Add #[rustc_legacy_const_generics]

compiler/rustc_ast_lowering/src/expr.rs

@@ -9,7 +9,9 @@ use rustc_data_structures::thin_vec::ThinVec;
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def::Res;
+use rustc_hir::definitions::DefPathData;
 use rustc_session::parse::feature_err;
+use rustc_span::hygiene::ExpnId;
 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
 use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::{hygiene::ForLoopLoc, DUMMY_SP};
@@ -42,8 +44,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
                 ExprKind::Call(ref f, ref args) => {
-                    let f = self.lower_expr(f);
-                    hir::ExprKind::Call(f, self.lower_exprs(args))
+                    if let Some(legacy_args) = self.legacy_const_generic_args(f) {
+                        self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
+                    } else {
+                        let f = self.lower_expr(f);
+                        hir::ExprKind::Call(f, self.lower_exprs(args))
+                    }
                 }
                 ExprKind::MethodCall(ref seg, ref args, span) => {
                     let hir_seg = self.arena.alloc(self.lower_path_segment(
@@ -292,6 +298,86 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
+    /// Checks if an expression refers to a function marked with
+    /// `#[rustc_legacy_const_generics]` and returns the argument index list
+    /// from the attribute.
+    fn legacy_const_generic_args(&mut self, expr: &Expr) -> Option<Vec<usize>> {
+        if let ExprKind::Path(None, path) = &expr.kind {
+            if path.segments.last().unwrap().args.is_some() {
+                return None;
+            }
+            if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
+                if partial_res.unresolved_segments() != 0 {
+                    return None;
+                }
+                if let Res::Def(hir::def::DefKind::Fn, def_id) = partial_res.base_res() {
+                    let attrs = self.item_attrs(def_id);
+                    let attr = attrs
+                        .iter()
+                        .find(|a| self.sess.check_name(a, sym::rustc_legacy_const_generics))?;
+                    let mut ret = vec![];
+                    for meta in attr.meta_item_list()? {
+                        match meta.literal()?.kind {
+                            LitKind::Int(a, _) => {
+                                ret.push(a as usize);
+                            }
+                            _ => panic!("invalid arg index"),
+                        }
+                    }
+                    return Some(ret);
+                }
+            }
+        }
+        None
+    }
+
+    fn lower_legacy_const_generics(
+        &mut self,
+        mut f: Expr,
+        args: Vec<AstP<Expr>>,
+        legacy_args_idx: &[usize],
+    ) -> hir::ExprKind<'hir> {
+        let path = match f.kind {
+            ExprKind::Path(None, ref mut path) => path,
+            _ => unreachable!(),
+        };
+
+        // Split the arguments into const generics and normal arguments
+        let mut real_args = vec![];
+        let mut generic_args = vec![];
+        for (idx, arg) in args.into_iter().enumerate() {
+            if legacy_args_idx.contains(&idx) {
+                let parent_def_id = self.current_hir_id_owner.last().unwrap().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(),
+                    arg.span,
+                );
+
+                let anon_const = AnonConst { id: node_id, value: arg };
+                generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
+            } else {
+                real_args.push(arg);
+            }
+        }
+
+        // Add generic args to the last element of the path
+        path.segments.last_mut().unwrap().args =
+            Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
+                span: DUMMY_SP,
+                args: generic_args,
+            })));
+
+        // Now lower everything as normal.
+        let f = self.lower_expr(&f);
+        hir::ExprKind::Call(f, self.lower_exprs(&real_args))
+    }
+
     /// Emit an error and lower `ast::ExprKind::Let(pat, scrutinee)` into:
     /// ```rust
     /// match scrutinee { pats => true, _ => false }

compiler/rustc_ast_lowering/src/lib.rs

@@ -175,6 +175,8 @@ pub trait ResolverAstLowering {
 
     fn item_generics_num_lifetimes(&self, def: DefId, sess: &Session) -> usize;
 
