Only use ... instead of _ for type elision

`_` might confuse people into believing that the type isn't known,
while `...` is not used anywhere else for types and is not valid
syntax, making it more likely to convey the right understanding.

Author: estebank

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

@@ -1765,11 +1765,7 @@ impl<'tcx> Printer<'tcx> for FmtPrinter<'_, 'tcx> {
             self.pretty_print_type(ty)
         } else {
             self.truncated = true;
-            if let ty::Adt(_, substs) = ty.kind() && substs.len() > 0 {
-                write!(self, "...")?;
-            } else {
-                write!(self, "_")?;
-            }
+            write!(self, "...")?;
             Ok(self)
         }
     }

src/test/ui/issues/issue-37311-type-length-limit/issue-37311.stderr

@@ -1,4 +1,4 @@
-error: reached the recursion limit while instantiating `<(&(&(_, _), _), _) as Foo>::recurse`
+error: reached the recursion limit while instantiating `<(&(&(..., ...), ...), ...) as Foo>::recurse`
   --> $DIR/issue-37311.rs:17:9
    |
 LL |         (self, self).recurse();

src/test/ui/issues/issue-67552.stderr

@@ -1,4 +1,4 @@
-error: reached the recursion limit while instantiating `rec::<&mut &mut &mut &mut &mut _>`
+error: reached the recursion limit while instantiating `rec::<&mut &mut &mut &mut &mut ...>`
   --> $DIR/issue-67552.rs:29:9
    |
 LL |         rec(identity(&mut it))

src/test/ui/recursion/issue-83150.stderr

@@ -14,7 +14,7 @@ error[E0275]: overflow evaluating the requirement `<std::ops::Range<u8> as Itera
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`issue_83150`)
    = note: required for `Map<&mut std::ops::Range<u8>, [closure@$DIR/issue-83150.rs:12:24: 12:27]>` to implement `Iterator`
    = note: 64 redundant requirements hidden
-   = note: required for `&mut Map<&mut Map<&mut Map<_, _>, _>, _>` to implement `Iterator`
+   = note: required for `&mut Map<&mut Map<&mut Map<..., ...>, ...>, ...>` to implement `Iterator`
    = note: the full type name has been written to '$TEST_BUILD_DIR/recursion/issue-83150/issue-83150.long-type-hash.txt'
 
 error: aborting due to previous error; 1 warning emitted

src/test/ui/traits/issue-91949-hangs-on-recursion.stderr

@@ -17,7 +17,7 @@ error[E0275]: overflow evaluating the requirement `(): Sized`
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "512"]` attribute to your crate (`issue_91949_hangs_on_recursion`)
    = note: required for `std::iter::Empty<()>` to implement `Iterator`
    = note: 171 redundant requirements hidden
-   = note: required for `IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<..., _>>, _>>` to implement `Iterator`
+   = note: required for `IteratorOfWrapped<(), Map<IteratorOfWrapped<(), Map<..., ...>>, ...>>` to implement `Iterator`
    = note: the full type name has been written to '$TEST_BUILD_DIR/traits/issue-91949-hangs-on-recursion/issue-91949-hangs-on-recursion.long-type-hash.txt'
 
 error: aborting due to previous error; 1 warning emitted

src/test/ui/type_length_limit.stderr

@@ -1,4 +1,4 @@
-error: reached the type-length limit while instantiating `std::mem::drop::<Option<((((_, _, _), _, _), _, _), _, _)>>`
+error: reached the type-length limit while instantiating `std::mem::drop::<Option<((((..., ..., ...), ..., ...), ..., ...), ..., ...)>>`
   --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
    |
 LL | pub fn drop<T>(_x: T) {}