Some((instance, call_span)) = scope_data.inlined {
-                        frames.push(FrameInfo { span, instance });
-                        span = call_span;
+                    while let Some(instance) = scope_data.inlined {
+                        frames.push(FrameInfo { span, instance: instance.node });
+                        span = instance.span;
                         scope_data = &frame.body.source_scopes[scope_data.parent_scope.unwrap()];
                     }
                     span

compiler/rustc_data_structures/src/intern.rs

@@ -24,6 +24,10 @@ mod private {
 #[rustc_pass_by_value]
 pub struct Interned<'a, T>(pub &'a T, pub private::PrivateZst);
 
+pub trait Internable<'a, I>: Sized {
+    fn intern(self, interner: I) -> Interned<'a, Self>;
+}
+
 impl<'a, T> Interned<'a, T> {
     /// Create a new `Interned` value. The value referred to *must* be interned
     /// and thus be unique, and it *must* remain unique in the future. This

compiler/rustc_hir/Cargo.toml

@@ -15,6 +15,7 @@ rustc_macros = { path = "../rustc_macros" }
 rustc_serialize = { path = "../rustc_serialize" }
 rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }
+rustc_type_ir = { path = "../rustc_type_ir" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 tracing = "0.1"
 # tidy-alphabetical-end

compiler/rustc_hir/src/hir.rs

@@ -3256,7 +3256,10 @@ impl<'hir> Item<'hir> {
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
-#[derive(Encodable, Decodable, HashStable_Generic)]
+#[derive(Encodable, Decodable, HashStable_Generic, TypeFoldable, TypeVisitable)]
+#[skip_traversal(
+    but_impl_despite_trivial_because = "`Unsafety` impls `Relate`, which is a subtrait of `TypeFoldable`."
+)]
 pub enum Unsafety {
     Unsafe,
     Normal,

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -1057,7 +1057,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     tcx,
                     predicates
                         .iter()
-                        .filter_map(|(p, _)| Some(p.as_trait_clause()?.map_bound(|t| t.trait_ref))),
+                        .filter_map(|p| Some(p.node.as_trait_clause()?.map_bound(|t| t.trait_ref))),
                     assoc_name,
                 )
             },

compiler/rustc_hir_analysis/src/astconv/object_safety.rs

@@ -57,15 +57,15 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
         let mut trait_bounds = vec![];
         let mut projection_bounds = vec![];
-        for (pred, span) in bounds.clauses() {
-            let bound_pred = pred.kind();
+        for pred in bounds.clauses() {
+            let bound_pred = pred.node.kind();
             match bound_pred.skip_binder() {
                 ty::ClauseKind::Trait(trait_pred) => {
                     assert_eq!(trait_pred.polarity, ty::ImplPolarity::Positive);
-                    trait_bounds.push((bound_pred.rebind(trait_pred.trait_ref), span));
+                    trait_bounds.push((bound_pred.rebind(trait_pred.trait_ref), pred.span));
                 }
                 ty::ClauseKind::Projection(proj) => {
-                    projection_bounds.push((bound_pred.rebind(proj), span));
+                    projection_bounds.push((bound_pred.rebind(proj), pred.span));
                 }
                 ty::ClauseKind::TypeOutlives(_) => {
                     // Do nothing, we deal with regions separately

compiler/rustc_hir_analysis/src/bounds.rs

@@ -23,7 +23,7 @@ use rustc_span::Span;
 /// include the self type (e.g., `trait_bounds`) but in others we do not
 #[derive(Default, PartialEq, Eq, Clone, Debug)]
 pub struct Bounds<'tcx> {
-    pub clauses: Vec<(ty::Clause<'tcx>, Span)>,
+    pub clauses: Vec<ty::Spanned<ty::Clause<'tcx>>>,
 }
 
 impl<'tcx> Bounds<'tcx> {
@@ -33,8 +33,10 @@ impl<'tcx> Bounds<'tcx> {
         region: ty::PolyTypeOutlivesPredicate<'tcx>,
         span: Span,
     ) {
-        self.clauses
-            .push((region.map_bound(|p| ty::ClauseKind::TypeOutlives(p)).to_predicate(tcx), span));
+        self.clauses.push(ty::Spanned {
+            node: region.map_bound(|p| ty::ClauseKind::TypeOutlives(p)).to_predicate(tcx),
+            span,
+        });
     }
 
     pub fn push_trait_bound(
@@ -72,14 +74,14 @@ impl<'tcx> Bounds<'tcx> {
         span: Span,
         polarity: ty::ImplPolarity,
     ) {
-        self.clauses.push((
-            trait_ref
+        self.clauses.push(ty::Spanned {
+            node: trait_ref
                 .map_bound(|trait_ref| {
                     ty::ClauseKind::Trait(ty::TraitPredicate { trait_ref, polarity })
                 })
                 .to_predicate(tcx),
             span,
-        ));
+        });
     }
 
     pub fn push_projection_bound(
@@ -88,20 +90,20 @@ impl<'tcx> Bounds<'tcx> {
         projection: ty::PolyProjectionPredicate<'tcx>,
         span: Span,
     ) {
-        self.clauses.push((
-            projection.map_bound(|proj| ty::ClauseKind::Projection(proj)).to_predicate(tcx),
+        self.clauses.push(ty::Spanned {
+            node: projection.map_bound(|proj| ty::ClauseKind::Projection(proj)).to_predicate(tcx),
             span,
-        ));
+        });
     }
 
     pub fn push_sized(&mut self, tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span) {
         let sized_def_id = tcx.require_lang_item(LangItem::Sized, Some(span));
         let trait_ref = ty::TraitRef::new(tcx, sized_def_id, [ty]);
         // Preferable to put this obligation first, since we report better errors for sized ambiguity.
-        self.clauses.insert(0, (trait_ref.to_predicate(tcx), span));
+        self.clauses.insert(0, ty::Spanned { node: trait_ref.to_predicate(tcx), span });
     }
 
-    pub fn clauses(&self) -> impl Iterator<Item = (ty::Clause<'tcx>, Span)> + '_ {
+    pub fn clauses(&self) -> impl Iterator<Item = ty::Spanned<ty::Clause<'tcx>>> + '_ {
         self.clauses.iter().cloned()
     }
 }