Add min_generic_const_args and gate new const path lowering with it

Author: camelid

compiler/rustc_ast_lowering/src/lib.rs

@@ -2063,8 +2063,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         ty_id: NodeId,
         span: Span,
     ) -> &'hir hir::ConstArg<'hir> {
+        let tcx = self.tcx;
+
         // FIXME(min_generic_const_args): we only allow one-segment const paths for now
-        let ct_kind = if path.is_potential_trivial_const_arg() {
+        let ct_kind = if path.is_potential_trivial_const_arg()
+            && (tcx.features().min_generic_const_args()
+                || matches!(res, Res::Def(DefKind::ConstParam, _)))
+        {
             let qpath = self.lower_qpath(
                 ty_id,
                 &None,
@@ -2127,6 +2132,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
     #[instrument(level = "debug", skip(self))]
     fn lower_anon_const_to_const_arg_direct(&mut self, anon: &AnonConst) -> hir::ConstArg<'hir> {
+        let tcx = self.tcx;
         // Unwrap a block, so that e.g. `{ P }` is recognised as a parameter. Const arguments
         // currently have to be wrapped in curly brackets, so it's necessary to special-case.
         let expr = if let ExprKind::Block(block, _) = &anon.value.kind
@@ -2138,9 +2144,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         } else {
             &anon.value
         };
+        let maybe_res =
+            self.resolver.get_partial_res(expr.id).and_then(|partial_res| partial_res.full_res());
         // FIXME(min_generic_const_args): we only allow one-segment const paths for now
         if let ExprKind::Path(None, path) = &expr.kind
             && path.is_potential_trivial_const_arg()
+            && (tcx.features().min_generic_const_args()
+                || matches!(maybe_res, Some(Res::Def(DefKind::ConstParam, _))))
         {
             let qpath = self.lower_qpath(
                 expr.id,

compiler/rustc_feature/src/unstable.rs

@@ -529,6 +529,8 @@ declare_features! (
     (unstable, macro_metavar_expr_concat, "1.81.0", Some(124225)),
     /// Allows `#[marker]` on certain traits allowing overlapping implementations.
     (unstable, marker_trait_attr, "1.30.0", Some(29864)),
+    /// Enables the generic const args MVP (only bare paths, not arbitrary computation).
+    (incomplete, min_generic_const_args, "CURRENT_RUSTC_VERSION", Some(132980)),
     /// A minimal, sound subset of specialization intended to be used by the
     /// standard library until the soundness issues with specialization
     /// are fixed.

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -121,6 +121,16 @@ fn generic_arg_mismatch_err(
             {
                 err.help(format!("`{}` is a function item, not a type", tcx.item_name(id)));
                 err.help("function item types cannot be named directly");
+            } else if let hir::ConstArgKind::Anon(anon) = cnst.kind
+                && let body = tcx.hir().body(anon.body)
+                && let rustc_hir::ExprKind::Path(rustc_hir::QPath::Resolved(_, path)) =
+                    body.value.kind
+                && let Res::Def(DefKind::Fn { .. }, id) = path.res
+            {
+                // FIXME(min_generic_const_args): this branch is dead once new const path lowering
+                // (for single-segment paths) is no longer gated
+                err.help(format!("`{}` is a function item, not a type", tcx.item_name(id)));
+                err.help("function item types cannot be named directly");
             }
         }
         _ => {}

compiler/rustc_span/src/symbol.rs

@@ -1225,6 +1225,7 @@ symbols! {
         min_const_generics,
         min_const_unsafe_fn,
         min_exhaustive_patterns,
+        min_generic_const_args,
         min_specialization,
         min_type_alias_impl_trait,
         minnumf128,

tests/ui/const-generics/adt_const_params/non_valtreeable_const_arg-2.rs

@@ -15,4 +15,5 @@ impl Wrapper<{ bar() }> {
 
 fn main() {
     Wrapper::<function>::call;
+    //~^ ERROR: the function or associated item `call` exists for struct `Wrapper<function>`,
 }

tests/ui/const-generics/adt_const_params/non_valtreeable_const_arg-2.stderr

@@ -15,7 +15,25 @@ error[E0741]: using function pointers as const generic parameters is forbidden
 LL | struct Wrapper<const F: fn()>;
    |                         ^^^^
 
-error: aborting due to 2 previous errors
+error[E0599]: the function or associated item `call` exists for struct `Wrapper<function>`, but its trait bounds were not satisfied
+  --> $DIR/non_valtreeable_const_arg-2.rs:17:26
+   |
+LL | struct Wrapper<const F: fn()>;
+   | ----------------------------- function or associated item `call` not found for this struct because it doesn't satisfy `Wrapper<function>: Fn<_>`
+...
+LL |     Wrapper::<function>::call;
+   |                          ^^^^ function or associated item cannot be called on `Wrapper<function>` due to unsatisfied trait bounds
+   |
+   = note: the following trait bounds were not satisfied:
+           `Wrapper<function>: Fn<_>`
+           which is required by `&Wrapper<function>: Fn<_>`
+note: the trait `Fn` must be implemented
+  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
+   = help: items from traits can only be used if the trait is implemented and in scope
+   = note: the following trait defines an item `call`, perhaps you need to implement it:
+           candidate #1: `Fn`
+
+error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0425, E0741.
+Some errors have detailed explanations: E0425, E0599, E0741.
 For more information about an error, try `rustc --explain E0425`.

