Support function args const generics

compiler/rustc_ast/src/ast.rs

@@ -446,6 +446,8 @@ pub enum GenericParamKind {
         span: Span,
         /// Optional default value for the const generic param.
         default: Option<AnonConst>,
+        #[visitable(ignore)]
+        arg_pos: Option<u32>,
     },
 }
 
@@ -4489,7 +4491,7 @@ mod size_asserts {
     static_assert_size!(GenericArg, 24);
     static_assert_size!(GenericArgs, 40);
     static_assert_size!(GenericBound, 80);
-    static_assert_size!(GenericParam, 80);
+    static_assert_size!(GenericParam, 88);
     static_assert_size!(Generics, 40);
     static_assert_size!(Impl, 80);
     static_assert_size!(Item, 144);

compiler/rustc_ast_lowering/src/delegation/generics.rs

@@ -641,6 +641,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::GenericParamKind::Const {
                         ty: self.arena.alloc(hir::Ty { kind, hir_id, span }),
                         default: None,
+                        arg_pos: None,
                     }
                 }
             };

compiler/rustc_ast_lowering/src/lib.rs

@@ -2373,7 +2373,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
                 (hir::ParamName::Plain(self.lower_ident(param.ident)), kind)
             }
-            GenericParamKind::Const { ty, span: _, default } => {
+            GenericParamKind::Const { ty, span: _, default, arg_pos } => {
                 let ty = self.lower_ty_alloc(
                     ty,
                     ImplTraitContext::Disallowed(ImplTraitPosition::GenericDefault),
@@ -2406,7 +2406,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
                 (
                     hir::ParamName::Plain(self.lower_ident(param.ident)),
-                    hir::GenericParamKind::Const { ty, default },
+                    hir::GenericParamKind::Const { ty, default, arg_pos: *arg_pos },
                 )
             }
         }

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1383,11 +1383,11 @@ fn validate_generic_param_order(dcx: DiagCtxtHandle<'_>, generics: &[GenericPara
                 }
                 GenericParamKind::Type { default: None } => (),
                 GenericParamKind::Lifetime => (),
-                GenericParamKind::Const { ty: _, span: _, default: Some(default) } => {
+                GenericParamKind::Const { ty: _, span: _, default: Some(default), arg_pos: _ } => {
                     ordered_params += " = ";
                     ordered_params += &pprust::expr_to_string(&default.value);
                 }
-                GenericParamKind::Const { ty: _, span: _, default: None } => (),
+                GenericParamKind::Const { ty: _, span: _, default: None, arg_pos: _ } => (),
             }
             first = false;
         }

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -442,6 +442,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(final_associated_functions, "`final` on trait functions is experimental");
     gate_all!(fn_delegation, "functions delegation is not yet fully implemented");
     gate_all!(frontmatter, "frontmatters are experimental");
+    gate_all!(function_arg_const_generics, "function arg const generics are experimental");
     gate_all!(gen_blocks, "gen blocks are experimental");
     gate_all!(generic_const_items, "generic const items are experimental");
     gate_all!(global_registration, "global registration is experimental");

compiler/rustc_builtin_macros/src/deriving/coerce_pointee.rs

@@ -117,14 +117,15 @@ pub(crate) fn expand_deriving_coerce_pointee(
                                 GenericParamKind::Type { default: _ } => {
                                     cx.typaram(p.span(), p.ident, p.bounds.clone(), None)
                                 }
-                                GenericParamKind::Const { ty, span: _, default: _ } => cx
-                                    .const_param(
+                                GenericParamKind::Const { ty, span: _, default: _, arg_pos: _ } => {
+                                    cx.const_param(
                                         p.span(),
                                         p.ident,
                                         p.bounds.clone(),
                                         ty.clone(),
                                         None,
-                                    ),
+                                    )
+                                }
                             })
                             .collect(),
                         where_clause: generics.where_clause.clone(),

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -635,6 +635,7 @@ impl<'a> TraitDef<'a> {
 
