never patterns: suggest ! patterns on non-exhaustive matches

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -939,43 +939,30 @@ fn report_non_exhaustive_match<'p, 'tcx>(
     };
     // In the case of an empty match, replace the '`_` not covered' diagnostic with something more
     // informative.
-    let mut err;
-    let pattern;
-    let patterns_len;
     if is_empty_match && !non_empty_enum {
         return cx.tcx.dcx().emit_err(NonExhaustivePatternsTypeNotEmpty {
             cx,
             expr_span,
             span: sp,
             ty: scrut_ty,
         });
-    } else {
-        // FIXME: migration of this diagnostic will require list support
-        let joined_patterns = joined_uncovered_patterns(cx, &witnesses);
-        err = create_e0004(
-            cx.tcx.sess,
-            sp,
-            format!("non-exhaustive patterns: {joined_patterns} not covered"),
-        );
-        err.span_label(
-            sp,
-            format!(
-                "pattern{} {} not covered",
-                rustc_errors::pluralize!(witnesses.len()),
-                joined_patterns
-            ),
-        );
-        patterns_len = witnesses.len();
-        pattern = if witnesses.len() < 4 {
-            witnesses
-                .iter()
-                .map(|witness| cx.hoist_witness_pat(witness).to_string())
-                .collect::<Vec<String>>()
-                .join(" | ")
-        } else {
-            "_".to_string()
-        };
-    };
+    }
+
+    // FIXME: migration of this diagnostic will require list support
+    let joined_patterns = joined_uncovered_patterns(cx, &witnesses);
+    let mut err = create_e0004(
+        cx.tcx.sess,
+        sp,
+        format!("non-exhaustive patterns: {joined_patterns} not covered"),
+    );
+    err.span_label(
+        sp,
+        format!(
+            "pattern{} {} not covered",
+            rustc_errors::pluralize!(witnesses.len()),
+            joined_patterns
+        ),
+    );
 
     // Point at the definition of non-covered `enum` variants.
     if let Some(AdtDefinedHere { adt_def_span, ty, variants }) =
@@ -1022,6 +1009,23 @@ fn report_non_exhaustive_match<'p, 'tcx>(
         }
     }
 
+    // Whether we suggest the actual missing patterns or `_`.
+    let suggest_the_witnesses = witnesses.len() < 4;
+    let suggested_arm = if suggest_the_witnesses {
+        let pattern = witnesses
+            .iter()
+            .map(|witness| cx.hoist_witness_pat(witness).to_string())
+            .collect::<Vec<String>>()
+            .join(" | ");
+        if witnesses.iter().all(|p| p.is_never_pattern()) && cx.tcx.features().never_patterns {
+            // Arms with a never pattern don't take a body.
+            pattern
+        } else {
+            format!("{pattern} => todo!()")
+        }
+    } else {
+        format!("_ => todo!()")
+    };
     let mut suggestion = None;
     let sm = cx.tcx.sess.source_map();
     match arms {
@@ -1034,7 +1038,7 @@ fn report_non_exhaustive_match<'p, 'tcx>(
             };
             suggestion = Some((
                 sp.shrink_to_hi().with_hi(expr_span.hi()),
-                format!(" {{{indentation}{more}{pattern} => todo!(),{indentation}}}",),
+                format!(" {{{indentation}{more}{suggested_arm},{indentation}}}",),
             ));
         }
         [only] => {
@@ -1060,7 +1064,7 @@ fn report_non_exhaustive_match<'p, 'tcx>(
             };
             suggestion = Some((
                 only.span.shrink_to_hi(),
-                format!("{comma}{pre_indentation}{pattern} => todo!()"),
+                format!("{comma}{pre_indentation}{suggested_arm}"),
             ));
         }
         [.., prev, last] => {
@@ -1083,7 +1087,7 @@ fn report_non_exhaustive_match<'p, 'tcx>(
                 if let Some(spacing) = spacing {
                     suggestion = Some((
                         last.span.shrink_to_hi(),
-                        format!("{comma}{spacing}{pattern} => todo!()"),
+                        format!("{comma}{spacing}{suggested_arm}"),
                     ));
                 }
             }
@@ -1094,13 +1098,13 @@ fn report_non_exhaustive_match<'p, 'tcx>(
     let msg = format!(
         "ensure that all possible cases are being handled by adding a match arm with a wildcard \
          pattern{}{}",
-        if patterns_len > 1 && patterns_len < 4 && suggestion.is_some() {
+        if witnesses.len() > 1 && suggest_the_witnesses && suggestion.is_some() {
             ", a match arm with multiple or-patterns"
         } else {
             // we are either not suggesting anything, or suggesting `_`
             ""
         },
-        match patterns_len {
+        match witnesses.len() {
             // non-exhaustive enum case
             0 if suggestion.is_some() => " as shown",
             0 => "",

