Add hir::GenericArg::Infer

compiler/rustc_ast/src/ast.rs

@@ -336,14 +336,17 @@ pub enum ParamKindOrd {
     // is active. Specifically, if it's only `min_const_generics`, it will still require
     // ordering consts after types.
     Const { unordered: bool },
+    // `Infer` is not actually constructed directly from the AST, but is implicitly constructed
+    // during HIR lowering, and `ParamKindOrd` will implicitly order inferred variables last.
+    Infer,
 }
 
 impl Ord for ParamKindOrd {
     fn cmp(&self, other: &Self) -> Ordering {
         use ParamKindOrd::*;
         let to_int = |v| match v {
             Lifetime => 0,
-            Type | Const { unordered: true } => 1,
+            Infer | Type | Const { unordered: true } => 1,
             // technically both consts should be ordered equally,
             // but only one is ever encountered at a time, so this is
             // fine.
@@ -371,6 +374,7 @@ impl fmt::Display for ParamKindOrd {
             ParamKindOrd::Lifetime => "lifetime".fmt(f),
             ParamKindOrd::Type => "type".fmt(f),
             ParamKindOrd::Const { .. } => "const".fmt(f),
+            ParamKindOrd::Infer => "infer".fmt(f),
         }
     }
 }

compiler/rustc_ast_lowering/src/lib.rs

@@ -51,7 +51,7 @@ use rustc_hir::def::{DefKind, Namespace, PartialRes, PerNS, Res};
 use rustc_hir::def_id::{DefId, DefIdMap, DefPathHash, LocalDefId, CRATE_DEF_ID};
 use rustc_hir::definitions::{DefKey, DefPathData, Definitions};
 use rustc_hir::intravisit;
-use rustc_hir::{ConstArg, GenericArg, ParamName};
+use rustc_hir::{ConstArg, GenericArg, InferKind, ParamName};
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_session::lint::builtin::{BARE_TRAIT_OBJECTS, MISSING_ABI};
 use rustc_session::lint::{BuiltinLintDiagnostics, LintBuffer};
@@ -1218,48 +1218,59 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         match arg {
             ast::GenericArg::Lifetime(lt) => GenericArg::Lifetime(self.lower_lifetime(&lt)),
             ast::GenericArg::Type(ty) => {
-                // We parse const arguments as path types as we cannot distinguish them during
-                // parsing. We try to resolve that ambiguity by attempting resolution in both the
-                // type and value namespaces. If we resolved the path in the value namespace, we
-                // transform it into a generic const argument.
-                if let TyKind::Path(ref qself, ref path) = ty.kind {
-                    if let Some(partial_res) = self.resolver.get_partial_res(ty.id) {
-                        let res = partial_res.base_res();
-                        if !res.matches_ns(Namespace::TypeNS) {
-                            debug!(
-                                "lower_generic_arg: Lowering type argument as const argument: {:?}",
-                                ty,
-                            );
-
-                            // Construct a AnonConst where the expr is the "ty"'s path.
-
-                            let parent_def_id = self.current_hir_id_owner.0;
-                            let node_id = self.resolver.next_node_id();
-
-                            // Add a definition for the in-band const def.
-                            self.resolver.create_def(
-                                parent_def_id,
-                                node_id,
-                                DefPathData::AnonConst,
-                                ExpnId::root(),
-                                ty.span,
-                            );
-
-                            let path_expr = Expr {
-                                id: ty.id,
-                                kind: ExprKind::Path(qself.clone(), path.clone()),
-                                span: ty.span,
-                                attrs: AttrVec::new(),
-                                tokens: None,
-                            };
-
-                            let ct = self.with_new_scopes(|this| hir::AnonConst {
-                                hir_id: this.lower_node_id(node_id),
-                                body: this.lower_const_body(path_expr.span, Some(&path_expr)),
-                            });
-                            return GenericArg::Const(ConstArg { value: ct, span: ty.span });
+                match ty.kind {
+                    TyKind::Infer if self.sess.features_untracked().generic_arg_infer => {
+                        let hir_id = self.lower_node_id(ty.id);
+                        return GenericArg::Infer(hir::InferArg {
+                            hir_id,
+                            span: ty.span,
+                            kind: InferKind::Type,
+                        });
+                    }
+                    // We parse const arguments as path types as we cannot distinguish them during
+                    // parsing. We try to resolve that ambiguity by attempting resolution in both the
+                    // type and value namespaces. If we resolved the path in the value namespace, we
+                    // transform it into a generic const argument.
+                    TyKind::Path(ref qself, ref path) => {
+                        if let Some(partial_res) = self.resolver.get_partial_res(ty.id) {
+                            let res = partial_res.base_res();
+                            if !res.matches_ns(Namespace::TypeNS) {
+                                debug!(
+                                    "lower_generic_arg: Lowering type argument as const argument: {:?}",
+                                    ty,
+                                );
+
+                                // Construct a AnonConst where the expr is the "ty"'s path.
+
+                                let parent_def_id = self.current_hir_id_owner.0;
+                                let node_id = self.resolver.next_node_id();
+
+                                // Add a definition for the in-band const def.
+                                self.resolver.create_def(
+                                    parent_def_id,
+                                    node_id,
+                                    DefPathData::AnonConst,
+                                    ExpnId::root(),
+                                    ty.span,
+                                );
+
+                                let path_expr = Expr {
+                                    id: ty.id,
+                                    kind: ExprKind::Path(qself.clone(), path.clone()),
+                                    span: ty.span,
+                                    attrs: AttrVec::new(),
+                                    tokens: None,
+                                };
+
+                                let ct = self.with_new_scopes(|this| hir::AnonConst {
+                                    hir_id: this.lower_node_id(node_id),
+                                    body: this.lower_const_body(path_expr.span, Some(&path_expr)),
+                                });
+                                return GenericArg::Const(ConstArg { value: ct, span: ty.span });
+                            }
                         }
                     }
+                    _ => {}
                 }
                 GenericArg::Type(self.lower_ty_direct(&ty, itctx))
             }

