allow special behavior when printing const infer

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -497,16 +497,32 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         let ty_to_string = |ty: Ty<'tcx>| -> String {
             let mut s = String::new();
             let mut printer = ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::TypeNS);
-            let mut inner = self.inner.borrow_mut();
-            let ty_vars = inner.type_variables();
-            let getter = move |ty_vid| {
-                let var_origin = ty_vars.var_origin(ty_vid);
-                if let TypeVariableOriginKind::TypeParameterDefinition(name, _) = var_origin.kind {
-                    return Some(name.to_string());
+            let ty_getter = move |ty_vid| {
+                if let TypeVariableOriginKind::TypeParameterDefinition(name, _) =
+                    self.inner.borrow_mut().type_variables().var_origin(ty_vid).kind
+                {
+                    Some(name.to_string())
+                } else {
+                    None
                 }
-                None
             };
-            printer.name_resolver = Some(Box::new(&getter));
+            printer.ty_infer_name_resolver = Some(Box::new(ty_getter));
+            let const_getter = move |ct_vid| {
+                if let ConstVariableOriginKind::ConstParameterDefinition(name, _) = self
+                    .inner
+                    .borrow_mut()
+                    .const_unification_table()
+                    .probe_value(ct_vid)
+                    .origin
+                    .kind
+                {
+                    return Some(name.to_string());
+                } else {
+                    None
+                }
+            };
+            printer.const_infer_name_resolver = Some(Box::new(const_getter));
+
             let _ = if let ty::FnDef(..) = ty.kind() {
                 // We don't want the regular output for `fn`s because it includes its path in
                 // invalid pseudo-syntax, we want the `fn`-pointer output instead.

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

@@ -606,7 +606,7 @@ pub trait PrettyPrinter<'tcx>:
             ty::Infer(infer_ty) => {
                 let verbose = self.tcx().sess.verbose();
                 if let ty::TyVar(ty_vid) = infer_ty {
-                    if let Some(name) = self.infer_ty_name(ty_vid) {
+                    if let Some(name) = self.ty_infer_name(ty_vid) {
                         p!(write("{}", name))
                     } else {
                         if verbose {
@@ -1015,7 +1015,11 @@ pub trait PrettyPrinter<'tcx>:
         }
     }
 
-    fn infer_ty_name(&self, _: ty::TyVid) -> Option<String> {
+    fn ty_infer_name(&self, _: ty::TyVid) -> Option<String> {
+        None
+    }
+
+    fn const_infer_name(&self, _: ty::ConstVid<'tcx>) -> Option<String> {
         None
     }
 
@@ -1203,7 +1207,14 @@ pub trait PrettyPrinter<'tcx>:
                     }
                 }
             }
-            ty::ConstKind::Infer(..) => print_underscore!(),
+            ty::ConstKind::Infer(infer_ct) => {
+                match infer_ct {
+                    ty::InferConst::Var(ct_vid)
+                        if let Some(name) = self.const_infer_name(ct_vid) =>
+                            p!(write("{}", name)),
+                    _ => print_underscore!(),
+                }
+            }
             ty::ConstKind::Param(ParamConst { name, .. }) => p!(write("{}", name)),
             ty::ConstKind::Value(value) => {
                 return self.pretty_print_const_value(value, ct.ty(), print_ty);
@@ -1551,7 +1562,8 @@ pub struct FmtPrinterData<'a, 'tcx, F> {
 
     pub region_highlight_mode: RegionHighlightMode<'tcx>,
 
-    pub name_resolver: Option<Box<&'a dyn Fn(ty::TyVid) -> Option<String>>>,
+    pub ty_infer_name_resolver: Option<Box<dyn Fn(ty::TyVid) -> Option<String> + 'a>>,
+    pub const_infer_name_resolver: Option<Box<dyn Fn(ty::ConstVid<'tcx>) -> Option<String> + 'a>>,
 }
 
 impl<'a, 'tcx, F> Deref for FmtPrinter<'a, 'tcx, F> {
@@ -1580,7 +1592,8 @@ impl<'a, 'tcx, F> FmtPrinter<'a, 'tcx, F> {
             binder_depth: 0,
             printed_type_count: 0,
             region_highlight_mode: RegionHighlightMode::new(tcx),
-            name_resolver: None,
+            ty_infer_name_resolver: None,
+            const_infer_name_resolver: None,
         }))
     }
 }
@@ -1835,8 +1848,12 @@ impl<'tcx, F: fmt::Write> Printer<'tcx> for FmtPrinter<'_, 'tcx, F> {
 }
 
 impl<'tcx, F: fmt::Write> PrettyPrinter<'tcx> for FmtPrinter<'_, 'tcx, F> {
-    fn infer_ty_name(&self, id: ty::TyVid) -> Option<String> {
-        self.0.name_resolver.as_ref().and_then(|func| func(id))
+    fn ty_infer_name(&self, id: ty::TyVid) -> Option<String> {
+        self.0.ty_infer_name_resolver.as_ref().and_then(|func| func(id))
+    }
+
+    fn const_infer_name(&self, id: ty::ConstVid<'tcx>) -> Option<String> {
+        self.0.const_infer_name_resolver.as_ref().and_then(|func| func(id))
     }
 
     fn print_value_path(

src/test/ui/const-generics/defaults/doesnt_infer.rs

@@ -9,5 +9,5 @@ impl<const N: u32> Foo<N> {
 fn main() {
     let foo = Foo::<1>::foo();
     let foo = Foo::foo();
-    //~^ error: type annotations needed for `Foo<{_: u32}>`
+    //~^ error: type annotations needed for `Foo<N>`
 }

src/test/ui/const-generics/defaults/doesnt_infer.stderr

@@ -1,4 +1,4 @@
-error[E0282]: type annotations needed for `Foo<{_: u32}>`
+error[E0282]: type annotations needed for `Foo<N>`
   --> $DIR/doesnt_infer.rs:11:15
    |
 LL |     let foo = Foo::foo();

src/test/ui/const-generics/generic_arg_infer/issue-91614.rs

@@ -4,5 +4,5 @@ use std::simd::Mask;
 
 fn main() {
     let y = Mask::<_, _>::splat(false);
-    //~^ error: type annotations needed for `Mask<_, {_: usize}>`
+    //~^ ERROR: type annotations needed for
 }

src/test/ui/const-generics/generic_arg_infer/issue-91614.stderr

@@ -1,4 +1,4 @@
-error[E0283]: type annotations needed for `Mask<_, {_: usize}>`
+error[E0283]: type annotations needed for `Mask<_, LANES>`
   --> $DIR/issue-91614.rs:6:13
    |
 LL |     let y = Mask::<_, _>::splat(false);