                         // We can't have default values inside impl block
                         default: None,
+                        arg_pos: None,
                     };
                     let mut param_clone = param.clone();
                     param_clone.kind = const_nodefault_kind;

compiler/rustc_builtin_macros/src/deriving/reborrow.rs

@@ -172,7 +172,7 @@ fn impl_generics(cx: &ExtCtxt<'_>, generics: &Generics) -> Generics {
                 GenericParamKind::Type { default: _ } => {
                     cx.typaram(param.span(), param.ident, param.bounds.clone(), None)
                 }
-                GenericParamKind::Const { ty, span: _, default: _ } => cx.const_param(
+                GenericParamKind::Const { ty, span: _, default: _, arg_pos: _ } => cx.const_param(
                     param.span(),
                     param.ident,
                     param.bounds.clone(),

compiler/rustc_expand/src/build.rs

@@ -174,7 +174,7 @@ impl<'a> ExtCtxt<'a> {
             attrs: AttrVec::new(),
             bounds,
             is_placeholder: false,
-            kind: ast::GenericParamKind::Const { ty, span: DUMMY_SP, default },
+            kind: ast::GenericParamKind::Const { ty, span: DUMMY_SP, default, arg_pos: None },
             colon_span: None,
         }
     }

compiler/rustc_feature/src/unstable.rs

@@ -546,6 +546,8 @@ declare_features! (
     (internal, freeze_impls, "1.78.0", Some(121675)),
     /// Frontmatter `---` blocks for use by external tools.
     (unstable, frontmatter, "1.88.0", Some(136889)),
+    /// Allow function args to be const generics
+    (incomplete, function_arg_const_generics, "CURRENT_RUSTC_VERSION", Some(163261)),
     /// Allows defining gen blocks and `gen fn`.
     (unstable, gen_blocks, "1.75.0", Some(117078)),
     /// Allows using generics in more complex const expressions, based on definitional equality.
@@ -859,6 +861,7 @@ pub const INCOMPATIBLE_FEATURES: &[(Symbol, Symbol)] = &[
 
 /// Some features require one or more other features to be enabled.
 pub const DEPENDENT_FEATURES: &[(Symbol, &[Symbol])] = &[
+    (sym::function_arg_const_generics, &[sym::min_generic_const_args]),
     (sym::generic_const_args, &[sym::min_generic_const_args]),
     (sym::macroless_generic_const_args, &[sym::min_generic_const_args]),
     (sym::macroless_const_item_generic_const_args, &[sym::min_generic_const_args]),

compiler/rustc_hir/src/hir.rs

@@ -848,6 +848,7 @@ pub enum GenericParamKind<'hir> {
         ty: &'hir Ty<'hir>,
         /// Optional default value for the const generic param
         default: Option<&'hir ConstArg<'hir>>,
+        arg_pos: Option<u32>,
     },
 }
 

compiler/rustc_hir/src/intravisit.rs

@@ -1169,7 +1169,7 @@ pub fn walk_generic_param<'v, V: Visitor<'v>>(
         GenericParamKind::Type { ref default, .. } => {
             visit_opt!(visitor, visit_ty_unambig, default)
         }
-        GenericParamKind::Const { ref ty, ref default } => {
+        GenericParamKind::Const { ref ty, ref default, arg_pos: _ } => {
             try_visit!(visitor.visit_ty_unambig(ty));
             if let Some(default) = default {
                 try_visit!(visitor.visit_const_param_default(*hir_id, default));

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -310,7 +310,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
 
                 ty::GenericParamDefKind::Type { has_default: default.is_some(), synthetic }
             }
-            GenericParamKind::Const { ty: _, default } => {
+            GenericParamKind::Const { ty: _, default, arg_pos } => {
                 if default.is_some() {
                     match param_default_policy.expect("no policy for generic param default") {
                         ParamDefaultPolicy::Allowed => {}
@@ -321,7 +321,7 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                     }
                 }
 
-                ty::GenericParamDefKind::Const { has_default: default.is_some() }
+                ty::GenericParamDefKind::Const { has_default: default.is_some(), arg_pos }
             }
         };
         Some(ty::GenericParamDef {
@@ -524,7 +524,7 @@ impl<'v> Visitor<'v> for AnonConstInParamTyDetector {
     type Result = ControlFlow<()>;
 
     fn visit_generic_param(&mut self, p: &'v hir::GenericParam<'v>) -> Self::Result {
-        if let GenericParamKind::Const { ty, default: _ } = p.kind {
+        if let GenericParamKind::Const { ty, default: _, arg_pos: _ } = p.kind {
             let prev = self.in_param_ty;
             self.in_param_ty = true;
             let res = self.visit_ty_unambig(ty);

