Auto merge of #96964 - oli-obk:const_trait_mvp, r=compiler-errors

Replace `#[default_method_body_is_const]` with `#[const_trait]`

pulled out of #96077

related issues:  #67792 and #92158

cc `@fee1-dead`

This is groundwork to only allowing `impl const Trait` for traits that are marked with `#[const_trait]`. This is necessary to prevent adding a new default method from becoming a breaking change (as it could be a non-const fn).

Author: bors

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -277,8 +277,8 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
             // sensitive check here.  But we can at least rule out functions that are not const
             // at all.
             if !ecx.tcx.is_const_fn_raw(def.did) {
-                // allow calling functions marked with #[default_method_body_is_const].
-                if !ecx.tcx.has_attr(def.did, sym::default_method_body_is_const) {
+                // allow calling functions inside a trait marked with #[const_trait].
+                if !ecx.tcx.is_const_default_method(def.did) {
                     // We certainly do *not* want to actually call the fn
                     // though, so be sure we return here.
                     throw_unsup_format!("calling non-const function `{}`", instance)

compiler/rustc_const_eval/src/lib.rs

@@ -9,6 +9,7 @@ Rust MIR: a lowered representation of Rust.
 #![feature(control_flow_enum)]
 #![feature(decl_macro)]
 #![feature(exact_size_is_empty)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]

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

@@ -711,8 +711,6 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     }
                 };
 
-                let mut nonconst_call_permission = false;
-
                 // Attempting to call a trait method?
                 if let Some(trait_id) = tcx.trait_of_item(callee) {
                     trace!("attempting to call a trait method");
@@ -774,13 +772,12 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                             }
                         }
                         _ if !tcx.is_const_fn_raw(callee) => {
-                            // At this point, it is only legal when the caller is marked with
-                            // #[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()
-                                && tcx.has_attr(caller.to_def_id(), sym::default_method_body_is_const)
-                                && callee_trait == tcx.trait_of_item(caller)
+                            // At this point, it is only legal when the caller is in a trait
+                            // marked with #[const_trait], and the callee is in the same trait.
+                            let mut nonconst_call_permission = false;
+                            if let Some(callee_trait) = tcx.trait_of_item(callee)
+                                && tcx.has_attr(callee_trait, sym::const_trait)
+                                && Some(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)
                             {
@@ -874,16 +871,10 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 let is_intrinsic = tcx.is_intrinsic(callee);
 
                 if !tcx.is_const_fn_raw(callee) {
-                    if tcx.trait_of_item(callee).is_some() {
-                        if tcx.has_attr(callee, sym::default_method_body_is_const) {
-                            // To get to here we must have already found a const impl for the
-                            // trait, but for it to still be non-const can be that the impl is
-                            // using default method bodies.
-                            nonconst_call_permission = true;
-                        }
-                    }
-
-                    if !nonconst_call_permission {
+                    if !tcx.is_const_default_method(callee) {
+                        // To get to here we must have already found a const impl for the
+                        // trait, but for it to still be non-const can be that the impl is
+                        // using default method bodies.
                         self.check_op(ops::FnCallNonConst {
                             caller,
                             callee,

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

@@ -9,7 +9,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::mir;
 use rustc_middle::ty::{self, TyCtxt};
-use rustc_span::{sym, Symbol};
+use rustc_span::Symbol;
 
 pub use self::qualifs::Qualif;
 
@@ -84,10 +84,10 @@ pub fn rustc_allow_const_fn_unstable(
 // functions are subject to more stringent restrictions than "const-unstable" functions: They
 // cannot use unstable features and can only call other "const-stable" functions.
 pub fn is_const_stable_const_fn(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
-    // A default body marked const is not const-stable because const
+    // A default body in a `#[const_trait]` is not const-stable because const
     // trait fns currently cannot be const-stable. We shouldn't
     // restrict default bodies to only call const-stable functions.
-    if tcx.has_attr(def_id, sym::default_method_body_is_const) {
+    if tcx.is_const_default_method(def_id) {
         return false;
     }
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -473,9 +473,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     // RFC 2632
     gated!(
-        default_method_body_is_const, Normal, template!(Word), WarnFollowing, const_trait_impl,
-        "`default_method_body_is_const` is a temporary placeholder for declaring default bodies \
-        as `const`, which may be removed or renamed in the future."
+        const_trait, Normal, template!(Word), WarnFollowing, const_trait_impl,
+        "`const` is a temporary placeholder for marking a trait that is suitable for `const` \
+        `impls` and all default bodies as `const`, which may be removed or renamed in the \
+        future."
     ),
     // lang-team MCP 147
     gated!(

compiler/rustc_hir/src/def.rs

@@ -92,6 +92,7 @@ pub enum DefKind {
     /// [RFC 2593]: https://github.com/rust-lang/rfcs/pull/2593
     Ctor(CtorOf, CtorKind),
     /// Associated function: `impl MyStruct { fn associated() {} }`
+    /// or `trait Foo { fn associated() {} }`
     AssocFn,
     /// Associated constant: `trait MyTrait { const ASSOC: usize; }`
     AssocConst,

