From 81fa794af9c515eb07a1b6956dc048473770b8ae Mon Sep 17 00:00:00 2001
From: varkor <github@varkor.com>
Date: Tue, 28 May 2019 20:12:48 +0100
Subject: [PATCH] Specify tuple element in lint message

---
 src/librustc_lint/unused.rs            |  9 ++++++---
 src/test/ui/lint/must_use-tuple.rs     | 12 ++++++------
 src/test/ui/lint/must_use-tuple.stderr |  8 ++++----
 3 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index bbec42b238f..d540b3f7e40 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -48,7 +48,7 @@ fn check_stmt(&mut self, cx: &LateContext<'_, '_>, s: &hir::Stmt) {
         }
 
         let ty = cx.tables.expr_ty(&expr);
-        let type_permits_lack_of_use = check_must_use_ty(cx, ty, &expr, s.span);
+        let type_permits_lack_of_use = check_must_use_ty(cx, ty, &expr, s.span, "");
 
         let mut fn_warned = false;
         let mut op_warned = false;
@@ -133,6 +133,7 @@ fn check_must_use_ty<'tcx>(
             ty: Ty<'tcx>,
             expr: &hir::Expr,
             span: Span,
+            descr_post_path: &str,
         ) -> bool {
             if ty.is_unit() || cx.tcx.is_ty_uninhabited_from(
                 cx.tcx.hir().get_module_parent_by_hir_id(expr.hir_id), ty)
@@ -141,7 +142,7 @@ fn check_must_use_ty<'tcx>(
             }
 
             match ty.sty {
-                ty::Adt(def, _) => check_must_use_def(cx, def.did, span, "", ""),
+                ty::Adt(def, _) => check_must_use_def(cx, def.did, span, "", descr_post_path),
                 ty::Opaque(def, _) => {
                     let mut has_emitted = false;
                     for (predicate, _) in &cx.tcx.predicates_of(def).predicates {
@@ -178,7 +179,9 @@ fn check_must_use_ty<'tcx>(
                         vec![]
                     };
                     for (i, ty) in tys.iter().map(|k| k.expect_ty()).enumerate() {
-                        if check_must_use_ty(cx, ty, expr, *spans.get(i).unwrap_or(&span)) {
+                        let descr_post_path = &format!(" in tuple element {}", i);
+                        let span = *spans.get(i).unwrap_or(&span);
+                        if check_must_use_ty(cx, ty, expr, span, descr_post_path) {
                             has_emitted = true;
                         }
                     }
diff --git a/src/test/ui/lint/must_use-tuple.rs b/src/test/ui/lint/must_use-tuple.rs
index 3091dbbdf98..efbe0ff9c6c 100644
--- a/src/test/ui/lint/must_use-tuple.rs
+++ b/src/test/ui/lint/must_use-tuple.rs
@@ -1,15 +1,15 @@
 #![deny(unused_must_use)]
 
-fn foo() -> Result<(), ()> {
-    Ok::<(), ()>(())
+fn foo() -> (Result<(), ()>, ()) {
+    (Ok::<(), ()>(()), ())
 }
 
 fn main() {
-    (Ok::<(), ()>(()),); //~ ERROR unused `std::result::Result` that must be used
+    (Ok::<(), ()>(()),); //~ ERROR unused `std::result::Result`
 
     (Ok::<(), ()>(()), 0, Ok::<(), ()>(()), 5);
-    //~^ ERROR unused `std::result::Result` that must be used
-    //~^^ ERROR unused `std::result::Result` that must be used
+    //~^ ERROR unused `std::result::Result`
+    //~^^ ERROR unused `std::result::Result`
 
-    foo(); //~ ERROR unused `std::result::Result` that must be used
+    foo(); //~ ERROR unused `std::result::Result`
 }
diff --git a/src/test/ui/lint/must_use-tuple.stderr b/src/test/ui/lint/must_use-tuple.stderr
index dee53e70edd..4efcb8703aa 100644
--- a/src/test/ui/lint/must_use-tuple.stderr
+++ b/src/test/ui/lint/must_use-tuple.stderr
@@ -1,4 +1,4 @@
-error: unused `std::result::Result` that must be used
+error: unused `std::result::Result` in tuple element 0 that must be used
   --> $DIR/must_use-tuple.rs:8:6
    |
 LL |     (Ok::<(), ()>(()),);
@@ -11,7 +11,7 @@ LL | #![deny(unused_must_use)]
    |         ^^^^^^^^^^^^^^^
    = note: this `Result` may be an `Err` variant, which should be handled
 
-error: unused `std::result::Result` that must be used
+error: unused `std::result::Result` in tuple element 0 that must be used
   --> $DIR/must_use-tuple.rs:10:6
    |
 LL |     (Ok::<(), ()>(()), 0, Ok::<(), ()>(()), 5);
@@ -19,7 +19,7 @@ LL |     (Ok::<(), ()>(()), 0, Ok::<(), ()>(()), 5);
    |
    = note: this `Result` may be an `Err` variant, which should be handled
 
-error: unused `std::result::Result` that must be used
+error: unused `std::result::Result` in tuple element 2 that must be used
   --> $DIR/must_use-tuple.rs:10:27
    |
 LL |     (Ok::<(), ()>(()), 0, Ok::<(), ()>(()), 5);
@@ -27,7 +27,7 @@ LL |     (Ok::<(), ()>(()), 0, Ok::<(), ()>(()), 5);
    |
    = note: this `Result` may be an `Err` variant, which should be handled
 
-error: unused `std::result::Result` that must be used
+error: unused `std::result::Result` in tuple element 0 that must be used
   --> $DIR/must_use-tuple.rs:14:5
    |
 LL |     foo();
-- 
2.44.0