+    fn item_attrs(&self, def_id: DefId, sess: &Session) -> Vec<ast::Attribute>;
+
     /// Obtains resolution for a `NodeId` with a single resolution.
     fn get_partial_res(&mut self, id: NodeId) -> Option<PartialRes>;
 
@@ -2826,6 +2828,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             )
         }
     }
+
+    fn item_attrs(&self, def_id: DefId) -> Vec<ast::Attribute> {
+        if let Some(_local_def_id) = def_id.as_local() {
+            // TODO: This doesn't actually work, items doesn't include everything?
+            //self.items[&hir::ItemId { def_id: local_def_id }].attrs.into()
+            Vec::new()
+        } else {
+            self.resolver.item_attrs(def_id, self.sess)
+        }
+    }
 }
 
 fn body_ids(bodies: &BTreeMap<hir::BodyId, hir::Body<'_>>) -> Vec<hir::BodyId> {

compiler/rustc_feature/src/builtin_attrs.rs

@@ -470,6 +470,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
 
     rustc_attr!(rustc_promotable, AssumedUsed, template!(Word), IMPL_DETAIL),
     rustc_attr!(rustc_args_required_const, AssumedUsed, template!(List: "N"), INTERNAL_UNSTABLE),
+    rustc_attr!(rustc_legacy_const_generics, AssumedUsed, template!(List: "N"), INTERNAL_UNSTABLE),
 
     // ==========================================================================
     // Internal attributes, Layout related:

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -468,6 +468,10 @@ impl CStore {
     pub fn num_def_ids(&self, cnum: CrateNum) -> usize {
         self.get_crate_data(cnum).num_def_ids()
     }
+
+    pub fn item_attrs(&self, def_id: DefId, sess: &Session) -> Vec<ast::Attribute> {
+        self.get_crate_data(def_id.krate).get_item_attrs(def_id.index, sess).collect()
+    }
 }
 
 impl CrateStore for CStore {

compiler/rustc_passes/src/check_attr.rs

@@ -91,6 +91,8 @@ impl CheckAttrVisitor<'tcx> {
                 self.check_rustc_allow_const_fn_unstable(hir_id, &attr, span, target)
             } else if self.tcx.sess.check_name(attr, sym::naked) {
                 self.check_naked(hir_id, attr, span, target)
+            } else if self.tcx.sess.check_name(attr, sym::rustc_legacy_const_generics) {
+                self.check_rustc_legacy_const_generics(&attr, span, target, item)
             } else {
                 // lint-only checks
                 if self.tcx.sess.check_name(attr, sym::cold) {
@@ -750,6 +752,79 @@ impl CheckAttrVisitor<'tcx> {
         }
     }
 
+    /// Checks if `#[rustc_legacy_const_generics]` is applied to a function and has a valid argument.
+    fn check_rustc_legacy_const_generics(
+        &self,
+        attr: &Attribute,
+        span: &Span,
+        target: Target,
+        item: Option<ItemLike<'_>>,
+    ) -> bool {
+        let is_function = matches!(target, Target::Fn | Target::Method(..) | Target::ForeignFn);
+        if !is_function {
+            self.tcx
+                .sess
+                .struct_span_err(attr.span, "attribute should be applied to a function")
+                .span_label(*span, "not a function")
+                .emit();
+            return false;
+        }
+
+        let list = match attr.meta_item_list() {
+            // The attribute form is validated on AST.
+            None => return false,
+            Some(it) => it,
+        };
+
+        let mut invalid_args = vec![];
+        for meta in list {
+            if let Some(LitKind::Int(val, _)) = meta.literal().map(|lit| &lit.kind) {
+                if let Some(ItemLike::Item(Item {
+                    kind: ItemKind::Fn(FnSig { decl, .. }, generics, _),
+                    ..
+                }))
+                | Some(ItemLike::ForeignItem(ForeignItem {
+                    kind: ForeignItemKind::Fn(decl, _, generics),
+                    ..
+                })) = item
+                {
+                    let arg_count = decl.inputs.len() as u128 + generics.params.len() as u128;
+                    if *val >= arg_count {
+                        let span = meta.span();
+                        self.tcx
+                            .sess
+                            .struct_span_err(span, "index exceeds number of arguments")
+                            .span_label(
+                                span,
+                                format!(
+                                    "there {} only {} argument{}",
+                                    if arg_count != 1 { "are" } else { "is" },
+                                    arg_count,
+                                    pluralize!(arg_count)
+                                ),
+                            )
+                            .emit();
+                        return false;
+                    }
+                } else {
+                    bug!("should be a function item");
+                }
+            } else {
+                invalid_args.push(meta.span());
+            }
+        }
+
+        if !invalid_args.is_empty() {
+            self.tcx
+                .sess
+                .struct_span_err(invalid_args, "arguments should be non-negative integers")
+                .emit();
+            false
+        } else {
+            true
+        }
+    }
+
     /// Checks if `#[link_section]` is applied to a function or static.
     fn check_link_section(&self, hir_id: HirId, attr: &Attribute, span: &Span, target: Target) {
         match target {

