Effects: don't print host param in diagnostics

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -17,6 +17,7 @@ use rustc_hir::LangItem;
 use rustc_session::config::TrimmedDefPaths;
 use rustc_session::cstore::{ExternCrate, ExternCrateSource};
 use rustc_session::Limit;
+use rustc_span::sym;
 use rustc_span::symbol::{kw, Ident, Symbol};
 use rustc_span::FileNameDisplayPreference;
 use rustc_target::abi::Size;
@@ -2017,11 +2018,37 @@ impl<'tcx> Printer<'tcx> for FmtPrinter<'_, 'tcx> {
     ) -> Result<Self::Path, Self::Error> {
         self = print_prefix(self)?;
 
-        if args.first().is_some() {
+        let tcx = self.tcx;
+
+        let args = args.iter().copied();
+
+        let args: Vec<_> = if !tcx.sess.verbose() {
+            // skip host param as those are printed as `~const`
+            args.filter(|arg| match arg.unpack() {
+                // FIXME(effects) there should be a better way than just matching the name
+                GenericArgKind::Const(c)
+                    if tcx.features().effects
+                        && matches!(
+                            c.kind(),
+                            ty::ConstKind::Param(ty::ParamConst { name: sym::host, .. })
+                        ) =>
+                {
+                    false
+                }
+                _ => true,
+            })
+            .collect()
+        } else {
+            // If -Zverbose is passed, we should print the host parameter instead
+            // of eating it.
+            args.collect()
+        };
+
+        if !args.is_empty() {
             if self.in_value {
                 write!(self, "::")?;
             }
-            self.generic_delimiters(|cx| cx.comma_sep(args.iter().cloned()))
+            self.generic_delimiters(|cx| cx.comma_sep(args.into_iter()))
         } else {
             Ok(self)
         }

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

@@ -1,13 +1,12 @@
-// known-bug: #110395
-#![feature(const_trait_impl)]
+#![feature(const_trait_impl, effects)]
 
 #[const_trait]
 pub trait Tr {
     fn a(&self) {}
 
     fn b(&self) {
         ().a()
-        //FIXME ~^ ERROR the trait bound
+        //~^ ERROR the trait bound
     }
 }
 

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

@@ -1,11 +1,11 @@
-error[E0015]: cannot call non-const fn `<() as Tr>::a` in constant functions
-  --> $DIR/default-method-body-is-const-same-trait-ck.rs:9:12
+error[E0277]: the trait bound `(): ~const Tr` is not satisfied
+  --> $DIR/default-method-body-is-const-same-trait-ck.rs:8:12
    |
 LL |         ().a()
-   |            ^^^
+   |            ^ the trait `~const Tr` is not implemented for `()`
    |
-   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+   = help: the trait `Tr` is implemented for `()`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0015`.
+For more information about this error, try `rustc --explain E0277`.