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -681,15 +681,19 @@ pub enum Constructor<Cx: TypeCx> {
     Or,
     /// Wildcard pattern.
     Wildcard,
+    /// Never pattern. Only used in `WitnessPat`. An actual never pattern should be lowered as
+    /// `Wildcard`.
+    Never,
     /// Fake extra constructor for enums that aren't allowed to be matched exhaustively. Also used
-    /// for those types for which we cannot list constructors explicitly, like `f64` and `str`.
+    /// for those types for which we cannot list constructors explicitly, like `f64` and `str`. Only
+    /// used in `WitnessPat`.
     NonExhaustive,
-    /// Fake extra constructor for variants that should not be mentioned in diagnostics.
-    /// We use this for variants behind an unstable gate as well as
-    /// `#[doc(hidden)]` ones.
+    /// Fake extra constructor for variants that should not be mentioned in diagnostics. We use this
+    /// for variants behind an unstable gate as well as `#[doc(hidden)]` ones. Only used in
+    /// `WitnessPat`.
     Hidden,
     /// Fake extra constructor for constructors that are not seen in the matrix, as explained at the
-    /// top of the file.
+    /// top of the file. Only used for specialization.
     Missing,
     /// Fake extra constructor that indicates and empty field that is private. When we encounter one
     /// we skip the column entirely so we don't observe its emptiness. Only used for specialization.
@@ -711,6 +715,7 @@ impl<Cx: TypeCx> Clone for Constructor<Cx> {
             Constructor::Str(value) => Constructor::Str(value.clone()),
             Constructor::Opaque(inner) => Constructor::Opaque(inner.clone()),
             Constructor::Or => Constructor::Or,
+            Constructor::Never => Constructor::Never,
             Constructor::Wildcard => Constructor::Wildcard,
             Constructor::NonExhaustive => Constructor::NonExhaustive,
             Constructor::Hidden => Constructor::Hidden,
@@ -1043,10 +1048,32 @@ impl<Cx: TypeCx> ConstructorSet<Cx> {
                 // In a `MaybeInvalid` place even an empty pattern may be reachable. We therefore
                 // add a dummy empty constructor here, which will be ignored if the place is
                 // `ValidOnly`.
-                missing_empty.push(NonExhaustive);
+                missing_empty.push(Never);
             }
         }
 
         SplitConstructorSet { present, missing, missing_empty }
     }
+
+    /// Whether this set only contains empty constructors.
+    pub(crate) fn all_empty(&self) -> bool {
+        match self {
+            ConstructorSet::Bool
+            | ConstructorSet::Integers { .. }
+            | ConstructorSet::Ref
+            | ConstructorSet::Union
+            | ConstructorSet::Unlistable => false,
+            ConstructorSet::NoConstructors => true,
+            ConstructorSet::Struct { empty } => *empty,
+            ConstructorSet::Variants { variants, non_exhaustive } => {
+                !*non_exhaustive
+                    && variants
+                        .iter()
+                        .all(|visibility| matches!(visibility, VariantVisibility::Empty))
+            }
+            ConstructorSet::Slice { array_len, subtype_is_empty } => {
+                *subtype_is_empty && matches!(array_len, Some(1..))
+            }
+        }
+    }
 }

