hir_analysis: add #![rustc_no_implicit_bounds]

Adds a new `rustc_attrs` attribute that stops rustc from adding any
default bounds. Useful for tests where default bounds just add noise and
make debugging harder.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1108,6 +1108,10 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         TEST, rustc_insignificant_dtor, Normal, template!(Word),
         WarnFollowing, EncodeCrossCrate::Yes
     ),
+    rustc_attr!(
+        TEST, rustc_no_implicit_bounds, CrateLevel, template!(Word),
+        WarnFollowing, EncodeCrossCrate::No
+    ),
     rustc_attr!(
         TEST, rustc_strict_coherence, Normal, template!(Word),
         WarnFollowing, EncodeCrossCrate::Yes

compiler/rustc_hir_analysis/src/hir_ty_lowering/bounds.rs

@@ -5,14 +5,14 @@ use rustc_errors::codes::*;
 use rustc_errors::struct_span_code_err;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
-use rustc_hir::def_id::{DefId, LocalDefId};
+use rustc_hir::def_id::{CRATE_DEF_ID, DefId, LocalDefId};
 use rustc_hir::{AmbigArg, LangItem, PolyTraitRef};
 use rustc_middle::bug;
 use rustc_middle::ty::{
     self as ty, IsSuggestable, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
     TypeVisitor, Upcast,
 };
-use rustc_span::{ErrorGuaranteed, Ident, Span, Symbol, kw};
+use rustc_span::{ErrorGuaranteed, Ident, Span, Symbol, kw, sym};
 use rustc_trait_selection::traits;
 use smallvec::SmallVec;
 use tracing::{debug, instrument};
@@ -188,6 +188,11 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     ) {
         let tcx = self.tcx();
 
+        // Skip adding any default bounds if `#![rustc_no_implicit_bounds]`
+        if tcx.has_attr(CRATE_DEF_ID, sym::rustc_no_implicit_bounds) {
+            return;
+        }
+
         let meta_sized_did = tcx.require_lang_item(LangItem::MetaSized, span);
         let pointee_sized_did = tcx.require_lang_item(LangItem::PointeeSized, span);
 
@@ -408,24 +413,21 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         let tcx = self.tcx();
         let trait_id = tcx.lang_items().get(trait_);
         if let Some(trait_id) = trait_id
-            && self.do_not_provide_default_trait_bound(
-                trait_id,
-                hir_bounds,
-                self_ty_where_predicates,
-            )
+            && self.should_add_default_traits(trait_id, hir_bounds, self_ty_where_predicates)
         {
             add_trait_bound(tcx, bounds, self_ty, trait_id, span);
         }
     }
 
-    fn do_not_provide_default_trait_bound<'a>(
+    /// Returns `true` if default trait bound should be added.
+    fn should_add_default_traits<'a>(
         &self,
         trait_def_id: DefId,
         hir_bounds: &'a [hir::GenericBound<'tcx>],
         self_ty_where_predicates: Option<(LocalDefId, &'tcx [hir::WherePredicate<'tcx>])>,
     ) -> bool {
         let collected = collect_bounds(hir_bounds, self_ty_where_predicates, trait_def_id);
-        !collected.any()
+        !self.tcx().has_attr(CRATE_DEF_ID, sym::rustc_no_implicit_bounds) && !collected.any()
     }
 
     /// Lower HIR bounds into `bounds` given the self type `param_ty` and the overarching late-bound vars if any.

compiler/rustc_span/src/symbol.rs

@@ -1866,6 +1866,7 @@ symbols! {
         rustc_never_returns_null_ptr,
         rustc_never_type_options,
         rustc_no_implicit_autorefs,
+        rustc_no_implicit_bounds,
         rustc_no_mir_inline,
         rustc_nonnull_optimization_guaranteed,
         rustc_nounwind,