Fix types being marked as dead when they are inferred generic arguments

Signed-off-by: Jonathan Brouwer <jonathantbrouwer@gmail.com>

compiler/rustc_passes/src/dead.rs

@@ -125,9 +125,13 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
             ) => {
                 self.check_def_id(def_id);
             }
-            _ if self.in_pat => {}
             Res::PrimTy(..) | Res::SelfCtor(..) | Res::Local(..) => {}
             Res::Def(DefKind::Ctor(CtorOf::Variant, ..), ctor_def_id) => {
+                // Using a variant in patterns should not make the variant live,
+                // since we can just remove the match arm that matches the pattern
+                if self.in_pat {
+                    return;
+                }
                 let variant_id = self.tcx.parent(ctor_def_id);
                 let enum_id = self.tcx.parent(variant_id);
                 self.check_def_id(enum_id);
@@ -136,6 +140,11 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
                 }
             }
             Res::Def(DefKind::Variant, variant_id) => {
+                // Using a variant in patterns should not make the variant live,
+                // since we can just remove the match arm that matches the pattern
+                if self.in_pat {
+                    return;
+                }
                 let enum_id = self.tcx.parent(variant_id);
                 self.check_def_id(enum_id);
                 if !self.ignore_variant_stack.contains(&variant_id) {

tests/ui/lint/dead-code/inferred-generic-arg.rs

@@ -0,0 +1,16 @@
+//@ check-pass
+
+#![deny(dead_code)]
+
+#[derive(Default)]
+struct Test {
+
+}
+
+fn main() {
+    if let Some::<Test>(test) = magic::<Test>() { }
+}
+
+fn magic<T: Default>() -> Option<T> {
+    Some(T::default())
+}