Postpone the evaluation of constant expressions that depend on inference variables

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -1,5 +1,6 @@
 use crate::infer::type_variable::TypeVariableOriginKind;
 use crate::infer::InferCtxt;
+use crate::rustc_middle::ty::TypeFoldable;
 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Namespace};
@@ -390,36 +391,75 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 }
             }
             GenericArgKind::Const(ct) => {
-                if let ty::ConstKind::Infer(InferConst::Var(vid)) = ct.val {
-                    let origin =
-                        self.inner.borrow_mut().const_unification_table().probe_value(vid).origin;
-                    if let ConstVariableOriginKind::ConstParameterDefinition(name, def_id) =
-                        origin.kind
-                    {
-                        return InferenceDiagnosticsData {
-                            name: name.to_string(),
+                match ct.val {
+                    ty::ConstKind::Infer(InferConst::Var(vid)) => {
+                        let origin = self
+                            .inner
+                            .borrow_mut()
+                            .const_unification_table()
+                            .probe_value(vid)
+                            .origin;
+                        if let ConstVariableOriginKind::ConstParameterDefinition(name, def_id) =
+                            origin.kind
+                        {
+                            return InferenceDiagnosticsData {
+                                name: name.to_string(),
+                                span: Some(origin.span),
+                                kind: UnderspecifiedArgKind::Const { is_parameter: true },
+                                parent: InferenceDiagnosticsParentData::for_def_id(
+                                    self.tcx, def_id,
+                                ),
+                            };
+                        }
+
+                        debug_assert!(!origin.span.is_dummy());
+                        let mut s = String::new();
+                        let mut printer =
+                            ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::ValueNS);
+                        if let Some(highlight) = highlight {
+                            printer.region_highlight_mode = highlight;
+                        }
+                        let _ = ct.print(printer);
+                        InferenceDiagnosticsData {
+                            name: s,
                             span: Some(origin.span),
-                            kind: UnderspecifiedArgKind::Const { is_parameter: true },
-                            parent: InferenceDiagnosticsParentData::for_def_id(self.tcx, def_id),
-                        };
+                            kind: UnderspecifiedArgKind::Const { is_parameter: false },
+                            parent: None,
+                        }
                     }
-
-                    debug_assert!(!origin.span.is_dummy());
-                    let mut s = String::new();
-                    let mut printer =
-                        ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::ValueNS);
-                    if let Some(highlight) = highlight {
-                        printer.region_highlight_mode = highlight;
+                    ty::ConstKind::Unevaluated(ty::Unevaluated {
+                        substs_: Some(substs), ..
+                    }) => {
+                        assert!(substs.has_infer_types_or_consts());
+
+                        // FIXME: We only use the first inference variable we encounter in
+                        // `substs` here, this gives insufficiently informative diagnostics
+                        // in case there are multiple inference variables
+                        for s in substs.iter() {
+                            match s.unpack() {
+                                GenericArgKind::Type(t) => match t.kind() {
+                                    ty::Infer(_) => {
+                                        return self.extract_inference_diagnostics_data(s, None);
+                                    }
+                                    _ => {}
+                                },
+                                GenericArgKind::Const(c) => match c.val {
+                                    ty::ConstKind::Infer(InferConst::Var(_)) => {
+                                        return self.extract_inference_diagnostics_data(s, None);
+                                    }
+                                    _ => {}
+                                },
+                                _ => {}
+                            }
+                        }
+                        bug!(
+                            "expected an inference variable in substs of unevaluated const {:?}",
+                            ct
+                        );
                     }
-                    let _ = ct.print(printer);
-                    InferenceDiagnosticsData {
-                        name: s,
-                        span: Some(origin.span),
-                        kind: UnderspecifiedArgKind::Const { is_parameter: false },
-                        parent: None,
+                    _ => {
+                        bug!("unexpect const: {:?}", ct);
                     }
-                } else {
-                    bug!("unexpect const: {:?}", ct);
                 }
             }
             GenericArgKind::Lifetime(_) => bug!("unexpected lifetime"),

