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`.

tests/ui/const-generics/const-param-type-depends-on-const-param.rs

@@ -10,8 +10,10 @@
 
 pub struct Dependent<const N: usize, const X: [u8; N]>([(); N]);
 //~^ ERROR: the type of const parameters must not depend on other generic parameters
+//[min]~^^ ERROR `[u8; N]` is forbidden
 
 pub struct SelfDependent<const N: [u8; N]>;
 //~^ ERROR: the type of const parameters must not depend on other generic parameters
+//[min]~^^ ERROR `[u8; N]` is forbidden
 
 fn main() {}

tests/ui/const-generics/fn-const-param-infer.adt_const_params.stderr

@@ -13,7 +13,18 @@ LL |     let _ = Checked::<{ generic_arg::<u32> }>;
    = note: expected fn pointer `fn(usize) -> _`
                  found fn item `fn(u32) -> _ {generic_arg::<u32>}`
 
-error: aborting due to 2 previous errors
+error[E0282]: type annotations needed
+  --> $DIR/fn-const-param-infer.rs:35:23
+   |
+LL |     let _ = Checked::<generic>;
+   |                       ^^^^^^^ cannot infer type of the type parameter `T` declared on the function `generic`
+   |
+help: consider specifying the generic argument
+   |
+LL |     let _ = Checked::<generic::<T>>;
+   |                              +++++
+
+error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0308, E0741.
-For more information about an error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0282, E0308, E0741.
+For more information about an error, try `rustc --explain E0282`.

tests/ui/const-generics/fn-const-param-infer.full.stderr

@@ -13,7 +13,18 @@ LL |     let _ = Checked::<{ generic_arg::<u32> }>;
    = note: expected fn pointer `fn(usize) -> _`
                  found fn item `fn(u32) -> _ {generic_arg::<u32>}`
 
-error: aborting due to 2 previous errors
+error[E0282]: type annotations needed
+  --> $DIR/fn-const-param-infer.rs:35:23
+   |
+LL |     let _ = Checked::<generic>;
+   |                       ^^^^^^^ cannot infer type of the type parameter `T` declared on the function `generic`
+   |
+help: consider specifying the generic argument
+   |
+LL |     let _ = Checked::<generic::<T>>;
+   |                              +++++
+
+error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0308, E0741.
-For more information about an error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0282, E0308, E0741.
+For more information about an error, try `rustc --explain E0282`.

tests/ui/const-generics/fn-const-param-infer.min.stderr

@@ -15,6 +15,18 @@ LL |     let _ = Checked::<{ generic_arg::<u32> }>;
    = note: expected fn pointer `fn(usize) -> _`
                  found fn item `fn(u32) -> _ {generic_arg::<u32>}`
 
-error: aborting due to 2 previous errors
+error[E0282]: type annotations needed
+  --> $DIR/fn-const-param-infer.rs:35:23
+   |
+LL |     let _ = Checked::<generic>;
+   |                       ^^^^^^^ cannot infer type of the type parameter `T` declared on the function `generic`
+   |
+help: consider specifying the generic argument
+   |
+LL |     let _ = Checked::<generic::<T>>;
+   |                              +++++
+
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0282, E0308.
+For more information about an error, try `rustc --explain E0282`.

tests/ui/const-generics/fn-const-param-infer.rs

@@ -32,7 +32,7 @@ fn main() {
     let _ = Checked::<{ generic_arg::<usize> }>;
     let _ = Checked::<{ generic_arg::<u32> }>; //~ ERROR: mismatched types
 
-    let _ = Checked::<generic>;
+    let _ = Checked::<generic>; //~ ERROR: type annotations needed
     let _ = Checked::<{ generic::<u16> }>;
     let _: Checked<{ generic::<u16> }> = Checked::<{ generic::<u16> }>;
     let _: Checked<{ generic::<u32> }> = Checked::<{ generic::<u16> }>;

tests/ui/const-generics/generic_const_exprs/adt_wf_hang.rs

@@ -7,8 +7,8 @@
 struct U;
 
 struct S<const N: U>()
-//~^ ERROR: `U` must implement `ConstParamTy` to be used as the type of a const generic parameter
 where
     S<{ U }>:;
+//~^ ERROR: overflow evaluating the requirement `S<{ U }> well-formed`
 
 fn main() {}

tests/ui/const-generics/generic_const_exprs/adt_wf_hang.stderr

@@ -1,15 +1,18 @@
-error[E0741]: `U` must implement `ConstParamTy` to be used as the type of a const generic parameter
-  --> $DIR/adt_wf_hang.rs:9:19
+error[E0275]: overflow evaluating the requirement `S<{ U }> well-formed`
+  --> $DIR/adt_wf_hang.rs:11:5
    |
-LL | struct S<const N: U>()
-   |                   ^
-   |
-help: add `#[derive(ConstParamTy)]` to the struct
+LL |     S<{ U }>:;
+   |     ^^^^^^^^
    |
-LL + #[derive(ConstParamTy)]
-LL | struct U;
+note: required by a bound in `S`
+  --> $DIR/adt_wf_hang.rs:11:5
    |
+LL | struct S<const N: U>()
+   |        - required by a bound in this struct
+LL | where
+LL |     S<{ U }>:;
+   |     ^^^^^^^^ required by this bound in `S`
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0741`.
+For more information about this error, try `rustc --explain E0275`.

tests/ui/const-generics/generic_const_exprs/error_in_ty.rs

@@ -5,13 +5,16 @@
 
 pub struct A<const z: [usize; x]> {}
 //~^ ERROR: cannot find value `x` in this scope
+//~| ERROR: `[usize; x]` is forbidden as the type of a const generic parameter
 
 impl A<2> {
+    //~^ ERROR: mismatched types
     pub const fn B() {}
     //~^ ERROR: duplicate definitions
 }
 
 impl A<2> {
+    //~^ ERROR: mismatched types
     pub const fn B() {}
 }
 

tests/ui/const-generics/generic_const_exprs/error_in_ty.stderr

@@ -6,16 +6,40 @@ LL | pub struct A<const z: [usize; x]> {}
    |                    |
    |                    similarly named const parameter `z` defined here
 
+error: `[usize; x]` is forbidden as the type of a const generic parameter
+  --> $DIR/error_in_ty.rs:6:23
+   |
+LL | pub struct A<const z: [usize; x]> {}
+   |                       ^^^^^^^^^^
+   |
+   = 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[E0308]: mismatched types
+  --> $DIR/error_in_ty.rs:10:8
+   |
+LL | impl A<2> {
+   |        ^ expected `[usize; x]`, found integer
+
+error[E0308]: mismatched types
+  --> $DIR/error_in_ty.rs:16:8
+   |
+LL | impl A<2> {
+   |        ^ expected `[usize; x]`, found integer
+
 error[E0592]: duplicate definitions with name `B`
-  --> $DIR/error_in_ty.rs:10:5
+  --> $DIR/error_in_ty.rs:12:5
    |
 LL |     pub const fn B() {}
    |     ^^^^^^^^^^^^^^^^ duplicate definitions for `B`
 ...
 LL |     pub const fn B() {}
    |     ---------------- other definition for `B`
 
-error: aborting due to 2 previous errors
+error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0425, E0592.
-For more information about an error, try `rustc --explain E0425`.
+Some errors have detailed explanations: E0308, E0425, E0592.
+For more information about an error, try `rustc --explain E0308`.