Rollup merge of #56439 - JohnGinger:master, r=nikomatsakis

Clearer error message for dead assign

I'm not that this is the right place for this (if it needs an RFC or not).

I had the problem where I misunderstood the compiler lint message https://github.com/rust-lang/rust/issues/56436 and other people seem to have had the same problem https://www.reddit.com/r/rust/comments/8cy9p4/value_assigned_to_is_never_read/.

I think this new wording might be slightly clearer (and help out beginners like me). I'm very new though, so there might be some nuance I'm missing that would make this more confusing or a bad idea for other reasons.

I thought I would create a PR to make it easy to change the code if the consensus was that it would make sense to make a change.

If this is the wrong place for this sort of thing I'll happily delete/move it.

4 files changed, 19 insertions, 4 deletions

diff --git a/src/librustc/middle/liveness.rs b/src/librustc/middle/liveness.rs
index e576951417f..31a75bd106e 100644
--- a/src/librustc/middle/liveness.rs
+++ b/src/librustc/middle/liveness.rs
@@ -1657,11 +1657,15 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
     fn report_dead_assign(&self, hir_id: HirId, sp: Span, var: Variable, is_argument: bool) {
         if let Some(name) = self.should_warn(var) {
             if is_argument {
-                self.ir.tcx.lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
-                    &format!("value passed to `{}` is never read", name));
+                self.ir.tcx.struct_span_lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
+                &format!("value passed to `{}` is never read", name))
+                .help("maybe it is overwritten before being read?")
+                .emit();
             } else {
-                self.ir.tcx.lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
-                    &format!("value assigned to `{}` is never read", name));
+                self.ir.tcx.struct_span_lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
+                &format!("value assigned to `{}` is never read", name))
+                .help("maybe it is overwritten before being read?")
+                .emit();
             }
         }
     }
diff --git a/src/test/ui/lint/issue-47390-unused-variable-in-struct-pattern.stderr b/src/test/ui/lint/issue-47390-unused-variable-in-struct-pattern.stderr
index a8b0e3e4250..6b9e1dc7057 100644
--- a/src/test/ui/lint/issue-47390-unused-variable-in-struct-pattern.stderr
+++ b/src/test/ui/lint/issue-47390-unused-variable-in-struct-pattern.stderr
@@ -55,6 +55,7 @@ note: lint level defined here
 LL | #![warn(unused)] // UI tests pass `-A unused` (#43896)
    |         ^^^^^^
    = note: #[warn(unused_assignments)] implied by #[warn(unused)]
+   = help: maybe it is overwritten before being read?
 
 warning: unused variable: `fire`
   --> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:54:32
diff --git a/src/test/ui/liveness/liveness-dead.stderr b/src/test/ui/liveness/liveness-dead.stderr
index 6709fee0abb..6e43cccdccf 100644
--- a/src/test/ui/liveness/liveness-dead.stderr
+++ b/src/test/ui/liveness/liveness-dead.stderr
@@ -9,24 +9,31 @@ note: lint level defined here
    |
 LL | #![deny(unused_assignments)]
    |         ^^^^^^^^^^^^^^^^^^
+   = help: maybe it is overwritten before being read?
 
 error: value assigned to `x` is never read
   --> $DIR/liveness-dead.rs:27:5
    |
 LL |     x = 4; //~ ERROR: value assigned to `x` is never read
    |     ^
+   |
+   = help: maybe it is overwritten before being read?
 
 error: value passed to `x` is never read
   --> $DIR/liveness-dead.rs:30:11
    |
 LL | fn f4(mut x: i32) { //~ ERROR: value passed to `x` is never read
    |           ^
+   |
+   = help: maybe it is overwritten before being read?
 
 error: value assigned to `x` is never read
   --> $DIR/liveness-dead.rs:37:5
    |
 LL |     x = 4; //~ ERROR: value assigned to `x` is never read
    |     ^
+   |
+   = help: maybe it is overwritten before being read?
 
 error: aborting due to 4 previous errors
 
diff --git a/src/test/ui/liveness/liveness-unused.stderr b/src/test/ui/liveness/liveness-unused.stderr
index 2846f242fbe..35ccc79a19a 100644
--- a/src/test/ui/liveness/liveness-unused.stderr
+++ b/src/test/ui/liveness/liveness-unused.stderr
@@ -60,6 +60,7 @@ note: lint level defined here
    |
 LL | #![deny(unused_assignments)]
    |         ^^^^^^^^^^^^^^^^^^
+   = help: maybe it is overwritten before being read?
 
 error: variable `z` is assigned to, but never used
   --> $DIR/liveness-unused.rs:47:13
@@ -106,6 +107,8 @@ error: value assigned to `x` is never read
    |
 LL |         x = 0;  //~ ERROR value assigned to `x` is never read
    |         ^
+   |
+   = help: maybe it is overwritten before being read?
 
 error: aborting due to 13 previous errors