compiler/rustc_infer/src/infer/mod.rs

@@ -21,6 +21,7 @@ use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::infer::canonical::{Canonical, CanonicalVarValues};
 use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind, ToType};
+use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::mir::interpret::EvalToConstValueResult;
 use rustc_middle::traits::select;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
@@ -1584,13 +1585,27 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         unevaluated: ty::Unevaluated<'tcx>,
         span: Option<Span>,
     ) -> EvalToConstValueResult<'tcx> {
-        let mut original_values = OriginalQueryValues::default();
-        let canonical = self.canonicalize_query((param_env, unevaluated), &mut original_values);
+        let mut substs = unevaluated.substs(self.tcx);
+        substs = self.resolve_vars_if_possible(substs);
+
+        // Postpone the evaluation of constants whose substs depend on inference
+        // variables
+        if substs.has_infer_types_or_consts() {
+            return Err(ErrorHandled::TooGeneric);
+        }
+
+        let param_env_erased = self.tcx.erase_regions(param_env);
+        let substs_erased = self.tcx.erase_regions(substs);
+
+        let unevaluated = ty::Unevaluated {
+            def: unevaluated.def,
+            substs_: Some(substs_erased),
+            promoted: unevaluated.promoted,
+        };
 
-        let (param_env, unevaluated) = canonical.value;
         // The return value is the evaluated value which doesn't contain any reference to inference
         // variables, thus we don't need to substitute back the original values.
-        self.tcx.const_eval_resolve(param_env, unevaluated, span)
+        self.tcx.const_eval_resolve(param_env_erased, unevaluated, span)
     }
 
     /// If `typ` is a type variable of some kind, resolve it one level

src/test/ui/const-generics/generic_const_exprs/const_eval_resolve_canonical.rs

@@ -1,4 +1,3 @@
-// run-pass
 #![feature(generic_const_exprs)]
 #![allow(incomplete_features)]
 
@@ -22,8 +21,11 @@ where
 }
 
 fn main() {
-    // Test that we can correctly infer `T` which requires evaluating
-    // `{ N + 1 }` which has substs containing an inference var
+    // FIXME(generic_const_exprs): We can't correctly infer `T` which requires
+    // evaluating `{ N + 1 }` which has substs containing an inference var
     let mut _q = Default::default();
+    //~^ ERROR type annotations needed
+
     _q = foo::<_, 2>(_q);
+    //~^ ERROR type annotations needed
 }

src/test/ui/const-generics/generic_const_exprs/const_eval_resolve_canonical.stderr

@@ -0,0 +1,33 @@
+error[E0282]: type annotations needed
+  --> $DIR/const_eval_resolve_canonical.rs:26:9
+   |
+LL |     let mut _q = Default::default();
+   |         ^^^^^^ consider giving `_q` a type
+
+error[E0283]: type annotations needed
+  --> $DIR/const_eval_resolve_canonical.rs:29:10
+   |
+LL |     _q = foo::<_, 2>(_q);
+   |          ^^^^^^^^^^^ cannot infer type
+   |
+note: multiple `impl`s satisfying `(): Foo<{ N + 1 }>` found
+  --> $DIR/const_eval_resolve_canonical.rs:8:1
+   |
+LL | impl Foo<0> for () {
+   | ^^^^^^^^^^^^^^^^^^
+...
+LL | impl Foo<3> for () {
+   | ^^^^^^^^^^^^^^^^^^
+note: required by a bound in `foo`
+  --> $DIR/const_eval_resolve_canonical.rs:18:9
+   |
+LL | fn foo<T, const N: usize>(_: T) -> <() as Foo<{ N + 1 }>>::Assoc
+   |    --- required by a bound in this
+LL | where
+LL |     (): Foo<{ N + 1 }>,
+   |         ^^^^^^^^^^^^^^ required by this bound in `foo`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0282, E0283.
+For more information about an error, try `rustc --explain E0282`.

src/test/ui/const-generics/issues/issue-83249.rs

@@ -0,0 +1,23 @@
+#![allow(incomplete_features)]
+#![feature(generic_const_exprs)]
+
+trait Foo {
+    const N: usize;
+}
+
+impl Foo for u8 {
+    const N: usize = 1;
+}
+
+fn foo<T: Foo>(_: [u8; T::N]) -> T {
+    todo!()
+}
+
+pub fn bar() {
+    let _: u8 = foo([0; 1]);
+
+    let _ = foo([0; 1]);
+    //~^ ERROR type annotations needed
+}
+
+fn main() {}

src/test/ui/const-generics/issues/issue-83249.stderr

@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed
+  --> $DIR/issue-83249.rs:19:13
+   |
+LL |     let _ = foo([0; 1]);
+   |         -   ^^^ cannot infer type for type parameter `T` declared on the function `foo`
+   |         |
+   |         consider giving this pattern a type
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/const-generics/issues/issue-83288.rs

