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Rollup merge of rust-lang#56439 - JohnGinger:master, r=nikomatsakis
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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 rust-lang#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.
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pietroalbini authored Dec 11, 2018
2 parents a7e1d77 + c0e3f4b commit f7de3cc
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Showing 4 changed files with 19 additions and 4 deletions.
12 changes: 8 additions & 4 deletions src/librustc/middle/liveness.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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();
}
}
}
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down
7 changes: 7 additions & 0 deletions src/test/ui/liveness/liveness-dead.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -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

3 changes: 3 additions & 0 deletions src/test/ui/liveness/liveness-unused.stderr
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down Expand Up @@ -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

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