forbid generic params in complex consts

src/librustc_ast/ast.rs

@@ -1052,6 +1052,30 @@ impl Expr {
         }
     }
 
+    /// Is this expr either `N`, or `{ N }`.
+    ///
+    /// If this is not the case, name resolution does not resolve `N` when using
+    /// `feature(min_const_generics)` as more complex expressions are not supported.
+    pub fn is_potential_trivial_const_param(&self) -> bool {
+        let this = if let ExprKind::Block(ref block, None) = self.kind {
+            if block.stmts.len() == 1 {
+                if let StmtKind::Expr(ref expr) = block.stmts[0].kind { expr } else { self }
+            } else {
+                self
+            }
+        } else {
+            self
+        };
+
+        if let ExprKind::Path(None, ref path) = this.kind {
+            if path.segments.len() == 1 && path.segments[0].args.is_none() {
+                return true;
+            }
+        }
+
+        false
+    }
+
     pub fn to_bound(&self) -> Option<GenericBound> {
         match &self.kind {
             ExprKind::Path(None, path) => Some(GenericBound::Trait(

src/librustc_resolve/diagnostics.rs

@@ -466,6 +466,23 @@ impl<'a> Resolver<'a> {
                 );
                 err
             }
+            ResolutionError::ParamInNonTrivialAnonConst(name) => {
+                let mut err = self.session.struct_span_err(
+                    span,
+                    "generic parameters must not be used inside of non trivial constant values",
+                );
+                err.span_label(
+                    span,
+                    &format!(
+                        "non-trivial anonymous constants must not depend on the parameter `{}`",
+                        name
+                    ),
+                );
+                err.help(
+                    &format!("it is currently only allowed to use either `{0}` or `{{ {0} }}` as generic constants", name)
+                );
+                err
+            }
             ResolutionError::SelfInTyParamDefault => {
                 let mut err = struct_span_err!(
                     self.session,

src/librustc_resolve/late.rs

@@ -111,7 +111,7 @@ crate enum RibKind<'a> {
     ItemRibKind(HasGenericParams),
 
     /// We're in a constant item. Can't refer to dynamic stuff.
-    ConstantItemRibKind,
+    ConstantItemRibKind(bool),
 
     /// We passed through a module.
     ModuleRibKind(Module<'a>),
@@ -137,7 +137,7 @@ impl RibKind<'_> {
             NormalRibKind
             | ClosureOrAsyncRibKind
             | FnItemRibKind
-            | ConstantItemRibKind
+            | ConstantItemRibKind(_)
             | ModuleRibKind(_)
             | MacroDefinition(_)
             | ConstParamTyRibKind => false,
@@ -426,7 +426,7 @@ impl<'a, 'ast> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast> {
     }
     fn visit_anon_const(&mut self, constant: &'ast AnonConst) {
         debug!("visit_anon_const {:?}", constant);
-        self.with_constant_rib(|this| {
+        self.with_constant_rib(constant.value.is_potential_trivial_const_param(), |this| {
             visit::walk_anon_const(this, constant);
         });
     }
@@ -628,7 +628,7 @@ impl<'a, 'ast> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast> {
                         if !check_ns(TypeNS) && check_ns(ValueNS) {
                             // This must be equivalent to `visit_anon_const`, but we cannot call it
                             // directly due to visitor lifetimes so we have to copy-paste some code.
-                            self.with_constant_rib(|this| {
+                            self.with_constant_rib(true, |this| {
                                 this.smart_resolve_path(
                                     ty.id,
                                     qself.as_ref(),
@@ -829,7 +829,7 @@ impl<'a, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                 | ClosureOrAsyncRibKind
                 | FnItemRibKind
                 | ItemRibKind(..)
-                | ConstantItemRibKind
+                | ConstantItemRibKind(_)
                 | ModuleRibKind(..)
                 | ForwardTyParamBanRibKind
                 | ConstParamTyRibKind => {
@@ -948,7 +948,10 @@ impl<'a, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                                         // Only impose the restrictions of `ConstRibKind` for an
                                         // actual constant expression in a provided default.
                                         if let Some(expr) = default {
-                                            this.with_constant_rib(|this| this.visit_expr(expr));
+                                            this.with_constant_rib(
+                                                expr.is_potential_trivial_const_param(),
+                                                |this| this.visit_expr(expr),
+                                            );
                                         }
                                     }
                                     AssocItemKind::Fn(_, _, generics, _) => {
@@ -989,7 +992,9 @@ impl<'a, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                 self.with_item_rib(HasGenericParams::No, |this| {
                     this.visit_ty(ty);
                     if let Some(expr) = expr {
-                        this.with_constant_rib(|this| this.visit_expr(expr));
+                        this.with_constant_rib(expr.is_potential_trivial_const_param(), |this| {
+                            this.visit_expr(expr)
+                        });
                     }
                 });
             }
@@ -1086,11 +1091,11 @@ impl<'a, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
         self.with_rib(ValueNS, kind, |this| this.with_rib(TypeNS, kind, f))
     }
 
