Explain why we require _ for empty patterns

Author: Nadrieril

Diff for: compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -1019,6 +1019,10 @@ fn report_non_exhaustive_match<'p, 'tcx>(
                 err.note("`&str` cannot be matched exhaustively, so a wildcard `_` is necessary");
             } else if cx.is_foreign_non_exhaustive_enum(ty) {
                 err.note(format!("`{ty}` is marked as non-exhaustive, so a wildcard `_` is necessary to match exhaustively"));
+            } else if cx.is_uninhabited(ty.inner()) && cx.tcx.features().min_exhaustive_patterns {
+                // The type is uninhabited yet there is a witness: we must be in the `MaybeInvalid`
+                // case.
+                err.note(format!("`{ty}` is uninhabited but is not being matched by value, so a wildcard `_` is required"));
             }
         }
     }

Diff for: 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 for: 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 ~         &[] => (),