Implement Return Type Notation (RTN)'s path form in where clauses

compiler/rustc_ast_lowering/src/asm.rs

@@ -20,8 +20,8 @@ use super::errors::{
 };
 use super::LoweringContext;
 use crate::{
- fluent_generated as fluent, ImplTraitContext, ImplTraitPosition, ParamMode,
- ResolverAstLoweringExt,
+ fluent_generated as fluent, AllowReturnTypeNotation, ImplTraitContext, ImplTraitPosition,
+ ParamMode, ResolverAstLoweringExt,
 };
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
@@ -201,6 +201,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  &sym.qself,
  &sym.path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );

compiler/rustc_ast_lowering/src/delegation.rs

@@ -52,7 +52,7 @@ use rustc_target::spec::abi;
 use {rustc_ast as ast, rustc_hir as hir};
 
 use super::{GenericArgsMode, ImplTraitContext, LoweringContext, ParamMode};
-use crate::{ImplTraitPosition, ResolverAstLoweringExt};
+use crate::{AllowReturnTypeNotation, ImplTraitPosition, ResolverAstLoweringExt};
 
 pub(crate) struct DelegationResults<'hir> {
  pub body_id: hir::BodyId,
@@ -340,6 +340,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
  &delegation.qself,
  &delegation.path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );

compiler/rustc_ast_lowering/src/expr.rs

@@ -23,7 +23,7 @@ use super::{
  GenericArgsMode, ImplTraitContext, LoweringContext, ParamMode, ResolverAstLoweringExt,
 };
 use crate::errors::YieldInClosure;
-use crate::{fluent_generated, FnDeclKind, ImplTraitPosition};
+use crate::{fluent_generated, AllowReturnTypeNotation, FnDeclKind, ImplTraitPosition};
 
 impl<'hir> LoweringContext<'_, 'hir> {
  fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
@@ -281,6 +281,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
  qself,
  path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );
@@ -328,6 +329,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
  &se.qself,
  &se.path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  )),
@@ -1291,6 +1293,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
  qself,
  path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );
@@ -1311,6 +1314,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
  qself,
  path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );
@@ -1336,6 +1340,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
  &se.qself,
  &se.path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );

compiler/rustc_ast_lowering/src/lib.rs

@@ -485,9 +485,23 @@ enum ParamMode {
  Optional,
 }
 
+#[derive(Copy, Clone, Debug)]
+enum AllowReturnTypeNotation {
+ /// Only in types, since RTN is denied later during HIR lowering.
+ Yes,
+ /// All other positions (path expr, method, use tree).
+ No,
+}
+
 enum GenericArgsMode {
+ /// Allow paren sugar, don't allow RTN.
  ParenSugar,
+ /// Allow RTN, don't allow paren sugar.
+ ReturnTypeNotation,
+ // Error if parenthesized generics or RTN are encountered.
  Err,
+ /// Silence errors when lowering generics. Only used with `Res::Err`.
+ Silence,
 }
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
@@ -1226,7 +1240,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  }
 
  let id = self.lower_node_id(t.id);
- let qpath = self.lower_qpath(t.id, qself, path, param_mode, itctx, None);
+ let qpath = self.lower_qpath(
+ t.id,
+ qself,
+ path,
+ param_mode,
+ AllowReturnTypeNotation::Yes,
+ itctx,
+ None,
+ );
  self.ty_path(id, t.span, qpath)
  }
 
@@ -2203,6 +2225,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  &None,
  &p.path,
  ParamMode::Explicit,
+ AllowReturnTypeNotation::No,
  itctx,
  Some(modifiers),
  ) {
@@ -2341,6 +2364,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  &None,
  path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );
@@ -2419,6 +2443,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  qself,
  path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );

compiler/rustc_ast_lowering/src/pat.rs

@@ -11,7 +11,7 @@ use super::errors::{
  ArbitraryExpressionInPattern, ExtraDoubleDot, MisplacedDoubleDot, SubTupleBinding,
 };
 use super::{ImplTraitContext, LoweringContext, ParamMode, ResolverAstLoweringExt};
-use crate::ImplTraitPosition;
+use crate::{AllowReturnTypeNotation, ImplTraitPosition};
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
  pub(crate) fn lower_pat(&mut self, pattern: &Pat) -> &'hir hir::Pat<'hir> {
@@ -38,6 +38,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  qself,
  path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );
@@ -55,6 +56,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  qself,
  path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );
@@ -66,6 +68,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  qself,
  path,
  ParamMode::Optional,
+ AllowReturnTypeNotation::No,
  ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  None,
  );

