Account for #![feature(const_trait_impl)] state

Author: estebank

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -393,7 +393,13 @@ fn build_error_for_const_call<'tcx>(
                             `{trait_name}` is not const",
                         ),
                     );
-                    if parent.is_local() {
+                    if parent.is_local() && ccx.tcx.sess.is_nightly_build() {
+                        if !ccx.tcx.features().const_trait_impl() {
+                            err.help(
+                                "add `#![feature(const_trait_impl)]` to the crate attributes to \
+                                 enable `#[const_trait]`",
+                            );
+                        }
                         let indentation = ccx
                             .tcx
                             .sess
@@ -406,6 +412,8 @@ fn build_error_for_const_call<'tcx>(
                             format!("#[const_trait]\n{indentation}"),
                             Applicability::MachineApplicable,
                         );
+                    } else if !ccx.tcx.sess.is_nightly_build() {
+                        err.help("const traits are not yet supported on stable Rust");
                     }
                 }
             } else if ccx.tcx.constness(callee) != hir::Constness::Const {

tests/run-make/const-trait-stable-toolchain/const-super-trait-nightly-disabled.stderr

@@ -55,10 +55,24 @@ LL | #[const_trait] trait Bar: ~const Foo {}
 error[E0015]: cannot call non-const method `<T as Foo>::a` in constant functions
   --> const-super-trait.rs:10:7
    |
+LL | const fn foo<T: ~const Bar>(x: &T) {
+   | ---------------------------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants
 LL |     x.a();
    |       ^^^
    |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+note: method `a` is not const because trait `Foo` is not const
+  --> const-super-trait.rs:3:1
+   |
+LL | trait Foo {
+   | ^^^^^^^^^ this trait is not const
+LL |     fn a(&self);
+   |     ------------ this method is not const
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable `#[const_trait]`
+help: consider making trait `Foo` const
+   |
+LL + #[const_trait]
+LL | trait Foo {
+   |
 
 error: aborting due to 6 previous errors
 

tests/run-make/const-trait-stable-toolchain/const-super-trait-nightly-enabled.stderr

@@ -35,10 +35,23 @@ LL | #[const_trait] trait Bar: ~const Foo {}
 error[E0015]: cannot call non-const method `<T as Foo>::a` in constant functions
   --> const-super-trait.rs:10:7
    |
+LL | const fn foo<T: ~const Bar>(x: &T) {
+   | ---------------------------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants
 LL |     x.a();
    |       ^^^
    |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+note: method `a` is not const because trait `Foo` is not const
+  --> const-super-trait.rs:3:1
+   |
+LL | trait Foo {
+   | ^^^^^^^^^ this trait is not const
+LL |     fn a(&self);
+   |     ------------ this method is not const
+help: consider making trait `Foo` const
+   |
+LL + #[const_trait]
+LL | trait Foo {
+   |
 
 error: aborting due to 4 previous errors
 

tests/run-make/const-trait-stable-toolchain/const-super-trait-stable-disabled.stderr

@@ -53,10 +53,19 @@ note: `Bar` can't be used with `[const]` because it isn't `const`
 error[E0015]: cannot call non-const method `<T as Foo>::a` in constant functions
   --> const-super-trait.rs:10:7
    |
+9  | const fn foo<T: ~const Bar>(x: &T) {
+   | ---------------------------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants
 10 |     x.a();
    |       ^^^
    |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+note: method `a` is not const because trait `Foo` is not const
+  --> const-super-trait.rs:3:1
+   |
+3  | trait Foo {
+   | ^^^^^^^^^ this trait is not const
+4  |     fn a(&self);
+   |     ------------ this method is not const
+   = help: const traits are not yet supported on stable Rust
 
 error: aborting due to 6 previous errors
 

tests/run-make/const-trait-stable-toolchain/const-super-trait-stable-enabled.stderr

@@ -43,10 +43,19 @@ note: `Bar` can't be used with `[const]` because it isn't `const`
 error[E0015]: cannot call non-const method `<T as Foo>::a` in constant functions
   --> const-super-trait.rs:10:7
    |
+9  | const fn foo<T: ~const Bar>(x: &T) {
+   | ---------------------------------- calls in constant functions are limited to constant functions, tuple structs and tuple variants
 10 |     x.a();
    |       ^^^
    |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+note: method `a` is not const because trait `Foo` is not const
+  --> const-super-trait.rs:3:1
+   |
+3  | trait Foo {
+   | ^^^^^^^^^ this trait is not const
+4  |     fn a(&self);
+   |     ------------ this method is not const
+   = help: const traits are not yet supported on stable Rust
 
 error: aborting due to 5 previous errors
 

tests/ui/resolve/issue-39559-2.stderr

@@ -11,6 +11,7 @@ LL | trait Dim {
    | ^^^^^^^^^ this trait is not const
 LL |     fn dim() -> usize;
    |     ------------------ this associated function is not const
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable `#[const_trait]`
 help: consider making trait `Dim` const
    |
 LL + #[const_trait]
@@ -30,6 +31,7 @@ LL | trait Dim {
    | ^^^^^^^^^ this trait is not const
 LL |     fn dim() -> usize;
    |     ------------------ this associated function is not const
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable `#[const_trait]`
 help: consider making trait `Dim` const
    |
 LL + #[const_trait]

tests/ui/traits/const-traits/super-traits-fail-3.nnn.stderr

@@ -104,6 +104,7 @@ LL | trait Foo {
    | ^^^^^^^^^ this trait is not const
 LL |     fn a(&self);
    |     ------------ this method is not const
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable `#[const_trait]`
 help: consider making trait `Foo` const
    |
 LL + #[const_trait]

tests/ui/traits/const-traits/super-traits-fail-3.nny.stderr

@@ -104,6 +104,7 @@ LL | trait Foo {
    | ^^^^^^^^^ this trait is not const
 LL |     fn a(&self);
    |     ------------ this method is not const
+   = help: add `#![feature(const_trait_impl)]` to the crate attributes to enable `#[const_trait]`
 help: consider making trait `Foo` const
    |
 LL + #[const_trait]