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -451,8 +451,9 @@ pub(crate) fn check_generic_arg_count(
         .iter()
         .filter(|param| matches!(param.kind, ty::GenericParamDefKind::Type { synthetic: true, .. }))
         .count();
+    let arg_const_param_count = gen_params.own_arg_pos_consts().count();
     let named_type_param_count = param_counts.types - has_self as usize - synth_type_param_count;
-    let named_const_param_count = param_counts.consts;
+    let named_const_param_count = param_counts.consts - arg_const_param_count;
     let infer_lifetimes =
         (gen_pos != GenericArgPosition::Type || seg.infer_args) && !gen_args.has_lifetime_args();
 
@@ -628,7 +629,7 @@ pub(crate) fn check_generic_arg_count(
         let expected_min = if seg.infer_args {
             0
         } else {
-            param_counts.consts + named_type_param_count
+            named_const_param_count + named_type_param_count
                 - default_counts.types
                 - default_counts.consts
         };

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -2385,6 +2385,41 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         self.check_param_uses_if_mcg(ct, tcx.hir_span(path_hir_id), false)
     }
 
+    pub fn lower_const_arg_expr(&self, expr: &hir::Expr<'_>, ty: Ty<'tcx>) -> Const<'tcx> {
+        let tcx = self.tcx();
+        match expr.kind {
+            hir::ExprKind::Lit(lit) => {
+                self.lower_const_arg_literal(&lit.node, false, ty, expr.span)
+            }
+            hir::ExprKind::Unary(
+                hir::UnOp::Neg,
+                hir::Expr { kind: hir::ExprKind::Lit(lit), .. },
+            ) => self.lower_const_arg_literal(&lit.node, true, ty, expr.span),
+            hir::ExprKind::Path(hir::QPath::Resolved(_, hir::Path { res: Res::Local(_), .. })) => {
+                Const::new_error(tcx, self.dcx().span_err(expr.span, "Path error"))
+            }
+            hir::ExprKind::Path(hir::QPath::Resolved(maybe_qself, path)) => {
+                let opt_self_ty = maybe_qself.as_ref().map(|qself| self.lower_ty(qself));
+                self.lower_resolved_const_path(opt_self_ty, path, expr.hir_id)
+            }
+            hir::ExprKind::Path(hir::QPath::TypeRelative(hir_self_ty, segment)) => {
+                let self_ty = self.lower_ty(hir_self_ty);
+                self.lower_type_relative_const_path(
+                    self_ty,
+                    hir_self_ty,
+                    segment,
+                    expr.hir_id,
+                    expr.span,
+                )
+                .unwrap_or_else(|guard| Const::new_error(tcx, guard))
+            }
+            _ => Const::new_error(
+                tcx,
+                self.dcx().span_err(expr.span, "Some other variant we dont support"),
+            ),
+        }
+    }
+
     /// Lower a [`hir::ConstArg`] to a (type-level) [`ty::Const`].
     #[instrument(skip(self), level = "debug")]
     pub fn lower_const_arg(&self, const_arg: &hir::ConstArg<'_>, ty: Ty<'tcx>) -> Const<'tcx> {

compiler/rustc_hir_pretty/src/lib.rs

@@ -2475,7 +2475,7 @@ impl<'a> State<'a> {
                     self.print_type(default);
                 }
             }
-            GenericParamKind::Const { ty, ref default } => {
+            GenericParamKind::Const { ty, ref default, arg_pos: _ } => {
                 self.word_space(":");
                 self.print_type(ty);
                 if let Some(default) = default {

compiler/rustc_hir_typeck/src/callee.rs

@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::iter;
 
 use rustc_abi::{CanonAbi, ExternAbi};
@@ -665,11 +666,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         fn_id: SplatLoweringInfo<'tcx>,
         callee_generic_args: Option<GenericArgsRef<'tcx>>,
     ) {
+        let mut formal_inputs = Cow::Borrowed(fn_sig.inputs());
+        if let (SplatLoweringInfo::FnDef(def_id), Some(args)) = (fn_id, callee_generic_args) {
+            for (param, pos) in self.tcx.generics_of(def_id).own_arg_pos_consts() {
+                let ty = self.tcx.type_of(param.def_id).instantiate(self.tcx, args).skip_norm_wip();
+                let ty = self.normalize(call_expr.span, Unnormalized::new_wip(ty));
+                formal_inputs.to_mut().insert(pos as usize, ty);
+            }
+        }
+
         let do_check = || {
             self.check_argument_types(
                 call_expr.span,
                 call_expr,
-                fn_sig.inputs(),
+                &formal_inputs,
                 fn_sig.output(),
                 expected,
                 arg_exprs,

compiler/rustc_hir_typeck/src/expr.rs

@@ -5,6 +5,8 @@
 //!
 //! See [`rustc_hir_analysis::check`] for more context on type checking in general.
 
