Rollup merge of #139858 - oli-obk:new-const-traits-syntax, r=fee1-dead

New const traits syntax

This PR only affects the AST and doesn't actually change anything semantically.

All occurrences of `~const` outside of libcore have been replaced by `[const]`. Within libcore we have to wait for rustfmt to be bumped in the bootstrap compiler. This will happen "automatically" (when rustfmt is run) during the bootstrap bump, as rustfmt converts `~const` into `[const]`. After this we can remove the `~const` support from the parser

Caveat discovered during impl: there is no legacy bare trait object recovery for `[const] Trait` as that snippet in type position goes down the slice /array parsing code and will error

r? ``@fee1-dead``

cc ``@nikomatsakis`` ``@traviscross`` ``@compiler-errors``

compiler/rustc_hir_analysis/src/check/check.rs

@@ -344,7 +344,7 @@ fn check_opaque_meets_bounds<'tcx>(
     let misc_cause = ObligationCause::misc(span, def_id);
     // FIXME: We should just register the item bounds here, rather than equating.
     // FIXME(const_trait_impl): When we do that, please make sure to also register
-    // the `~const` bounds.
+    // the `[const]` bounds.
     match ocx.eq(&misc_cause, param_env, opaque_ty, hidden_ty) {
         Ok(()) => {}
         Err(ty_err) => {

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -264,9 +264,9 @@ fn compare_method_predicate_entailment<'tcx>(
     }
 
     // If we're within a const implementation, we need to make sure that the method
-    // does not assume stronger `~const` bounds than the trait definition.
+    // does not assume stronger `[const]` bounds than the trait definition.
     //
-    // This registers the `~const` bounds of the impl method, which we will prove
+    // This registers the `[const]` bounds of the impl method, which we will prove
     // using the hybrid param-env that we earlier augmented with the const conditions
     // from the impl header and trait method declaration.
     if is_conditionally_const {
@@ -2335,7 +2335,7 @@ pub(super) fn check_type_bounds<'tcx>(
     )
     .collect();
 
-    // Only in a const implementation do we need to check that the `~const` item bounds hold.
+    // Only in a const implementation do we need to check that the `[const]` item bounds hold.
     if tcx.is_conditionally_const(impl_ty_def_id) {
         obligations.extend(util::elaborate(
             tcx,

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1402,7 +1402,7 @@ fn check_impl<'tcx>(
                     }
                 }
 
-                // Ensure that the `~const` where clauses of the trait hold for the impl.
+                // Ensure that the `[const]` where clauses of the trait hold for the impl.
                 if tcx.is_conditionally_const(item.owner_id.def_id) {
                     for (bound, _) in
                         tcx.const_conditions(trait_ref.def_id).instantiate(tcx, trait_ref.args)