Report the selection error when possible

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -14,6 +14,7 @@ use rustc_middle::ty::{self, adjustment::PointerCast, Instance, InstanceDef, Ty,
 use rustc_middle::ty::{Binder, TraitPredicate, TraitRef};
 use rustc_mir_dataflow::{self, Analysis};
 use rustc_span::{sym, Span, Symbol};
+use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
 use rustc_trait_selection::traits::SelectionContext;
 
 use std::mem;
@@ -808,15 +809,13 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     }
 
                     let trait_ref = TraitRef::from_method(tcx, trait_id, substs);
-                    let obligation = Obligation::new(
+                    let poly_trait_pred = Binder::dummy(TraitPredicate {
-                        ObligationCause::dummy(),
-                        param_env,
-                        Binder::dummy(TraitPredicate {
                         trait_ref,
-                            constness: ty::BoundConstness::NotConst,
+                        constness: ty::BoundConstness::ConstIfConst,
                         polarity: ty::ImplPolarity::Positive,
-                        }),
+                    });
-                    );
+                    let obligation =
+                        Obligation::new(ObligationCause::dummy(), param_env, poly_trait_pred);
 
                     let implsrc = tcx.infer_ctxt().enter(|infcx| {
                         let mut selcx = SelectionContext::new(&infcx);
@@ -860,15 +859,37 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                             // #[default_method_body_is_const], and the callee is in the same
                             // trait.
                             let callee_trait = tcx.trait_of_item(callee);
-                            if callee_trait.is_some() {
+                            if callee_trait.is_some()
-                                if tcx.has_attr(caller, sym::default_method_body_is_const) {
+                                && tcx.has_attr(caller, sym::default_method_body_is_const)
-                                    if tcx.trait_of_item(caller) == callee_trait {
+                                && callee_trait == tcx.trait_of_item(caller)
+                                // Can only call methods when it's `<Self as TheTrait>::f`.
+                                && tcx.types.self_param == substs.type_at(0)
+                            {
                                 nonconst_call_permission = true;
                             }
-                                }
-                            }
 
                             if !nonconst_call_permission {
+                                let obligation = Obligation::new(
+                                    ObligationCause::dummy_with_span(*fn_span),
+                                    param_env,
+                                    tcx.mk_predicate(
+                                        poly_trait_pred.map_bound(ty::PredicateKind::Trait),
+                                    ),
+                                );
+
+                                // improve diagnostics by showing what failed. Our requirements are stricter this time
+                                // as we are going to error again anyways.
+                                tcx.infer_ctxt().enter(|infcx| {
+                                    if let Err(e) = implsrc {
+                                        infcx.report_selection_error(
+                                            obligation.clone(),
+                                            &obligation,
+                                            &e,
+                                            false,
+                                        );
+                                    }
+                                });
+
                                 self.check_op(ops::FnCallNonConst {
                                     caller,
                                     callee,

src/test/ui/rfc-2632-const-trait-impl/call-const-trait-method-fail.rs

@@ -22,7 +22,8 @@ pub const fn add_i32(a: i32, b: i32) -> i32 {
 
 pub const fn add_u32(a: u32, b: u32) -> u32 {
     a.plus(b)
-    //~^ ERROR cannot call non-const fn
+    //~^ ERROR the trait bound
+    //~| ERROR cannot call non-const fn
 }
 
 fn main() {}

src/test/ui/rfc-2632-const-trait-impl/call-const-trait-method-fail.stderr

@@ -1,3 +1,9 @@
+error[E0277]: the trait bound `u32: ~const Plus` is not satisfied
+  --> $DIR/call-const-trait-method-fail.rs:24:7
+   |
+LL |     a.plus(b)
+   |       ^^^^^^^ the trait `~const Plus` is not implemented for `u32`
+
 error[E0015]: cannot call non-const fn `<u32 as Plus>::plus` in constant functions
   --> $DIR/call-const-trait-method-fail.rs:24:7
    |
@@ -6,6 +12,7 @@ LL |     a.plus(b)
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0015`.
+Some errors have detailed explanations: E0015, E0277.
+For more information about an error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-fail.rs

@@ -3,7 +3,8 @@
 
 pub const fn equals_self<T: PartialEq>(t: &T) -> bool {
     *t == *t
-    //~^ ERROR cannot call non-const operator
+    //~^ ERROR can't compare
+    //~| ERROR cannot call non-const
 }
 
 fn main() {}

src/test/ui/rfc-2632-const-trait-impl/call-generic-method-fail.stderr

@@ -1,3 +1,9 @@
+error[E0277]: can't compare `T` with `T` in const contexts
+  --> $DIR/call-generic-method-fail.rs:5:5
+   |
+LL |     *t == *t
+   |     ^^^^^^^^ no implementation for `T == T`
+
 error[E0015]: cannot call non-const operator in constant functions
   --> $DIR/call-generic-method-fail.rs:5:5
    |
@@ -10,6 +16,7 @@ help: consider further restricting this bound
 LL | pub const fn equals_self<T: PartialEq + ~const std::cmp::PartialEq>(t: &T) -> bool {
    |                                       ++++++++++++++++++++++++++++
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0015`.
+Some errors have detailed explanations: E0015, E0277.
+For more information about an error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/const-default-method-bodies.rs

