constify Iterator, take IV

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -774,8 +774,8 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
 
                 // Attempting to call a trait method?
                 if let Some(trait_did) = tcx.trait_of_assoc(callee) {
-                    // We can't determine the actual callee here, so we have to do different checks
-                    // than usual.
+                    // We can't determine the actual callee (the underlying impl of the trait) here, so we have
+                    // to do different checks than usual.
 
                     trace!("attempting to call a trait method");
                     let is_const = tcx.constness(callee) == hir::Constness::Const;

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1333,7 +1333,7 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_non_const_trait_method, AttributeType::Normal, template!(Word),
         ErrorFollowing, EncodeCrossCrate::No,
         "`#[rustc_non_const_trait_method]` should only used by the standard library to mark trait methods \
-        as non-const to allow large traits to easier transition to const"
+        as non-const to allow large traits an easier transition to const"
     ),
 
     BuiltinAttribute {

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -218,7 +218,7 @@ fn compare_method_predicate_entailment<'tcx>(
         trait_m_predicates.instantiate_own(tcx, trait_to_impl_args).map(|(predicate, _)| predicate),
     );
 
-    let is_conditionally_const = tcx.is_conditionally_const(impl_def_id);
+    let is_conditionally_const = tcx.is_conditionally_const(impl_m.def_id);
     if is_conditionally_const {
         // Augment the hybrid param-env with the const conditions
         // of the impl header and the trait method.

compiler/rustc_middle/src/ty/mod.rs

@@ -2181,7 +2181,9 @@ impl<'tcx> TyCtxt<'tcx> {
                         self.constness(def_id) == hir::Constness::Const
                     }
                     DefKind::Impl { of_trait: true } => {
-                        let Some(trait_method_did) = self.trait_item_of(def_id) else { return false; };
+                        let Some(trait_method_did) = self.trait_item_of(def_id) else {
+                            return false;
+                        };
                         self.constness(trait_method_did) == hir::Constness::Const
                             && self.is_conditionally_const(parent_def_id)
                     }

src/doc/rustc-dev-guide/src/SUMMARY.md

@@ -198,7 +198,7 @@
     - [Inference details](./opaque-types-impl-trait-inference.md)
     - [Return Position Impl Trait In Trait](./return-position-impl-trait-in-trait.md)
     - [Region inference restrictions][opaque-infer]
-- [Const condition checking](./effects.md)
+- [Const traits and const condition checking](./effects.md)
 - [Pattern and exhaustiveness checking](./pat-exhaustive-checking.md)
 - [Unsafety checking](./unsafety-checking.md)
 - [MIR dataflow](./mir/dataflow.md)

src/doc/rustc-dev-guide/src/effects.md

@@ -1,4 +1,4 @@
-# Effects and const condition checking
+# Effects, const traits, and const condition checking
 
 ## The `HostEffect` predicate
 
@@ -154,3 +154,18 @@ be dropped at compile time.
 
 [old solver]: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_trait_selection/traits/effects.rs.html
 [new trait solver]: https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_next_trait_solver/solve/effect_goals.rs.html
+
+## More on const traits
+
+To be expanded later.
+
+### The `#[rustc_non_const_trait_method]` attribute
+
+This is intended for internal (standard library) usage only. With this attribute
+applied to a trait method, the compiler will not check the default body of this
+method for ability to run in compile time. Users of the trait will also not be
+allowed to use this trait method in const contexts. This attribute is primarily
+used for constifying large traits such as `Iterator` without having to make all
+its methods `const` at the same time.
+
+This attribute should not be present while stabilizing the trait as `const`.

tests/ui/traits/const-traits/partial/attr-gate.stderr

@@ -6,7 +6,7 @@ LL |     #[rustc_non_const_trait_method]
    |
    = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
    = note: the `#[rustc_non_const_trait_method]` attribute is an internal implementation detail that will never be stable
-   = note: `#[rustc_non_const_trait_method]` should only used by the standard library to mark trait methods as non-const to allow large traits to easier transition to const
+   = note: `#[rustc_non_const_trait_method]` should only used by the standard library to mark trait methods as non-const to allow large traits an easier transition to const
 
 error: aborting due to 1 previous error
 

tests/ui/traits/const-traits/partial/no-const-callers.rs

@@ -10,13 +10,22 @@ impl const A for () {
     fn a() {}
 }
 
+impl const A for u8 {
+    fn a() {}
+    fn b() { println!("hello"); }
+    //~^ ERROR: cannot call non-const function
+}
+
 const fn foo<T: [const] A>() {
     T::a();
     T::b();
     //~^ ERROR: cannot call non-const associated function
     <()>::a();
     <()>::b();
     //~^ ERROR: cannot call non-const associated function
+    u8::a();
+    u8::b();
+    //~^ ERROR: cannot call non-const associated function
 }
 
 fn main() {}

tests/ui/traits/const-traits/partial/no-const-callers.stderr

@@ -1,19 +1,38 @@
+error[E0015]: cannot call non-const function `std::io::_print` in constant functions
+  --> $DIR/no-const-callers.rs:15:14
+   |
+LL |     fn b() { println!("hello"); }
+   |              ^^^^^^^^^^^^^^^^^
+   |
+note: function `_print` is not const
+  --> $SRC_DIR/std/src/io/stdio.rs:LL:COL
+   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+   = note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
+
 error[E0015]: cannot call non-const associated function `<T as A>::b` in constant functions
-  --> $DIR/no-const-callers.rs:15:5
+  --> $DIR/no-const-callers.rs:21:5
    |
 LL |     T::b();
    |     ^^^^^^
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
 error[E0015]: cannot call non-const associated function `<() as A>::b` in constant functions
-  --> $DIR/no-const-callers.rs:18:5
+  --> $DIR/no-const-callers.rs:24:5
    |
 LL |     <()>::b();
    |     ^^^^^^^^^
    |
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
-error: aborting due to 2 previous errors
+error[E0015]: cannot call non-const associated function `<u8 as A>::b` in constant functions
+  --> $DIR/no-const-callers.rs:27:5
+   |
+LL |     u8::b();
+   |     ^^^^^^^
+   |
+   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0015`.