Properly reject non-const arguments

compiler/rustc_resolve/src/late.rs

@@ -2326,8 +2326,20 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
 
             ExprKind::Call(ref callee, ref arguments) => {
                 self.resolve_expr(callee, Some(expr));
-                for argument in arguments {
-                    self.resolve_expr(argument, None);
+                let const_args = self.legacy_const_generic_args(callee).unwrap_or(Vec::new());
+                for (idx, argument) in arguments.iter().enumerate() {
+                    if const_args.contains(&idx) {
+                        self.with_constant_rib(
+                            IsRepeatExpr::No,
+                            argument.is_potential_trivial_const_param(),
+                            None,
+                            |this| {
+                                this.resolve_expr(argument, None);
+                            },
+                        );
+                    } else {
+                        self.resolve_expr(argument, None);
+                    }
                 }
             }
             ExprKind::Type(ref type_expr, ref ty) => {
@@ -2406,6 +2418,42 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             Some((ident.name, ns)),
         )
     }
+
+    /// 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.r.get_partial_res(expr.id) {
+                if partial_res.unresolved_segments() != 0 {
+                    return None;
+                }
+                if let Res::Def(def::DefKind::Fn, def_id) = partial_res.base_res() {
+                    if def_id.is_local() {
+                        return None;
+                    }
+                    let attrs = self.r.cstore().item_attrs(def_id, self.r.session);
+                    let attr = attrs
+                        .iter()
+                        .find(|a| self.r.session.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
+    }
 }
 
 impl<'a> Resolver<'a> {

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

@@ -0,0 +1,16 @@
+// aux-build:legacy-const-generics.rs
+
+extern crate legacy_const_generics;
+
+fn foo<const N: usize>() {
+    let a = 1;
+    legacy_const_generics::foo(0, a, 2);
+    //~^ ERROR attempt to use a non-constant value in a constant
+
+    legacy_const_generics::foo(0, N, 2);
+
+    legacy_const_generics::foo(0, N + 1, 2);
+    //~^ ERROR generic parameters may not be used in const operations
+}
+
+fn main() {}

src/test/ui/legacy-const-generics-bad.stderr

@@ -0,0 +1,20 @@
+error[E0435]: attempt to use a non-constant value in a constant
+  --> $DIR/legacy-const-generics-bad.rs:7:35
+   |
+LL |     let a = 1;
+   |     ----- help: consider using `const` instead of `let`: `const a`
+LL |     legacy_const_generics::foo(0, a, 2);
+   |                                   ^ non-constant value
+
+error: generic parameters may not be used in const operations
+  --> $DIR/legacy-const-generics-bad.rs:12:35
+   |
+LL |     legacy_const_generics::foo(0, N + 1, 2);
+   |                                   ^ cannot perform const operation using `N`
+   |
+   = help: const parameters may only be used as standalone arguments, i.e. `N`
+   = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0435`.