compiler/rustc_feature/src/active.rs

@@ -684,6 +684,9 @@ declare_features! (
     /// Allows `cfg(target_abi = "...")`.
     (active, cfg_target_abi, "1.55.0", Some(80970), None),
 
+    /// Infer generic args for both consts and types.
+    (active, generic_arg_infer, "1.55.0", Some(85077), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_hir/src/hir.rs

@@ -253,11 +253,38 @@ pub struct ConstArg {
     pub span: Span,
 }
 
+#[derive(Copy, Clone, Encodable, Debug, HashStable_Generic)]
+pub enum InferKind {
+    Const,
+    Type,
+}
+
+impl InferKind {
+    #[inline]
+    pub fn is_type(self) -> bool {
+        matches!(self, InferKind::Type)
+    }
+}
+
+#[derive(Encodable, Debug, HashStable_Generic)]
+pub struct InferArg {
+    pub hir_id: HirId,
+    pub kind: InferKind,
+    pub span: Span,
+}
+
+impl InferArg {
+    pub fn to_ty(&self) -> Ty<'_> {
+        Ty { kind: TyKind::Infer, span: self.span, hir_id: self.hir_id }
+    }
+}
+
 #[derive(Debug, HashStable_Generic)]
 pub enum GenericArg<'hir> {
     Lifetime(Lifetime),
     Type(Ty<'hir>),
     Const(ConstArg),
+    Infer(InferArg),
 }
 
 impl GenericArg<'_> {
@@ -266,6 +293,7 @@ impl GenericArg<'_> {
             GenericArg::Lifetime(l) => l.span,
             GenericArg::Type(t) => t.span,
             GenericArg::Const(c) => c.span,
+            GenericArg::Infer(i) => i.span,
         }
     }
 
@@ -274,6 +302,7 @@ impl GenericArg<'_> {
             GenericArg::Lifetime(l) => l.hir_id,
             GenericArg::Type(t) => t.hir_id,
             GenericArg::Const(c) => c.value.hir_id,
+            GenericArg::Infer(i) => i.hir_id,
         }
     }
 
