add min_const_generics feature gate

Author: lcnr

src/librustc_ast_passes/ast_validation.rs

@@ -776,7 +776,13 @@ fn validate_generic_param_order<'a>(
             span,
             &format!(
                 "reorder the parameters: lifetimes, then types{}",
-                if sess.features_untracked().const_generics { ", then consts" } else { "" },
+                if sess.features_untracked().const_generics
+                    || sess.features_untracked().min_const_generics
+                {
+                    ", then consts"
+                } else {
+                    ""
+                },
             ),
             ordered_params.clone(),
             Applicability::MachineApplicable,

src/librustc_ast_passes/feature_gate.rs

@@ -526,12 +526,13 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_generic_param(&mut self, param: &'a GenericParam) {
         if let GenericParamKind::Const { .. } = param.kind {
-            gate_feature_post!(
+            gate_feature_fn!(
                 &self,
-                const_generics,
+                |x: &Features| x.const_generics || x.min_const_generics,
                 param.ident.span,
+                sym::const_generics,
                 "const generics are unstable"
-            )
+            );
         }
         visit::walk_generic_param(self, param)
     }

src/librustc_feature/active.rs

@@ -579,6 +579,9 @@ declare_features! (
     /// Alloc calling `transmute` in const fn
     (active, const_fn_transmute, "1.46.0", Some(53605), None),
 
+    /// The smallest useful subset of `const_generics`.
+    (active, min_const_generics, "1.46.0", None, None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

src/librustc_middle/ty/context.rs

@@ -1380,7 +1380,9 @@ impl<'tcx> TyCtxt<'tcx> {
     /// we still evaluate them eagerly.
     #[inline]
     pub fn lazy_normalization(self) -> bool {
-        self.features().const_generics || self.features().lazy_normalization_consts
+        let features = self.features();
+        // Note: We do not enable lazy normalization for `features.min_const_generics`.
+        features.const_generics || features.lazy_normalization_consts
     }
 
     #[inline]

src/librustc_span/symbol.rs

@@ -672,6 +672,7 @@ symbols! {
         min_align_of,
         min_align_of_val,
         min_const_fn,
+        min_const_generics,
         min_const_unsafe_fn,
         min_specialization,
         minnumf32,

src/librustc_typeck/collect.rs

@@ -1238,6 +1238,9 @@ fn generics_of(tcx: TyCtxt<'_>, def_id: DefId) -> ty::Generics {
                 // HACK(eddyb) this provides the correct generics when
                 // `feature(const_generics)` is enabled, so that const expressions
                 // used with const generics, e.g. `Foo<{N+1}>`, can work at all.
+                //
+                // Note that we do not supply the parent generics when using
+                // `feature(min_const_generics)`.
                 Some(parent_def_id.to_def_id())
             } else {
                 let parent_node = tcx.hir().get(tcx.hir().get_parent_node(hir_id));