@@ -0,0 +1,69 @@
+// build-pass
+
+#![allow(incomplete_features)]
+#![feature(generic_const_exprs)]
+
+use std::{marker::PhantomData, ops::Mul};
+
+pub enum Nil {}
+pub struct Cons<T, L> {
+    _phantom: PhantomData<(T, L)>,
+}
+
+pub trait Indices<const N: usize> {
+    const RANK: usize;
+    const NUM_ELEMS: usize;
+}
+
+impl<const N: usize> Indices<N> for Nil {
+    const RANK: usize = 0;
+    const NUM_ELEMS: usize = 1;
+}
+
+impl<T, I: Indices<N>, const N: usize> Indices<N> for Cons<T, I> {
+    const RANK: usize = I::RANK + 1;
+    const NUM_ELEMS: usize = I::NUM_ELEMS * N;
+}
+
+pub trait Concat<J> {
+    type Output;
+}
+
+impl<J> Concat<J> for Nil {
+    type Output = J;
+}
+
+impl<T, I, J> Concat<J> for Cons<T, I>
+where
+    I: Concat<J>,
+{
+    type Output = Cons<T, <I as Concat<J>>::Output>;
+}
+
+pub struct Tensor<I: Indices<N>, const N: usize>
+where
+    [u8; I::NUM_ELEMS]: Sized,
+{
+    pub data: [u8; I::NUM_ELEMS],
+    _phantom: PhantomData<I>,
+}
+
+impl<I: Indices<N>, J: Indices<N>, const N: usize> Mul<Tensor<J, N>> for Tensor<I, N>
+where
+    I: Concat<J>,
+    <I as Concat<J>>::Output: Indices<N>,
+    [u8; I::NUM_ELEMS]: Sized,
+    [u8; J::NUM_ELEMS]: Sized,
+    [u8; <I as Concat<J>>::Output::NUM_ELEMS]: Sized,
+{
+    type Output = Tensor<<I as Concat<J>>::Output, N>;
+
+    fn mul(self, _rhs: Tensor<J, N>) -> Self::Output {
+        Tensor {
+            data: [0u8; <I as Concat<J>>::Output::NUM_ELEMS],
+            _phantom: PhantomData,
+        }
+    }
+}
+
+fn main() {}

src/test/ui/const-generics/issues/issue-87470.rs

@@ -0,0 +1,24 @@
+// build-pass
+
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+
+pub trait TraitWithConst {
+    const SOME_CONST: usize;
+}
+
+pub trait OtherTrait: TraitWithConst {
+    fn some_fn(self) -> [u8 ; <Self as TraitWithConst>::SOME_CONST];
+}
+
+impl TraitWithConst for f32 {
+    const SOME_CONST: usize = 32;
+}
+
+impl OtherTrait for f32 {
+    fn some_fn(self) -> [u8 ; <Self as TraitWithConst>::SOME_CONST] {
+        [0; 32]
+    }
+}
+
+fn main() {}

src/test/ui/const-generics/issues/issue-87964.rs

@@ -0,0 +1,29 @@
+// build-pass
+
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+
+pub trait Target {
+    const LENGTH: usize;
+}
+
+
+pub struct Container<T: Target>
+where
+    [(); T::LENGTH]: Sized,
+{
+    _target: T,
+}
+
+impl<T: Target> Container<T>
+where
+    [(); T::LENGTH]: Sized,
+{
+    pub fn start(
+        _target: T,
+    ) -> Container<T> {
+        Container { _target }
+    }
+}
+
+fn main() {}

src/test/ui/const-generics/issues/issue-89146.rs

@@ -0,0 +1,26 @@
+// build-pass
+
+#![allow(incomplete_features)]
+#![feature(generic_const_exprs)]
+
+pub trait Foo {
+    const SIZE: usize;
+
+    fn to_bytes(&self) -> [u8; Self::SIZE];
+}
+
+pub fn bar<G: Foo>(a: &G) -> u8
+where
+    [(); G::SIZE]: Sized,
+{
+    deeper_bar(a)
+}
+
+fn deeper_bar<G: Foo>(a: &G) -> u8
+where
+    [(); G::SIZE]: Sized,
+{
+    a.to_bytes()[0]
+}
+
+fn main() {}