tests/ui/const-generics/bad-generic-in-copy-impl.rs

@@ -1,9 +1,8 @@
 #[derive(Copy, Clone)]
-//~^ ERROR the trait `Copy` cannot be implemented for this type
 pub struct Foo {
     x: [u8; SIZE],
-    //~^ ERROR the constant `1` is not of type `usize`
-    //~| ERROR the constant `1` is not of type `usize`
+    //~^ ERROR mismatched types
+    //~| ERROR mismatched types
 }
 
 const SIZE: u32 = 1;

tests/ui/const-generics/bad-generic-in-copy-impl.stderr

@@ -1,36 +1,17 @@
-error: the constant `1` is not of type `usize`
-  --> $DIR/bad-generic-in-copy-impl.rs:4:8
+error[E0308]: mismatched types
+  --> $DIR/bad-generic-in-copy-impl.rs:3:13
    |
 LL |     x: [u8; SIZE],
-   |        ^^^^^^^^^^ expected `usize`, found `u32`
+   |             ^^^^ expected `usize`, found `u32`
 
-error[E0204]: the trait `Copy` cannot be implemented for this type
-  --> $DIR/bad-generic-in-copy-impl.rs:1:10
+error[E0308]: mismatched types
+  --> $DIR/bad-generic-in-copy-impl.rs:3:13
    |
-LL | #[derive(Copy, Clone)]
-   |          ^^^^
-...
 LL |     x: [u8; SIZE],
-   |     ------------- this field does not implement `Copy`
+   |             ^^^^ expected `usize`, found `u32`
    |
-note: the `Copy` impl for `[u8; 1]` requires that `the constant `1` has type `usize``
-  --> $DIR/bad-generic-in-copy-impl.rs:4:8
-   |
-LL |     x: [u8; SIZE],
-   |        ^^^^^^^^^^
-   = note: this error originates in the derive macro `Copy` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error: the constant `1` is not of type `usize`
-  --> $DIR/bad-generic-in-copy-impl.rs:4:5
-   |
-LL | #[derive(Copy, Clone)]
-   |                ----- in this derive macro expansion
-...
-LL |     x: [u8; SIZE],
-   |     ^^^^^^^^^^^^^ expected `usize`, found `u32`
-   |
-   = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0204`.
+For more information about this error, try `rustc --explain E0308`.

tests/ui/const-generics/const-param-type-depends-on-const-param.full.stderr

@@ -7,7 +7,7 @@ LL | pub struct Dependent<const N: usize, const X: [u8; N]>([(); N]);
    = note: const parameters may not be used in the type of const parameters
 
 error[E0770]: the type of const parameters must not depend on other generic parameters
-  --> $DIR/const-param-type-depends-on-const-param.rs:14:40
+  --> $DIR/const-param-type-depends-on-const-param.rs:15:40
    |
 LL | pub struct SelfDependent<const N: [u8; N]>;
    |                                        ^ the type must not depend on the parameter `N`

tests/ui/const-generics/const-param-type-depends-on-const-param.min.stderr

@@ -7,13 +7,37 @@ LL | pub struct Dependent<const N: usize, const X: [u8; N]>([(); N]);
    = note: const parameters may not be used in the type of const parameters
 
 error[E0770]: the type of const parameters must not depend on other generic parameters
-  --> $DIR/const-param-type-depends-on-const-param.rs:14:40
+  --> $DIR/const-param-type-depends-on-const-param.rs:15:40
    |
 LL | pub struct SelfDependent<const N: [u8; N]>;
    |                                        ^ the type must not depend on the parameter `N`
    |
    = note: const parameters may not be used in the type of const parameters
 
-error: aborting due to 2 previous errors
+error: `[u8; N]` is forbidden as the type of a const generic parameter
+  --> $DIR/const-param-type-depends-on-const-param.rs:11:47
+   |
+LL | pub struct Dependent<const N: usize, const X: [u8; N]>([(); N]);
+   |                                               ^^^^^^^
+   |
+   = note: the only supported types are integers, `bool`, and `char`
+help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
+   |
+LL + #![feature(adt_const_params)]
+   |
+
+error: `[u8; N]` is forbidden as the type of a const generic parameter
+  --> $DIR/const-param-type-depends-on-const-param.rs:15:35
+   |
+LL | pub struct SelfDependent<const N: [u8; N]>;
+   |                                   ^^^^^^^
+   |
+   = note: the only supported types are integers, `bool`, and `char`
+help: add `#![feature(adt_const_params)]` to the crate attributes to enable more complex and user defined types
+   |
+LL + #![feature(adt_const_params)]
+   |
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0770`.