Fix union handling in exhaustiveness

compiler/rustc_middle/src/thir.rs

@@ -1120,7 +1120,8 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                             printed += 1;
                         }
 
-                        if printed < variant.fields.len() {
+                        let is_union = self.ty.ty_adt_def().is_some_and(|adt| adt.is_union());
+                        if printed < variant.fields.len() && (!is_union || printed == 0) {
                             write!(f, "{}..", start_or_comma())?;
                         }
 

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -140,6 +140,34 @@
 //! [`ConstructorSet::split`]. The invariants of [`SplitConstructorSet`] are also of interest.
 //!
 //!
+//! ## Unions
+//!
+//! Unions allow us to match a value via several overlapping representations at the same time. For
+//! example, the following is exhaustive because when seeing the value as a boolean we handled all
+//! possible cases (other cases such as `n == 3` would trigger UB).
+//!
+//! ```rust
+//! # fn main() {
+//! union U8AsBool {
+//!     n: u8,
+//!     b: bool,
+//! }
+//! let x = U8AsBool { n: 1 };
+//! unsafe {
+//!     match x {
+//!         U8AsBool { n: 2 } => {}
+//!         U8AsBool { b: true } => {}
+//!         U8AsBool { b: false } => {}
+//!     }
+//! }
+//! # }
+//! ```
+//!
+//! Pattern-matching has no knowledge that e.g. `false as u8 == 0`, so the values we consider in the
+//! algorithm look like `U8AsBool { b: true, n: 2 }`. In other words, for the most part a union is
+//! treated like a struct with the same fields. The difference lies in how we construct witnesses of
+//! non-exhaustiveness.
+//!
 //!
 //! ## Opaque patterns
 //!

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -1384,12 +1384,35 @@ fn push_pattern(&mut self, pat: WitnessPat<Cx>) {
     /// pats: [(false, "foo"), _, true]
     /// result: [Enum::Variant { a: (false, "foo"), b: _ }, true]
     /// ```
-    fn apply_constructor(&mut self, pcx: &PlaceCtxt<'_, Cx>, ctor: &Constructor<Cx>) {
+    fn apply_constructor(
+        mut self,
+        pcx: &PlaceCtxt<'_, Cx>,
+        ctor: &Constructor<Cx>,
+    ) -> SmallVec<[Self; 1]> {
         let len = self.0.len();
         let arity = pcx.ctor_arity(ctor);
-        let fields = self.0.drain((len - arity)..).rev().collect();
+        let fields: Vec<_> = self.0.drain((len - arity)..).rev().collect();
-        let pat = WitnessPat::new(ctor.clone(), fields, pcx.ty.clone());
+        if matches!(ctor, Constructor::UnionField)
-        self.0.push(pat);
+            && fields.iter().filter(|p| !matches!(p.ctor(), Constructor::Wildcard)).count() >= 2
+        {
+            // Convert a `Union { a: p, b: q }` witness into `Union { a: p }` and `Union { b: q }`.
+            // First add `Union { .. }` to `self`.
+            self.0.push(WitnessPat::wild_from_ctor(pcx.cx, ctor.clone(), pcx.ty.clone()));
+            fields
+                .into_iter()
+                .enumerate()
+                .filter(|(_, p)| !matches!(p.ctor(), Constructor::Wildcard))
+                .map(|(i, p)| {
+                    let mut ret = self.clone();
+                    // Fill the `i`th field of the union with `p`.
+                    ret.0.last_mut().unwrap().fields[i] = p;
+                    ret
+                })
+                .collect()
+        } else {
+            self.0.push(WitnessPat::new(ctor.clone(), fields, pcx.ty.clone()));
+            smallvec![self]
+        }
     }
 }
 