-    fn with_constant_rib(&mut self, f: impl FnOnce(&mut Self)) {
+    fn with_constant_rib(&mut self, trivial: bool, f: impl FnOnce(&mut Self)) {
         debug!("with_constant_rib");
-        self.with_rib(ValueNS, ConstantItemRibKind, |this| {
-            this.with_rib(TypeNS, ConstantItemRibKind, |this| {
-                this.with_label_rib(ConstantItemRibKind, f);
+        self.with_rib(ValueNS, ConstantItemRibKind(trivial), |this| {
+            this.with_rib(TypeNS, ConstantItemRibKind(trivial), |this| {
+                this.with_label_rib(ConstantItemRibKind(trivial), f);
             })
         });
     }
@@ -1220,7 +1225,7 @@ impl<'a, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                                 for item in impl_items {
                                     use crate::ResolutionError::*;
                                     match &item.kind {
-                                        AssocItemKind::Const(..) => {
+                                        AssocItemKind::Const(_default, _ty, expr) => {
                                             debug!("resolve_implementation AssocItemKind::Const",);
                                             // If this is a trait impl, ensure the const
                                             // exists in trait
@@ -1231,9 +1236,18 @@ impl<'a, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                                                 |n, s| ConstNotMemberOfTrait(n, s),
                                             );
 
-                                            this.with_constant_rib(|this| {
-                                                visit::walk_assoc_item(this, item, AssocCtxt::Impl)
-                                            });
+                                            this.with_constant_rib(
+                                                expr.as_ref().map_or(false, |e| {
+                                                    e.is_potential_trivial_const_param()
+                                                }),
+                                                |this| {
+                                                    visit::walk_assoc_item(
+                                                        this,
+                                                        item,
+                                                        AssocCtxt::Impl,
+                                                    )
+                                                },
+                                            );
                                         }
                                         AssocItemKind::Fn(_, _, generics, _) => {
                                             // We also need a new scope for the impl item type parameters.

src/librustc_resolve/lib.rs

@@ -218,6 +218,10 @@ enum ResolutionError<'a> {
     ParamInTyOfConstParam(Symbol),
     /// constant values inside of type parameter defaults must not depend on generic parameters.
     ParamInAnonConstInTyDefault(Symbol),
+    /// generic parameters must not be used inside of non trivial constant values.
+    ///
+    /// This error is only emitted when using `min_const_generics`.
+    ParamInNonTrivialAnonConst(Symbol),
     /// Error E0735: type parameters with a default cannot use `Self`
     SelfInTyParamDefault,
     /// Error E0767: use of unreachable label
@@ -2507,7 +2511,7 @@ impl<'a> Resolver<'a> {
                                 res_err = Some(CannotCaptureDynamicEnvironmentInFnItem);
                             }
                         }
-                        ConstantItemRibKind => {
+                        ConstantItemRibKind(_) => {
                             // Still doesn't deal with upvars
                             if record_used {
                                 self.report_error(span, AttemptToUseNonConstantValueInConstant);
@@ -2546,7 +2550,18 @@ impl<'a> Resolver<'a> {
                             in_ty_param_default = true;
                             continue;
                         }
-                        ConstantItemRibKind => {
+                        ConstantItemRibKind(trivial) => {
+                            // HACK(min_const_generics): We currently only allow `N` or `{ N }`.
+                            if !trivial && self.session.features_untracked().min_const_generics {
+                                if record_used {
+                                    self.report_error(
+                                        span,
+                                        ResolutionError::ParamInNonTrivialAnonConst(rib_ident.name),
+                                    );
+                                }
+                                return Res::Err;
+                            }
+
                             if in_ty_param_default {
                                 if record_used {
                                     self.report_error(
@@ -2612,7 +2627,18 @@ impl<'a> Resolver<'a> {
                             in_ty_param_default = true;
                             continue;
                         }
-                        ConstantItemRibKind => {
+                        ConstantItemRibKind(trivial) => {
+                            // HACK(min_const_generics): We currently only allow `N` or `{ N }`.
+                            if !trivial && self.session.features_untracked().min_const_generics {
+                                if record_used {
+                                    self.report_error(
+                                        span,
+                                        ResolutionError::ParamInNonTrivialAnonConst(rib_ident.name),
+                                    );
+                                }
+                                return Res::Err;
+                            }
+
                             if in_ty_param_default {
                                 if record_used {
                                     self.report_error(