compiler/rustc_resolve/src/lib.rs

@@ -1076,6 +1076,10 @@ impl ResolverAstLowering for Resolver<'_> {
         self.cstore().item_generics_num_lifetimes(def_id, sess)
     }
 
+    fn item_attrs(&self, def_id: DefId, sess: &Session) -> Vec<ast::Attribute> {
+        self.cstore().item_attrs(def_id, sess)
+    }
+
     fn get_partial_res(&mut self, id: NodeId) -> Option<PartialRes> {
         self.partial_res_map.get(&id).cloned()
     }

compiler/rustc_span/src/symbol.rs

@@ -973,6 +973,7 @@ symbols! {
         rustc_layout,
         rustc_layout_scalar_valid_range_end,
         rustc_layout_scalar_valid_range_start,
+        rustc_legacy_const_generics,
         rustc_macro_transparency,
         rustc_mir,
         rustc_nonnull_optimization_guaranteed,

src/test/ui/auxiliary/legacy-const-generics.rs

@@ -0,0 +1,6 @@
+#![feature(rustc_attrs)]
+
+#[rustc_legacy_const_generics(1)]
+pub fn foo<const Y: usize>(x: usize, z: usize) -> [usize; 3] {
+    [x, Y, z]
+}

src/test/ui/invalid/invalid-rustc_legacy_const_generics-arguments.rs

@@ -0,0 +1,35 @@
+#![feature(rustc_attrs)]
+
+#[rustc_legacy_const_generics(0)] //~ ERROR index exceeds number of arguments
+fn foo1() {}
+
+#[rustc_legacy_const_generics(1)] //~ ERROR index exceeds number of arguments
+fn foo2(_: u8) {}
+
+#[rustc_legacy_const_generics(2)] //~ ERROR index exceeds number of arguments
+fn foo3<const X: usize>(_: u8) {}
+
+#[rustc_legacy_const_generics(a)] //~ ERROR arguments should be non-negative integers
+fn foo4() {}
+
+#[rustc_legacy_const_generics(1, a, 2, b)] //~ ERROR arguments should be non-negative integers
+fn foo5(_: u8, _: u8, _: u8) {}
+
+#[rustc_legacy_const_generics(0)] //~ ERROR attribute should be applied to a function
+struct S;
+
+#[rustc_legacy_const_generics(0usize)] //~ ERROR suffixed literals are not allowed in attributes
+fn foo6(_: u8) {}
+
+extern {
+    #[rustc_legacy_const_generics(1)] //~ ERROR index exceeds number of arguments
+    fn foo7(_: u8);
+}
+
+#[rustc_legacy_const_generics] //~ ERROR malformed `rustc_legacy_const_generics` attribute
+fn bar1() {}
+
+#[rustc_legacy_const_generics = 1] //~ ERROR malformed `rustc_legacy_const_generics` attribute
+fn bar2() {}
+
+fn main() {}