@@ -290,6 +319,7 @@ impl GenericArg<'_> {
             GenericArg::Lifetime(_) => "lifetime",
             GenericArg::Type(_) => "type",
             GenericArg::Const(_) => "constant",
+            GenericArg::Infer(_) => "inferred",
         }
     }
 
@@ -300,6 +330,7 @@ impl GenericArg<'_> {
             GenericArg::Const(_) => {
                 ast::ParamKindOrd::Const { unordered: feats.unordered_const_ty_params() }
             }
+            GenericArg::Infer(_) => ast::ParamKindOrd::Infer,
         }
     }
 }
@@ -341,27 +372,36 @@ impl GenericArgs<'_> {
                         break;
                     }
                     GenericArg::Const(_) => {}
+                    GenericArg::Infer(_) => {}
                 }
             }
         }
         panic!("GenericArgs::inputs: not a `Fn(T) -> U`");
     }
 
-    pub fn own_counts(&self) -> GenericParamCount {
-        // We could cache this as a property of `GenericParamCount`, but
-        // the aim is to refactor this away entirely eventually and the
-        // presence of this method will be a constant reminder.
-        let mut own_counts: GenericParamCount = Default::default();
+    #[inline]
+    pub fn has_type_params(&self) -> bool {
+        self.args.iter().any(|arg| matches!(arg, GenericArg::Type(_)))
+    }
 
-        for arg in self.args {
-            match arg {
-                GenericArg::Lifetime(_) => own_counts.lifetimes += 1,
-                GenericArg::Type(_) => own_counts.types += 1,
-                GenericArg::Const(_) => own_counts.consts += 1,
-            };
-        }
+    #[inline]
+    pub fn num_type_params(&self) -> usize {
+        self.args.iter().filter(|arg| matches!(arg, GenericArg::Type(_))).count()
+    }
 
-        own_counts
+    #[inline]
+    pub fn num_lifetime_params(&self) -> usize {
+        self.args.iter().filter(|arg| matches!(arg, GenericArg::Lifetime(_))).count()
+    }
+
+    #[inline]
+    pub fn has_lifetime_params(&self) -> bool {
+        self.args.iter().any(|arg| matches!(arg, GenericArg::Lifetime(_)))
+    }
+
+    #[inline]
+    pub fn num_generic_params(&self) -> usize {
+        self.args.iter().filter(|arg| !matches!(arg, GenericArg::Lifetime(_))).count()
     }
 
     /// The span encompassing the text inside the surrounding brackets.
@@ -484,6 +524,7 @@ pub struct GenericParamCount {
     pub lifetimes: usize,
     pub types: usize,
     pub consts: usize,
+    pub infer: usize,
 }
 
 /// Represents lifetimes and type parameters attached to a declaration
@@ -2987,6 +3028,8 @@ pub enum Node<'hir> {
     Visibility(&'hir Visibility<'hir>),
 
     Crate(&'hir Mod<'hir>),
+
+    Infer(&'hir InferArg),
 }
 
 impl<'hir> Node<'hir> {
@@ -3055,6 +3098,7 @@ impl<'hir> Node<'hir> {
             | Node::Local(Local { hir_id, .. })
             | Node::Lifetime(Lifetime { hir_id, .. })
             | Node::Param(Param { hir_id, .. })
+            | Node::Infer(InferArg { hir_id, .. })
             | Node::GenericParam(GenericParam { hir_id, .. }) => Some(*hir_id),
             Node::TraitRef(TraitRef { hir_ref_id, .. }) => Some(*hir_ref_id),
             Node::PathSegment(PathSegment { hir_id, .. }) => *hir_id,

