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Clearer error message for dead assign #56439

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Merged
merged 5 commits into from
Dec 16, 2018
Merged

Clearer error message for dead assign #56439

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eaeebb5
Clearer error message for dead assign
JohnGinger Dec 2, 2018
54026c1
Fix line length
JohnGinger Dec 2, 2018
4cf5702
Fix stderr files
JohnGinger Dec 3, 2018
70536d4
Fix stderr file (unused variable)
JohnGinger Dec 3, 2018
c0e3f4b
Change to give a help message
JohnGinger Dec 7, 2018
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6 changes: 4 additions & 2 deletions src/librustc/middle/liveness.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1658,10 +1658,12 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
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));
&format!("value passed to `{}` is never read
(maybe it is overwritten before being read)", name));
} else {
self.ir.tcx.lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
&format!("value assigned to `{}` is never read", name));
&format!("value assigned to `{}` is never read
(maybe it is overwritten before being read)", name));
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Hmm, it's a nice idea to make a suggestion like this. It'd be nicer if we could find the overwriting assignment. But i'm reluctant to suggest trying to implement that, since this liveness code is ripe to be rewritten completely.

Still, I think that including the suggestion in a parenthetical makes it less likely to be noticed, and more likely to wrap around on the terminal.

A middle option would be to create a "help". You could change this code from self.ir.tcx.lint_hir(..) to:

let msg = self.ir.tcx.self.struct_span_lint_hir(...)
  .help("maybe it is overwritten before being read?")
  .emit();

and I think things would look better.

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Sounds good to me.

I could try rewriting the liveness code, but I don't think I know enough about how it works...

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I'd love to talk you through it =) but it'd obviously be a rather bigger endeavor :)

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I'm happy to try :)

}
}
}
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1 change: 1 addition & 0 deletions src/test/ui/lint/issue-47390-unused-variable-in-struct-pattern.stderr
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Expand Up @@ -44,6 +44,7 @@ LL | mut hours_are_suns,
= note: consider using `_hours_are_suns` instead

warning: value assigned to `hours_are_suns` is never read
(maybe it is overwritten before being read)
--> $DIR/issue-47390-unused-variable-in-struct-pattern.rs:50:9
|
LL | hours_are_suns = false;
Expand Down
4 changes: 4 additions & 0 deletions src/test/ui/liveness/liveness-dead.stderr
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@@ -1,4 +1,5 @@
error: value assigned to `x` is never read
(maybe it is overwritten before being read)
--> $DIR/liveness-dead.rs:19:13
|
LL | let mut x: isize = 3; //~ ERROR: value assigned to `x` is never read
Expand All @@ -11,18 +12,21 @@ LL | #![deny(unused_assignments)]
| ^^^^^^^^^^^^^^^^^^

error: value assigned to `x` is never read
(maybe it is overwritten before being read)
--> $DIR/liveness-dead.rs:27:5
|
LL | x = 4; //~ ERROR: value assigned to `x` is never read
| ^

error: value passed to `x` is never read
(maybe it is overwritten before being read)
--> $DIR/liveness-dead.rs:30:11
|
LL | fn f4(mut x: i32) { //~ ERROR: value passed to `x` is never read
| ^

error: value assigned to `x` is never read
(maybe it is overwritten before being read)
--> $DIR/liveness-dead.rs:37:5
|
LL | x = 4; //~ ERROR: value assigned to `x` is never read
Expand Down
2 changes: 2 additions & 0 deletions src/test/ui/liveness/liveness-unused.stderr
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Expand Up @@ -50,6 +50,7 @@ LL | let mut x = 3;
= note: consider using `_x` instead

error: value assigned to `x` is never read
(maybe it is overwritten before being read)
--> $DIR/liveness-unused.rs:42:5
|
LL | x += 4;
Expand Down Expand Up @@ -102,6 +103,7 @@ LL | let x;
= note: consider using `_x` instead

error: value assigned to `x` is never read
(maybe it is overwritten before being read)
--> $DIR/liveness-unused.rs:126:9
|
LL | x = 0; //~ ERROR value assigned to `x` is never read
Expand Down