@@ -1462,8 +1485,8 @@ fn apply_constructor(
             *self = ret;
         } else {
             // Any other constructor we unspecialize as expected.
-            for witness in self.0.iter_mut() {
+            for witness in std::mem::take(&mut self.0) {
-                witness.apply_constructor(pcx, ctor)
+                self.0.extend(witness.apply_constructor(pcx, ctor));
             }
         }
     }

tests/ui/pattern/usefulness/unions.rs

@@ -20,12 +20,13 @@ union U8AsBool {
             U8AsBool { n: 1.. } => {}
         }
         match x {
-            //~^ ERROR non-exhaustive patterns: `U8AsBool { n: 0_u8, b: false }` not covered
+            //~^ ERROR non-exhaustive patterns: `U8AsBool { n: 0_u8 }` and `U8AsBool { b: false }` not covered
             U8AsBool { b: true } => {}
             U8AsBool { n: 1.. } => {}
         }
+        // Our approach can report duplicate witnesses sometimes.
         match (x, true) {
-            //~^ ERROR non-exhaustive patterns: `(U8AsBool { n: 0_u8, b: false }, false)` and `(U8AsBool { n: 0_u8, b: true }, false)` not covered
+            //~^ ERROR non-exhaustive patterns: `(U8AsBool { n: 0_u8 }, false)`, `(U8AsBool { b: false }, false)`, `(U8AsBool { n: 0_u8 }, false)` and 1 more not covered
             (U8AsBool { b: true }, true) => {}
             (U8AsBool { b: false }, true) => {}
             (U8AsBool { n: 1.. }, true) => {}

tests/ui/pattern/usefulness/unions.stderr

@@ -1,8 +1,8 @@
-error[E0004]: non-exhaustive patterns: `U8AsBool { n: 0_u8, b: false }` not covered
+error[E0004]: non-exhaustive patterns: `U8AsBool { n: 0_u8 }` and `U8AsBool { b: false }` not covered
   --> $DIR/unions.rs:22:15
    |
 LL |         match x {
-   |               ^ pattern `U8AsBool { n: 0_u8, b: false }` not covered
+   |               ^ patterns `U8AsBool { n: 0_u8 }` and `U8AsBool { b: false }` not covered
    |
 note: `U8AsBool` defined here
   --> $DIR/unions.rs:3:11
@@ -10,23 +10,23 @@ note: `U8AsBool` defined here
 LL |     union U8AsBool {
    |           ^^^^^^^^
    = note: the matched value is of type `U8AsBool`
-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 ~             U8AsBool { n: 1.. } => {},
-LL +             U8AsBool { n: 0_u8, b: false } => todo!()
-   |
-
-error[E0004]: non-exhaustive patterns: `(U8AsBool { n: 0_u8, b: false }, false)` and `(U8AsBool { n: 0_u8, b: true }, false)` not covered
-  --> $DIR/unions.rs:27:15
-   |
-LL |         match (x, true) {
-   |               ^^^^^^^^^ patterns `(U8AsBool { n: 0_u8, b: false }, false)` and `(U8AsBool { n: 0_u8, b: true }, false)` not covered
-   |
-   = note: the matched value is of type `(U8AsBool, bool)`
 help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern, a match arm with multiple or-patterns as shown, or multiple match arms
    |
+LL ~             U8AsBool { n: 1.. } => {},
+LL +             U8AsBool { n: 0_u8 } | U8AsBool { b: false } => todo!()
+   |
+
+error[E0004]: non-exhaustive patterns: `(U8AsBool { n: 0_u8 }, false)`, `(U8AsBool { b: false }, false)`, `(U8AsBool { n: 0_u8 }, false)` and 1 more not covered
+  --> $DIR/unions.rs:28:15
+   |
+LL |         match (x, true) {
+   |               ^^^^^^^^^ patterns `(U8AsBool { n: 0_u8 }, false)`, `(U8AsBool { b: false }, false)`, `(U8AsBool { n: 0_u8 }, false)` and 1 more not covered
+   |
+   = note: the matched value is of type `(U8AsBool, bool)`
+help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown, or multiple match arms
+   |
 LL ~             (U8AsBool { n: 1.. }, true) => {},
-LL +             (U8AsBool { n: 0_u8, b: false }, false) | (U8AsBool { n: 0_u8, b: true }, false) => todo!()
+LL +             _ => todo!()
    |
 
 error: aborting due to 2 previous errors