compiler/rustc_ast_lowering/src/path.rs

@@ -5,6 +5,7 @@ use rustc_hir::def::{DefKind, PartialRes, Res};
 use rustc_hir::def_id::DefId;
 use rustc_hir::GenericArg;
 use rustc_middle::span_bug;
+use rustc_session::parse::add_feature_diagnostics;
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{BytePos, DesugaringKind, Span, Symbol, DUMMY_SP};
 use smallvec::{smallvec, SmallVec};
@@ -15,10 +16,9 @@ use super::errors::{
  GenericTypeWithParentheses, UseAngleBrackets,
 };
 use super::{
- GenericArgsCtor, GenericArgsMode, ImplTraitContext, LifetimeRes, LoweringContext, ParamMode,
- ResolverAstLoweringExt,
+ AllowReturnTypeNotation, GenericArgsCtor, GenericArgsMode, ImplTraitContext, ImplTraitPosition,
+ LifetimeRes, LoweringContext, ParamMode, ResolverAstLoweringExt,
 };
-use crate::ImplTraitPosition;
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
  #[instrument(level = "trace", skip(self))]
@@ -28,6 +28,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  qself: &Option<ptr::P<QSelf>>,
  p: &Path,
  param_mode: ParamMode,
+ allow_return_type_notation: AllowReturnTypeNotation,
  itctx: ImplTraitContext,
  // modifiers of the impl/bound if this is a trait path
  modifiers: Option<ast::TraitBoundModifiers>,
@@ -103,8 +104,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  {
  GenericArgsMode::ParenSugar
  }
+ Res::Def(DefKind::AssocFn, _) if i + 1 == proj_start => {
+ match allow_return_type_notation {
+ AllowReturnTypeNotation::Yes => GenericArgsMode::ReturnTypeNotation,
+ AllowReturnTypeNotation::No => GenericArgsMode::Err,
+ }
+ }
  // Avoid duplicated errors.
- Res::Err => GenericArgsMode::ParenSugar,
+ Res::Err => GenericArgsMode::Silence,
  // An error
  _ => GenericArgsMode::Err,
  };
@@ -164,11 +171,20 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  // 3. `<<std::vec::Vec<T>>::IntoIter>::Item`
  // * final path is `<<<std::vec::Vec<T>>::IntoIter>::Item>::clone`
  for (i, segment) in p.segments.iter().enumerate().skip(proj_start) {
+ // If this is a type-dependent `T::method(..)`.
+ let generic_args_mode = if i + 1 == p.segments.len()
+ && matches!(allow_return_type_notation, AllowReturnTypeNotation::Yes)
+ {
+ GenericArgsMode::ReturnTypeNotation
+ } else {
+ GenericArgsMode::Err
+ };
+
  let hir_segment = self.arena.alloc(self.lower_path_segment(
  p.span,
  segment,
  param_mode,
- GenericArgsMode::Err,
+ generic_args_mode,
  itctx,
  None,
  ));
@@ -238,11 +254,46 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  self.lower_angle_bracketed_parameter_data(data, param_mode, itctx)
  }
  GenericArgs::Parenthesized(data) => match generic_args_mode {
- GenericArgsMode::ParenSugar => self.lower_parenthesized_parameter_data(
- data,
- itctx,
- bound_modifier_allowed_features,
- ),
+ GenericArgsMode::ReturnTypeNotation => {
+ let mut err = if !data.inputs.is_empty() {
+ self.dcx().create_err(BadReturnTypeNotation::Inputs {
+ span: data.inputs_span,
+ })
+ } else if let FnRetTy::Ty(ty) = &data.output {
+ self.dcx().create_err(BadReturnTypeNotation::Output {
+ span: data.inputs_span.shrink_to_hi().to(ty.span),
+ })
+ } else {
+ self.dcx().create_err(BadReturnTypeNotation::NeedsDots {
+ span: data.inputs_span,
+ })
+ };
+ if !self.tcx.features().return_type_notation
+ && self.tcx.sess.is_nightly_build()
+ {
+ add_feature_diagnostics(
+ &mut err,
+ &self.tcx.sess,
+ sym::return_type_notation,
+ );
+ }
+ err.emit();
+ (
+ GenericArgsCtor {
+ args: Default::default(),
+ constraints: &[],
+ parenthesized: hir::GenericArgsParentheses::ReturnTypeNotation,
+ span: path_span,
+ },
+ false,
+ )
+ }
+ GenericArgsMode::ParenSugar | GenericArgsMode::Silence => self
+ .lower_parenthesized_parameter_data(
+ data,
+ itctx,
+ bound_modifier_allowed_features,
+ ),
  GenericArgsMode::Err => {
  // Suggest replacing parentheses with angle brackets `Trait(params...)` to `Trait<params...>`
  let sub = if !data.inputs.is_empty() {
@@ -279,7 +330,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
  }
  },
  GenericArgs::ParenthesizedElided(span) => {
- self.dcx().emit_err(BadReturnTypeNotation::Position { span: *span });
+ match generic_args_mode {
+ GenericArgsMode::ReturnTypeNotation | GenericArgsMode::Silence => {
+ // Ok
+ }
+ GenericArgsMode::ParenSugar | GenericArgsMode::Err => {
+ self.dcx().emit_err(BadReturnTypeNotation::Position { span: *span });
+ }
+ }
  (
  GenericArgsCtor {
  args: Default::default(),