+use std::borrow::Cow;
+
 use rustc_abi::{FIRST_VARIANT, FieldIdx};
 use rustc_ast as ast;
 use rustc_ast::util::parser::ExprPrecedence;
@@ -618,7 +620,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     call_expr_and_args.map_or(expr.span, |(e, _)| e.span),
                     expr.span,
                     expr.hir_id,
-                    call_expr_and_args.is_some(),
+                    call_expr_and_args.map(|(_, args)| args),
                 )
                 .0
             }
@@ -1489,14 +1491,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
                 // Handle splatted method arguments
                 // self is already handled as `rcvr`, so it's never splatted here
-                let method_inputs = &method.sig.inputs()[1..];
+                let mut method_inputs = Cow::Borrowed(&method.sig.inputs()[1..]);
+                for (param, pos) in self.tcx.generics_of(method.def_id).own_arg_pos_consts() {
+                    let ty = self
+                        .tcx
+                        .type_of(param.def_id)
+                        .instantiate(self.tcx, method.args)
+                        .skip_norm_wip();
+                    method_inputs.to_mut().insert(pos as usize - 1, ty);
+                }
+
                 let method_tuple_args_flag =
                     TupleArgumentsFlag::with_fn_sig_kind(method.sig.fn_sig_kind, true);
 
                 self.check_argument_types(
                     segment.ident.span,
                     expr,
-                    method_inputs,
+                    &method_inputs,
                     method.sig.output(),
                     expected,
                     args,

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -613,6 +613,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         ct
     }
 
+    pub(crate) fn lower_const_arg_expr(
+        &self,
+        expr: &hir::Expr<'_>,
+        ty: Ty<'tcx>,
+    ) -> ty::Const<'tcx> {
+        let ct = self.lowerer().lower_const_arg_expr(expr, ty);
+        self.register_wf_obligation(ct.into(), expr.span, ObligationCauseCode::WellFormed(None));
+        ct
+    }
+
     // If the type given by the user has free regions, save it for later, since
     // NLL would like to enforce those. Also pass in types that involve
     // projections, since those can resolve to `'static` bounds (modulo #54940,
@@ -1006,8 +1016,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         span: Span,
         path_span: Span,
         hir_id: HirId,
-        has_args: bool,
+        call_args: Option<&'tcx [hir::Expr<'tcx>]>,
     ) -> (Ty<'tcx>, Res) {
+        let has_args = call_args.is_some();
         let tcx = self.tcx;
 
         let generic_segments = match res {
@@ -1325,6 +1336,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             generic_segments: &'a [GenericPathSegment],
             infer_args_for_err: &'a FxHashSet<usize>,
             segments: &'tcx [hir::PathSegment<'tcx>],
+            call_args: Option<&'tcx [hir::Expr<'tcx>]>,
         }
         impl<'a, 'tcx> GenericArgsLowerer<'a, 'tcx> for CtorGenericArgsCtxt<'a, 'tcx> {
             fn args_for_def_id(
@@ -1393,6 +1405,24 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 infer_args: bool,
             ) -> ty::GenericArg<'tcx> {
                 let tcx = self.fcx.tcx();
+                if let Some(pos) = param.kind.arg_pos() {
+                    let Some(args) = self.call_args else {
+                        let guard =
+                            self.fcx.dcx().span_err(self.span, "argument should be provided");
+                        return ty::Const::new_error(tcx, guard).into();
+                    };
+
+                    let Some(arg) = args.get(pos as usize) else {
+                        let guard = self
+                            .fcx
+                            .dcx()
+                            .span_delayed_bug(self.span, "missing argument for const param");
+                        return ty::Const::new_error(tcx, guard).into();
+                    };
+                    let ty =
+                        tcx.type_of(param.def_id).instantiate(tcx, preceding_args).skip_norm_wip();
+                    return self.fcx.lower_const_arg_expr(arg, ty).into();
+                }
                 if !infer_args && let Some(default) = param.default_value(tcx) {
                     // If we have a default, then it doesn't matter that we're not inferring
                     // the type/const arguments: We provide the default where any is missing.
@@ -1420,6 +1450,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     generic_segments: &generic_segments,
                     infer_args_for_err: &infer_args_for_err,
                     segments,
+                    call_args,
                 },
             )
         });