src/test/ui/invalid/invalid-rustc_legacy_const_generics-arguments.stderr

@@ -0,0 +1,66 @@
+error: suffixed literals are not allowed in attributes
+  --> $DIR/invalid-rustc_legacy_const_generics-arguments.rs:21:31
+   |
+LL | #[rustc_legacy_const_generics(0usize)]
+   |                               ^^^^^^
+   |
+   = help: instead of using a suffixed literal (`1u8`, `1.0f32`, etc.), use an unsuffixed version (`1`, `1.0`, etc.)
+
+error: malformed `rustc_legacy_const_generics` attribute input
+  --> $DIR/invalid-rustc_legacy_const_generics-arguments.rs:29:1
+   |
+LL | #[rustc_legacy_const_generics]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: must be of the form: `#[rustc_legacy_const_generics(N)]`
+
+error: malformed `rustc_legacy_const_generics` attribute input
+  --> $DIR/invalid-rustc_legacy_const_generics-arguments.rs:32:1
+   |
+LL | #[rustc_legacy_const_generics = 1]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: must be of the form: `#[rustc_legacy_const_generics(N)]`
+
+error: index exceeds number of arguments
+  --> $DIR/invalid-rustc_legacy_const_generics-arguments.rs:3:31
+   |
+LL | #[rustc_legacy_const_generics(0)]
+   |                               ^ there are only 0 arguments
+
+error: index exceeds number of arguments
+  --> $DIR/invalid-rustc_legacy_const_generics-arguments.rs:6:31
+   |
+LL | #[rustc_legacy_const_generics(1)]
+   |                               ^ there is only 1 argument
+
+error: index exceeds number of arguments
+  --> $DIR/invalid-rustc_legacy_const_generics-arguments.rs:9:31
+   |
+LL | #[rustc_legacy_const_generics(2)]
+   |                               ^ there are only 2 arguments
+
+error: arguments should be non-negative integers
+  --> $DIR/invalid-rustc_legacy_const_generics-arguments.rs:12:31
+   |
+LL | #[rustc_legacy_const_generics(a)]
+   |                               ^
+
+error: arguments should be non-negative integers
+  --> $DIR/invalid-rustc_legacy_const_generics-arguments.rs:15:34
+   |
+LL | #[rustc_legacy_const_generics(1, a, 2, b)]
+   |                                  ^     ^
+
+error: attribute should be applied to a function
+  --> $DIR/invalid-rustc_legacy_const_generics-arguments.rs:18:1
+   |
+LL | #[rustc_legacy_const_generics(0)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | struct S;
+   | --------- not a function
+
+error: index exceeds number of arguments
+  --> $DIR/invalid-rustc_legacy_const_generics-arguments.rs:25:35
+   |
+LL |     #[rustc_legacy_const_generics(1)]
+   |                                   ^ there is only 1 argument
+
+error: aborting due to 10 previous errors
+

src/test/ui/legacy-const-generics.rs

@@ -0,0 +1,18 @@
+// aux-build:legacy-const-generics.rs
+// run-pass
+
+#![feature(rustc_attrs)]
+
+extern crate legacy_const_generics;
+
+#[rustc_legacy_const_generics(1)]
+pub fn bar<const Y: usize>(x: usize, z: usize) -> [usize; 3] {
+    [x, Y, z]
+}
+
+fn main() {
+    assert_eq!(legacy_const_generics::foo(0 + 0, 1 + 1, 2 + 2), [0, 2, 4]);
+    assert_eq!(legacy_const_generics::foo::<{1 + 1}>(0 + 0, 2 + 2), [0, 2, 4]);
+    // TODO: Only works cross-crate
+    //assert_eq!(bar(0, 1, 2), [0, 1, 2]);
+}