compiler/rustc_hir_analysis/messages.ftl

@@ -37,7 +37,7 @@ hir_analysis_assoc_kind_mismatch = expected {$expected}, found {$got}
 
 hir_analysis_assoc_kind_mismatch_wrap_in_braces_sugg = consider adding braces here
 
-hir_analysis_associated_type_trait_uninferred_generic_params = cannot use the associated type of a trait with uninferred generic parameters
+hir_analysis_associated_type_trait_uninferred_generic_params = cannot use the associated {$what} of a trait with uninferred generic parameters
  .suggestion = use a fully qualified path with inferred lifetimes
 
 hir_analysis_associated_type_trait_uninferred_generic_params_multipart_suggestion = use a fully qualified path with explicit lifetimes
@@ -48,6 +48,8 @@ hir_analysis_auto_deref_reached_recursion_limit = reached the recursion limit wh
 
 hir_analysis_bad_precise_capture = expected {$kind} parameter in `use<...>` precise captures list, found {$found}
 
+hir_analysis_bad_return_type_notation_position = return type notation not allowed in this position yet
+
 hir_analysis_cannot_capture_late_bound_const =
  cannot capture late-bound const parameter in {$what}
  .label = parameter defined here

compiler/rustc_hir_analysis/src/collect.rs

@@ -460,7 +460,7 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
  [] => (generics.span, format!("<{lt_name}>")),
  [bound, ..] => (bound.span.shrink_to_lo(), format!("{lt_name}, ")),
  };
- mpart_sugg = Some(errors::AssociatedTypeTraitUninferredGenericParamsMultipartSuggestion {
+ mpart_sugg = Some(errors::AssociatedItemTraitUninferredGenericParamsMultipartSuggestion {
  fspan: lt_sp,
  first: sugg,
  sspan: span.with_hi(item_segment.ident.span.lo()),
@@ -502,11 +502,12 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
  }
  Ty::new_error(
  self.tcx(),
- self.tcx().dcx().emit_err(errors::AssociatedTypeTraitUninferredGenericParams {
+ self.tcx().dcx().emit_err(errors::AssociatedItemTraitUninferredGenericParams {
  span,
  inferred_sugg,
  bound,
  mpart_sugg,
+ what: "type",
  }),
  )
  }

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -240,7 +240,8 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
  for predicate in hir_generics.predicates {
  match predicate {
  hir::WherePredicate::BoundPredicate(bound_pred) => {
- let ty = icx.lower_ty(bound_pred.bounded_ty);
+ let ty = icx.lowerer().lower_ty_maybe_return_type_notation(bound_pred.bounded_ty);
+
  let bound_vars = tcx.late_bound_vars(bound_pred.hir_id);
  // Keep the type around in a dummy predicate, in case of no bounds.
  // That way, `where Ty:` is not a complete noop (see #53696) and `Ty`
@@ -770,6 +771,10 @@ impl<'tcx> ItemCtxt<'tcx> {
  continue;
  };
 
+ // Subtle: If we're collecting `SelfAndAssociatedTypeBounds`, then we
+ // want to only consider predicates with `Self: ...`, but we don't want
+ // `OnlySelfBounds(true)` since we want to collect the nested associated
+ // type bound as well.
  let (only_self_bounds, assoc_name) = match filter {
  PredicateFilter::All | PredicateFilter::SelfAndAssociatedTypeBounds => {
  (OnlySelfBounds(false), None)
@@ -780,14 +785,10 @@ impl<'tcx> ItemCtxt<'tcx> {
  }
  };
 
- // Subtle: If we're collecting `SelfAndAssociatedTypeBounds`, then we
- // want to only consider predicates with `Self: ...`, but we don't want
- // `OnlySelfBounds(true)` since we want to collect the nested associated
- // type bound as well.
  let bound_ty = if predicate.is_param_bound(param_def_id.to_def_id()) {
  ty
  } else if matches!(filter, PredicateFilter::All) {
- self.lower_ty(predicate.bounded_ty)
+ self.lowerer().lower_ty_maybe_return_type_notation(predicate.bounded_ty)
  } else {
  continue;
  };