src/test/ui/const-generics/min_const_generics/complex-expression.rs

@@ -0,0 +1,37 @@
+#![feature(min_const_generics)]
+
+fn test<const N: usize>() {}
+
+fn ok<const M: usize>() -> [u8; M] {
+    [0; { M }]
+}
+
+struct Break0<const N: usize>([u8; { N + 1 }]);
+//~^ ERROR generic parameters must not be used inside of non trivial constant values
+
+struct Break1<const N: usize>([u8; { { N } }]);
+//~^ ERROR generic parameters must not be used inside of non trivial constant values
+
+fn break2<const N: usize>() {
+    let _: [u8; N + 1];
+    //~^ ERROR generic parameters must not be used inside of non trivial constant values
+}
+
+fn break3<const N: usize>() {
+    let _ = [0; N + 1];
+    //~^ ERROR generic parameters must not be used inside of non trivial constant values
+}
+
+trait Foo {
+    const ASSOC: usize;
+}
+
+impl<const N: usize> Foo for [u8; N] {
+    const ASSOC: usize = N + 1;
+    //~^ ERROR generic parameters must not be used inside of non trivial constant values
+    // FIXME(min_const_generics): We probably have to allow this as we can
+    // already allow referencing type parameters here on stable.
+}
+
+
+fn main() {}

src/test/ui/const-generics/min_const_generics/complex-expression.stderr

@@ -0,0 +1,42 @@
+error: generic parameters must not be used inside of non trivial constant values
+  --> $DIR/complex-expression.rs:9:38
+   |
+LL | struct Break0<const N: usize>([u8; { N + 1 }]);
+   |                                      ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |
+   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+
+error: generic parameters must not be used inside of non trivial constant values
+  --> $DIR/complex-expression.rs:12:40
+   |
+LL | struct Break1<const N: usize>([u8; { { N } }]);
+   |                                        ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |
+   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+
+error: generic parameters must not be used inside of non trivial constant values
+  --> $DIR/complex-expression.rs:16:17
+   |
+LL |     let _: [u8; N + 1];
+   |                 ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |
+   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+
+error: generic parameters must not be used inside of non trivial constant values
+  --> $DIR/complex-expression.rs:21:17
+   |
+LL |     let _ = [0; N + 1];
+   |                 ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |
+   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+
+error: generic parameters must not be used inside of non trivial constant values
+  --> $DIR/complex-expression.rs:30:26
+   |
+LL |     const ASSOC: usize = N + 1;
+   |                          ^ non-trivial anonymous constants must not depend on the parameter `N`
+   |
+   = help: it is currently only allowed to use either `N` or `{ N }` as generic constants
+
+error: aborting due to 5 previous errors
+

src/test/ui/const-generics/min_const_generics/feature-gate-min_const_generics.rs

@@ -0,0 +1,4 @@
+fn test<const N: usize>() {}
+//~^ ERROR const generics are unstable
+
+fn main() {}

src/test/ui/const-generics/min_const_generics/feature-gate-min_const_generics.stderr

@@ -0,0 +1,12 @@
+error[E0658]: const generics are unstable
+  --> $DIR/feature-gate-min_const_generics.rs:1:15
+   |
+LL | fn test<const N: usize>() {}
+   |               ^
+   |
+   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information
+   = help: add `#![feature(const_generics)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.