compiler/rustc_hir/src/intravisit.rs

@@ -436,11 +436,15 @@ pub trait Visitor<'v>: Sized {
     fn visit_label(&mut self, label: &'v Label) {
         walk_label(self, label)
     }
+    fn visit_infer(&mut self, inf: &'v InferArg) {
+        walk_inf(self, inf);
+    }
     fn visit_generic_arg(&mut self, generic_arg: &'v GenericArg<'v>) {
         match generic_arg {
             GenericArg::Lifetime(lt) => self.visit_lifetime(lt),
             GenericArg::Type(ty) => self.visit_ty(ty),
             GenericArg::Const(ct) => self.visit_anon_const(&ct.value),
+            GenericArg::Infer(inf) => self.visit_infer(inf),
         }
     }
     fn visit_lifetime(&mut self, lifetime: &'v Lifetime) {
@@ -746,6 +750,10 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty<'v>) {
     }
 }
 
+pub fn walk_inf<'v, V: Visitor<'v>>(visitor: &mut V, inf: &'v InferArg) {
+    visitor.visit_id(inf.hir_id);
+}
+
 pub fn walk_qpath<'v, V: Visitor<'v>>(
     visitor: &mut V,
     qpath: &'v QPath<'v>,

compiler/rustc_hir_pretty/src/lib.rs

@@ -103,6 +103,7 @@ impl<'a> State<'a> {
             Node::TraitRef(a) => self.print_trait_ref(&a),
             Node::Binding(a) | Node::Pat(a) => self.print_pat(&a),
             Node::Arm(a) => self.print_arm(&a),
+            Node::Infer(_) => self.s.word("_"),
             Node::Block(a) => {
                 // Containing cbox, will be closed by print-block at `}`.
                 self.cbox(INDENT_UNIT);
@@ -437,14 +438,14 @@ impl<'a> State<'a> {
                 self.print_anon_const(e);
                 self.s.word(")");
             }
-            hir::TyKind::Infer => {
-                self.s.word("_");
-            }
             hir::TyKind::Err => {
                 self.popen();
                 self.s.word("/*ERROR*/");
                 self.pclose();
             }
+            hir::TyKind::Infer => {
+                self.s.word("_");
+            }
         }
         self.end()
     }
@@ -1851,6 +1852,7 @@ impl<'a> State<'a> {
                         GenericArg::Lifetime(_) => {}
                         GenericArg::Type(ty) => s.print_type(ty),
                         GenericArg::Const(ct) => s.print_anon_const(&ct.value),
+                        GenericArg::Infer(_inf) => s.word("_"),
                     },
                 );
             }

compiler/rustc_lint/src/late.rs

@@ -244,6 +244,11 @@ impl<'tcx, T: LateLintPass<'tcx>> hir_visit::Visitor<'tcx> for LateContextAndPas
         hir_visit::walk_ty(self, t);
     }
 
+    fn visit_infer(&mut self, inf: &'tcx hir::InferArg) {
+        lint_callback!(self, check_infer, inf);
+        hir_visit::walk_inf(self, inf);
+    }
+
     fn visit_name(&mut self, sp: Span, name: Symbol) {
         lint_callback!(self, check_name, sp, name);
     }

compiler/rustc_lint/src/passes.rs

@@ -33,6 +33,7 @@ macro_rules! late_lint_methods {
             fn check_expr(a: &$hir hir::Expr<$hir>);
             fn check_expr_post(a: &$hir hir::Expr<$hir>);
             fn check_ty(a: &$hir hir::Ty<$hir>);
+            fn check_infer(a: &$hir hir::InferArg);
             fn check_generic_arg(a: &$hir hir::GenericArg<$hir>);
             fn check_generic_param(a: &$hir hir::GenericParam<$hir>);
             fn check_generics(a: &$hir hir::Generics<$hir>);

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

@@ -404,6 +404,14 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         });
     }
 
+    fn visit_infer(&mut self, inf: &'hir InferArg) {
+        self.insert(inf.span, inf.hir_id, Node::Infer(inf));
+
+        self.with_parent(inf.hir_id, |this| {
+            intravisit::walk_inf(this, inf);
+        });
+    }
+
     fn visit_trait_ref(&mut self, tr: &'hir TraitRef<'hir>) {
         self.insert(tr.path.span, tr.hir_ref_id, Node::TraitRef(tr));