Make duplicate lang items fatal

compiler/rustc_passes/src/lang_items.rs

@@ -149,7 +149,9 @@ impl<'ast, 'tcx> LanguageItemCollector<'ast, 'tcx> {
                 }
             };
 
-            self.tcx.dcx().emit_err(DuplicateLangItem {
+            // When there's a duplicate lang item, something went very wrong and there's no value in recovering or doing anything.
+            // Give the user the one message to let them debug the mess they created and then wish them farewell.
+            self.tcx.dcx().emit_fatal(DuplicateLangItem {
                 local_span: item_span,
                 lang_item_name,
                 crate_name,

tests/ui/lang-items/duplicate.rs

@@ -0,0 +1,10 @@
+// normalize-stderr-test "loaded from .*libcore-.*.rlib" -> "loaded from SYSROOT/libcore-*.rlib"
+#![feature(lang_items)]
+
+#[lang = "sized"]
+trait Sized {}
+//~^ ERROR: duplicate lang item
+
+#[lang = "tuple_trait"]
+pub trait Tuple {}
+// no error

tests/ui/lang-items/duplicate.stderr

@@ -0,0 +1,13 @@
+error[E0152]: found duplicate lang item `sized`
+  --> $DIR/duplicate.rs:5:1
+   |
+LL | trait Sized {}
+   | ^^^^^^^^^^^^^^
+   |
+   = note: the lang item is first defined in crate `core` (which `std` depends on)
+   = note: first definition in `core` loaded from SYSROOT/libcore-*.rlib
+   = note: second definition in the local crate (`duplicate`)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0152`.