compiler/rustc_pattern_analysis/src/pat.rs

@@ -189,6 +189,7 @@ impl<Cx: TypeCx> fmt::Debug for DeconstructedPat<Cx> {
                 }
                 Ok(())
             }
+            Never => write!(f, "!"),
             Wildcard | Missing | NonExhaustive | Hidden | PrivateUninhabited => {
                 write!(f, "_ : {:?}", pat.ty())
             }
@@ -291,18 +292,24 @@ impl<Cx: TypeCx> WitnessPat<Cx> {
     pub(crate) fn new(ctor: Constructor<Cx>, fields: Vec<Self>, ty: Cx::Ty) -> Self {
         Self { ctor, fields, ty }
     }
-    pub(crate) fn wildcard(ty: Cx::Ty) -> Self {
-        Self::new(Wildcard, Vec::new(), ty)
+    /// Create a wildcard pattern for this type. If the type is empty, we create a `!` pattern.
+    pub(crate) fn wildcard(cx: &Cx, ty: Cx::Ty) -> Self {
+        let is_empty = cx.ctors_for_ty(&ty).is_ok_and(|ctors| ctors.all_empty());
+        let ctor = if is_empty { Never } else { Wildcard };
+        Self::new(ctor, Vec::new(), ty)
     }
 
     /// Construct a pattern that matches everything that starts with this constructor.
     /// For example, if `ctor` is a `Constructor::Variant` for `Option::Some`, we get the pattern
     /// `Some(_)`.
     pub(crate) fn wild_from_ctor(cx: &Cx, ctor: Constructor<Cx>, ty: Cx::Ty) -> Self {
+        if matches!(ctor, Wildcard) {
+            return Self::wildcard(cx, ty);
+        }
         let fields = cx
             .ctor_sub_tys(&ctor, &ty)
             .filter(|(_, PrivateUninhabitedField(skip))| !skip)
-            .map(|(ty, _)| Self::wildcard(ty))
+            .map(|(ty, _)| Self::wildcard(cx, ty))
             .collect();
         Self::new(ctor, fields, ty)
     }
@@ -314,6 +321,14 @@ impl<Cx: TypeCx> WitnessPat<Cx> {
         &self.ty
     }
 
+    pub fn is_never_pattern(&self) -> bool {
+        match self.ctor() {
+            Never => true,
+            Or => self.fields.iter().all(|p| p.is_never_pattern()),
+            _ => self.fields.iter().any(|p| p.is_never_pattern()),
+        }
+    }
+
     pub fn iter_fields(&self) -> impl Iterator<Item = &WitnessPat<Cx>> {
         self.fields.iter()
     }

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -251,7 +251,7 @@ impl<'p, 'tcx: 'p> RustcMatchCheckCtxt<'p, 'tcx> {
                 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty),
             },
             Bool(..) | IntRange(..) | F32Range(..) | F64Range(..) | Str(..) | Opaque(..)
-            | NonExhaustive | Hidden | Missing | PrivateUninhabited | Wildcard => &[],
+            | Never | NonExhaustive | Hidden | Missing | PrivateUninhabited | Wildcard => &[],
             Or => {
                 bug!("called `Fields::wildcards` on an `Or` ctor")
             }
@@ -279,7 +279,7 @@ impl<'p, 'tcx: 'p> RustcMatchCheckCtxt<'p, 'tcx> {
             Ref => 1,
             Slice(slice) => slice.arity(),
             Bool(..) | IntRange(..) | F32Range(..) | F64Range(..) | Str(..) | Opaque(..)
-            | NonExhaustive | Hidden | Missing | PrivateUninhabited | Wildcard => 0,
+            | Never | NonExhaustive | Hidden | Missing | PrivateUninhabited | Wildcard => 0,
             Or => bug!("The `Or` constructor doesn't have a fixed arity"),
         }
     }
@@ -809,7 +809,8 @@ impl<'p, 'tcx: 'p> RustcMatchCheckCtxt<'p, 'tcx> {
                 }
             }
             &Str(value) => PatKind::Constant { value },
-            Wildcard | NonExhaustive | Hidden | PrivateUninhabited => PatKind::Wild,
+            Never if self.tcx.features().never_patterns => PatKind::Never,
+            Never | Wildcard | NonExhaustive | Hidden | PrivateUninhabited => PatKind::Wild,
             Missing { .. } => bug!(
                 "trying to convert a `Missing` constructor into a `Pat`; this is probably a bug,
                 `Missing` should have been processed in `apply_constructors`"