Add note to non-exhaustive match on reference to empty

Rust prints "type `&A` is non-empty" even is A is empty.
This is the intended behavior, but can be confusing.
This commit adds a note to non-exhaustive pattern errors if they are a
reference to something uninhabited.

I did not add tests to check that the note is not shown for
non-references or inhabited references, because this is already done
in other tests.

Maybe the added test is superfluous, because
`always-inhabited-union-ref` already checks for this case.

This does not handle &&Void or &&&void etc. I could add those as special
cases as well and ignore people who need quadruple
references.

Fixes #78123
This commit is contained in:
Daniel Noom 2021-01-03 14:13:33 +01:00
parent 18cb4ad3b9
commit 998bf0ab88
4 changed files with 29 additions and 0 deletions

View File

@ -503,6 +503,11 @@ fn non_exhaustive_match<'p, 'tcx>(
));
}
}
if let ty::Ref(_, sub_ty, _) = scrut_ty.kind() {
if cx.tcx.is_ty_uninhabited_from(cx.module, sub_ty, cx.param_env) {
err.note("references are always considered inhabited");
}
}
err.emit();
}

View File

@ -6,6 +6,7 @@ LL | match uninhab_ref() {
|
= help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
= note: the matched value is of type `&!`
= note: references are always considered inhabited
error[E0004]: non-exhaustive patterns: type `Foo` is non-empty
--> $DIR/always-inhabited-union-ref.rs:27:11

View File

@ -0,0 +1,7 @@
enum A {}
fn f(a: &A) {
match a {} //~ ERROR non-exhaustive patterns: type `&A` is non-empty
}
fn main() {}

View File

@ -0,0 +1,16 @@
error[E0004]: non-exhaustive patterns: type `&A` is non-empty
--> $DIR/issue-78123-non-exhaustive-reference.rs:4:11
|
LL | enum A {}
| --------- `A` defined here
...
LL | match a {}
| ^
|
= help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
= note: the matched value is of type `&A`
= note: references are always considered inhabited
error: aborting due to previous error
For more information about this error, try `rustc --explain E0004`.