Rollup merge of rust-lang#139740 - jieyouxu:known-bug, r=nnethercote

Convert `tests/ui/lint/dead-code/self-assign.rs` to a known-bug test

I did a survey pass over `tests/`, and this test seems like the only candidate suitable for conversion into a known-bug test. (Other tests had varying degrees of other issues that known-bug would not be suitable.)

r? compiler

Author: jhpratt

tests/ui/lint/dead-code/self-assign.rs

@@ -1,19 +1,29 @@
-// Test that dead code warnings are issued for superfluous assignments of
-// fields or variables to themselves (issue #75356).
-
-//@ ignore-test FIXME(81658, 83171)
+//! Test that dead code warnings are issued for superfluous assignments of fields or variables to
+//! themselves (issue #75356).
+//!
+//! # History of this test (to aid relanding of a fixed version of #81473)
+//!
+//! - Original lint request was about self-assignments not triggering sth like `dead_code`.
+//! - `dead_code` lint expansion for self-assignments was implemented in #87129.
+//! - Unfortunately implementation components of #87129 had to be disabled as part of reverts
+//!   #86212, #83171 (to revert #81473) to address regressions #81626 and #81658.
+//! - Consequently, none of the following warnings are emitted.
 
 //@ check-pass
+
+// Implementation of self-assignment `dead_code` lint expansions disabled due to reverts.
+//@ known-bug: #75356
+
 #![allow(unused_assignments)]
 #![warn(dead_code)]
 
 fn main() {
     let mut x = 0;
     x = x;
-    //~^ WARNING: useless assignment of variable of type `i32` to itself
+    // FIXME ~^ WARNING: useless assignment of variable of type `i32` to itself
 
     x = (x);
-    //~^ WARNING: useless assignment of variable of type `i32` to itself
+    // FIXME ~^ WARNING: useless assignment of variable of type `i32` to itself
 
     x = {x};
     // block expressions don't count as self-assignments
@@ -22,10 +32,10 @@ fn main() {
     struct S<'a> { f: &'a str }
     let mut s = S { f: "abc" };
     s = s;
-    //~^ WARNING: useless assignment of variable of type `S` to itself
+    // FIXME ~^ WARNING: useless assignment of variable of type `S` to itself
 
     s.f = s.f;
-    //~^ WARNING: useless assignment of field of type `&str` to itself
+    // FIXME ~^ WARNING: useless assignment of field of type `&str` to itself
 
 
     struct N0 { x: Box<i32> }
@@ -34,11 +44,11 @@ fn main() {
     struct N3 { n: N2 };
     let mut n3 = N3 { n: N2(N1 { n: N0 { x: Box::new(42) } }) };
     n3.n.0.n.x = n3.n.0.n.x;
-    //~^ WARNING: useless assignment of field of type `Box<i32>` to itself
+    // FIXME ~^ WARNING: useless assignment of field of type `Box<i32>` to itself
 
     let mut t = (1, ((2, 3, (4, 5)),));
     t.1.0.2.1 = t.1.0.2.1;
-    //~^ WARNING: useless assignment of field of type `i32` to itself
+    // FIXME ~^ WARNING: useless assignment of field of type `i32` to itself
 
 
     let mut y = 0;