trait_sel: skip nominal_obligations for Sized

`nominal_obligations` calls `predicates_of` on a `Sized` obligation,
effectively elaborating the trait and making the well-formedness checking
machinery do a bunch of extra work checking a `MetaSized` obligation is
well-formed, but given that both `Sized` and `MetaSized` are built-ins,
if `Sized` is otherwise well-formed, so `MetaSized` will be.

Author: davidtwco

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -567,6 +567,14 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
         def_id: DefId,
         args: GenericArgsRef<'tcx>,
     ) -> PredicateObligations<'tcx> {
+        // PERF: `Sized`'s predicates include `MetaSized`, but both are compiler implemented marker
+        // traits, so `MetaSized` will always be WF if `Sized` is WF and vice-versa. Determining
+        // the nominal obligations of `Sized` would in-effect just elaborate `MetaSized` and make
+        // the compiler do a bunch of work needlessly.
+        if self.tcx().is_lang_item(def_id, LangItem::Sized) {
+            return Default::default();
+        }
+
         let predicates = self.tcx().predicates_of(def_id);
         let mut origins = vec![def_id; predicates.predicates.len()];
         let mut head = predicates;

tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.current.stderr

@@ -1,27 +1,11 @@
-error[E0308]: mismatched types
-  --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:13:23
-   |
-LL |     (MyType<'a, T>,): Sized,
-   |                       ^^^^^ lifetime mismatch
-   |
-   = note: expected trait `<MyType<'a, T> as Sized>`
-              found trait `<MyType<'static, T> as Sized>`
-note: the lifetime `'a` as defined here...
-  --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:11:8
-   |
-LL | fn foo<'a, T: ?Sized>()
-   |        ^^
-   = note: ...does not necessarily outlive the static lifetime
-
 error: lifetime may not live long enough
-  --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:22:5
+  --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:20:5
    |
 LL | fn foo<'a, T: ?Sized>()
    |        -- lifetime `'a` defined here
 ...
 LL |     is_sized::<(MyType<'a, T>,)>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'a` must outlive `'static`
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.next.stderr

@@ -1,25 +1,11 @@
-error[E0478]: lifetime bound not satisfied
-  --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:13:23
-   |
-LL |     (MyType<'a, T>,): Sized,
-   |                       ^^^^^
-   |
-note: lifetime parameter instantiated with the lifetime `'a` as defined here
-  --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:11:8
-   |
-LL | fn foo<'a, T: ?Sized>()
-   |        ^^
-   = note: but lifetime parameter must outlive the static lifetime
-
 error: lifetime may not live long enough
-  --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:22:5
+  --> $DIR/lifetime-incomplete-prefer-sized-builtin-over-wc.rs:20:5
    |
 LL | fn foo<'a, T: ?Sized>()
    |        -- lifetime `'a` defined here
 ...
 LL |     is_sized::<(MyType<'a, T>,)>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ requires that `'a` must outlive `'static`
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0478`.

tests/ui/traits/lifetime-incomplete-prefer-sized-builtin-over-wc.rs

@@ -11,8 +11,6 @@ fn is_sized<T>() {}
 fn foo<'a, T: ?Sized>()
 where
     (MyType<'a, T>,): Sized,
-    //[current]~^ ERROR mismatched types
-    //[next]~^^ ERROR lifetime bound not satisfied
     MyType<'static, T>: Sized,
 {
     // Preferring the builtin `Sized` impl of tuples