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -893,9 +893,9 @@ fn should_encode_mir(tcx: TyCtxt<'_>, def_id: LocalDefId) -> (bool, bool) {
             let needs_inline = (generics.requires_monomorphization(tcx)
                 || tcx.codegen_fn_attrs(def_id).requests_inline())
                 && tcx.sess.opts.output_types.should_codegen();
-            // The function has a `const` modifier or is annotated with `default_method_body_is_const`.
+            // The function has a `const` modifier or is in a `#[const_trait]`.
             let is_const_fn = tcx.is_const_fn_raw(def_id.to_def_id())
-                || tcx.has_attr(def_id.to_def_id(), sym::default_method_body_is_const);
+                || tcx.is_const_default_method(def_id.to_def_id());
             let always_encode_mir = tcx.sess.opts.debugging_opts.always_encode_mir;
             (is_const_fn, needs_inline || always_encode_mir)
         }

compiler/rustc_middle/src/hir/map/mod.rs

@@ -494,9 +494,7 @@ impl<'hir> Map<'hir> {
             BodyOwnerKind::Fn if self.tcx.is_const_fn_raw(def_id.to_def_id()) => {
                 ConstContext::ConstFn
             }
-            BodyOwnerKind::Fn
-                if self.tcx.has_attr(def_id.to_def_id(), sym::default_method_body_is_const) =>
-            {
+            BodyOwnerKind::Fn if self.tcx.is_const_default_method(def_id.to_def_id()) => {
                 ConstContext::ConstFn
             }
             BodyOwnerKind::Fn | BodyOwnerKind::Closure => return None,

compiler/rustc_middle/src/ty/mod.rs

@@ -2303,6 +2303,11 @@ impl<'tcx> TyCtxt<'tcx> {
         matches!(self.def_kind(def_id), DefKind::Fn | DefKind::AssocFn | DefKind::Ctor(..))
             && self.impl_constness(def_id) == hir::Constness::Const
     }
+
+    #[inline]
+    pub fn is_const_default_method(self, def_id: DefId) -> bool {
+        matches!(self.trait_of_item(def_id), Some(trait_id) if self.has_attr(trait_id, sym::const_trait))
+    }
 }
 
 /// Yields the parent function's `LocalDefId` if `def_id` is an `impl Trait` definition.

compiler/rustc_passes/src/check_attr.rs

@@ -122,9 +122,7 @@ impl CheckAttrVisitor<'_> {
                 | sym::rustc_if_this_changed
                 | sym::rustc_then_this_would_need => self.check_rustc_dirty_clean(&attr),
                 sym::cmse_nonsecure_entry => self.check_cmse_nonsecure_entry(attr, span, target),
-                sym::default_method_body_is_const => {
-                    self.check_default_method_body_is_const(attr, span, target)
-                }
+                sym::const_trait => self.check_const_trait(attr, span, target),
                 sym::must_not_suspend => self.check_must_not_suspend(&attr, span, target),
                 sym::must_use => self.check_must_use(hir_id, &attr, span, target),
                 sym::rustc_pass_by_value => self.check_pass_by_value(&attr, span, target),
@@ -2097,23 +2095,14 @@ impl CheckAttrVisitor<'_> {
         }
     }
 
-    /// default_method_body_is_const should only be applied to trait methods with default bodies.
-    fn check_default_method_body_is_const(
-        &self,
-        attr: &Attribute,
-        span: Span,
-        target: Target,
-    ) -> bool {
+    /// `#[const_trait]` only applies to traits.
+    fn check_const_trait(&self, attr: &Attribute, _span: Span, target: Target) -> bool {
         match target {
-            Target::Method(MethodKind::Trait { body: true }) => true,
+            Target::Trait => true,
             _ => {
                 self.tcx
                     .sess
-                    .struct_span_err(
-                        attr.span,
-                        "attribute should be applied to a trait method with body",
-                    )
-                    .span_label(span, "not a trait method or missing a body")
+                    .struct_span_err(attr.span, "attribute should be applied to a trait")
                     .emit();
                 false
             }
@@ -2207,6 +2196,8 @@ impl CheckAttrVisitor<'_> {
                 "attribute `{}` without any lints has no effect",
                 attr.name_or_empty()
             )
+        } else if attr.name_or_empty() == sym::default_method_body_is_const {
+            format!("`default_method_body_is_const` has been replaced with `#[const_trait]` on traits")
         } else {
             return;
         };