Rollup merge of #128033 - Nadrieril:explain-empty-wildcards, r=compiler-errors

Explain why we require `_` for empty patterns

This adds a note to the "non-exhaustive patterns" diagnostic to explain why we sometimes require extra `_` patterns on empty types. This is one of the two diagnostic improvements I wanted to do before [stabilizing `min_exhaustive_patterns`](https://github.com/rust-lang/rust/pull/122792).

r? ``@compiler-errors``

2 files changed, 10 insertions, 0 deletions

diff --git a/tests/ui/pattern/usefulness/empty-types.min_exh_pats.stderr b/tests/ui/pattern/usefulness/empty-types.min_exh_pats.stderr
index 6e50dfe6a26..9b57c895eea 100644
--- a/tests/ui/pattern/usefulness/empty-types.min_exh_pats.stderr
+++ b/tests/ui/pattern/usefulness/empty-types.min_exh_pats.stderr
@@ -204,6 +204,7 @@ note: `Option<Void>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Option<Void>`
+   = note: `Void` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~             None => {},
@@ -349,6 +350,7 @@ LL |     match slice_never {
    |           ^^^^^^^^^^^ pattern `&[_, ..]` not covered
    |
    = note: the matched value is of type `&[!]`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         [] => {},
@@ -484,6 +486,7 @@ note: `Option<!>` defined here
    |
    = note: not covered
    = note: the matched value is of type `&Option<!>`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         &None => {},
@@ -502,6 +505,7 @@ note: `Option<!>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Option<!>`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         None => {},
@@ -520,6 +524,7 @@ note: `Result<!, !>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Result<!, !>`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         Ok(_) => {},
@@ -538,6 +543,7 @@ note: `Result<!, !>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Result<!, !>`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         Ok(_a) => {},
@@ -589,6 +595,7 @@ LL |     match ref_never {
    |           ^^^^^^^^^ pattern `&_` not covered
    |
    = note: the matched value is of type `&!`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
    = note: references are always considered inhabited
    = note: match arms with guards don't count towards exhaustivity
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
@@ -609,6 +616,7 @@ note: `Result<!, !>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Result<!, !>`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         Err(_) => {},
@@ -627,6 +635,7 @@ note: `Option<Result<!, !>>` defined here
    |
    = note: not covered
    = note: the matched value is of type `Option<Result<!, !>>`
+   = note: `Result<!, !>` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         None => {},
diff --git a/tests/ui/pattern/usefulness/slice_of_empty.min_exhaustive_patterns.stderr b/tests/ui/pattern/usefulness/slice_of_empty.min_exhaustive_patterns.stderr
index a1239466c9c..f24ce154d14 100644
--- a/tests/ui/pattern/usefulness/slice_of_empty.min_exhaustive_patterns.stderr
+++ b/tests/ui/pattern/usefulness/slice_of_empty.min_exhaustive_patterns.stderr
@@ -5,6 +5,7 @@ LL |     match nevers {
    |           ^^^^^^ pattern `&[_, ..]` not covered
    |
    = note: the matched value is of type `&[!]`
+   = note: `!` is uninhabited but is not being matched by value, so a wildcard `_` is required
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
 LL ~         &[] => (),