@@ -23,7 +23,8 @@ impl const ConstDefaultFn for ConstImpl {
 
 const fn test() {
     NonConstImpl.a();
-    //~^ ERROR cannot call non-const fn
+    //~^ ERROR the trait bound
+    //~| ERROR cannot call non-const fn
     ConstImpl.a();
 }
 

src/test/ui/rfc-2632-const-trait-impl/const-default-method-bodies.stderr

@@ -1,3 +1,9 @@
+error[E0277]: the trait bound `NonConstImpl: ~const ConstDefaultFn` is not satisfied
+  --> $DIR/const-default-method-bodies.rs:25:18
+   |
+LL |     NonConstImpl.a();
+   |                  ^^^ the trait `~const ConstDefaultFn` is not implemented for `NonConstImpl`
+
 error[E0015]: cannot call non-const fn `<NonConstImpl as ConstDefaultFn>::a` in constant functions
   --> $DIR/const-default-method-bodies.rs:25:18
    |
@@ -6,6 +12,7 @@ LL |     NonConstImpl.a();
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0015`.
+Some errors have detailed explanations: E0015, E0277.
+For more information about an error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/cross-crate.gated.stderr

@@ -1,3 +1,9 @@
+error[E0277]: the trait bound `cross_crate::NonConst: ~const cross_crate::MyTrait` is not satisfied
+  --> $DIR/cross-crate.rs:15:14
+   |
+LL |     NonConst.func();
+   |              ^^^^^^ the trait `~const cross_crate::MyTrait` is not implemented for `cross_crate::NonConst`
+
 error[E0015]: cannot call non-const fn `<cross_crate::NonConst as cross_crate::MyTrait>::func` in constant functions
   --> $DIR/cross-crate.rs:15:14
    |
@@ -6,6 +12,7 @@ LL |     NonConst.func();
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0015`.
+Some errors have detailed explanations: E0015, E0277.
+For more information about an error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/cross-crate.rs

@@ -12,8 +12,8 @@ fn non_const_context() {
 }
 
 const fn const_context() {
-    NonConst.func();
+    NonConst.func(); //~ ERROR: cannot call non-const fn
-    //~^ ERROR: cannot call non-const fn
+    //[gated]~^ ERROR: the trait bound
     Const.func();
     //[stock]~^ ERROR: cannot call non-const fn
 }

src/test/ui/rfc-2632-const-trait-impl/default-method-body-is-const-same-trait-ck.rs

@@ -8,7 +8,8 @@ pub trait Tr {
     #[default_method_body_is_const]
     fn b(&self) {
         ().a()
-        //~^ ERROR calls in constant functions are limited
+        //~^ ERROR the trait bound
+        //~| ERROR cannot call
     }
 }
 

src/test/ui/rfc-2632-const-trait-impl/default-method-body-is-const-same-trait-ck.stderr

@@ -1,9 +1,18 @@
-error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+error[E0277]: the trait bound `(): ~const Tr` is not satisfied
-  --> $DIR/default-method-body-is-const-same-trait-ck.rs:10:9
+  --> $DIR/default-method-body-is-const-same-trait-ck.rs:10:12
    |
 LL |         ().a()
-   |         ^^^^^^
+   |            ^^^ the trait `~const Tr` is not implemented for `()`
 
-error: aborting due to previous error
+error[E0015]: cannot call non-const fn `<() as Tr>::a` in constant functions
+  --> $DIR/default-method-body-is-const-same-trait-ck.rs:10:12
+   |
+LL |         ().a()
+   |            ^^^
+   |
+   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
-For more information about this error, try `rustc --explain E0015`.
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0015, E0277.
+For more information about an error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/issue-88155.rs

@@ -7,7 +7,8 @@ pub trait A {
 
 pub const fn foo<T: A>() -> bool {
     T::assoc()
-    //~^ ERROR cannot call non-const fn
+    //~^ ERROR the trait bound
+    //~| ERROR cannot call non-const fn
 }
 
 fn main() {}

src/test/ui/rfc-2632-const-trait-impl/issue-88155.stderr

@@ -1,3 +1,9 @@
+error[E0277]: the trait bound `T: ~const A` is not satisfied
+  --> $DIR/issue-88155.rs:9:5
+   |
+LL |     T::assoc()
+   |     ^^^^^^^^^^ the trait `~const A` is not implemented for `T`
+
 error[E0015]: cannot call non-const fn `<T as A>::assoc` in constant functions
   --> $DIR/issue-88155.rs:9:5
    |
@@ -6,6 +12,7 @@ LL |     T::assoc()
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
-error: aborting due to previous error
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0015`.
+Some errors have detailed explanations: E0015, E0277.
+For more information about an error, try `rustc --explain E0015`.