discriminant reads: make semantics independent of module/crate

Author: meithecatte

compiler/rustc_mir_build/src/builder/matches/match_pair.rs

@@ -295,22 +295,18 @@ impl<'tcx> MatchPairTree<'tcx> {
                 }
             }
 
-            PatKind::Variant { adt_def, variant_index, args, ref subpatterns } => {
+            PatKind::Variant { adt_def, variant_index, args: _, ref subpatterns } => {
                 let downcast_place = place_builder.downcast(adt_def, variant_index); // `(x as Variant)`
                 cx.field_match_pairs(&mut subpairs, extra_data, downcast_place, subpatterns);
 
-                let irrefutable = adt_def.variants().iter_enumerated().all(|(i, v)| {
-                    i == variant_index
-                        || !v.inhabited_predicate(cx.tcx, adt_def).instantiate(cx.tcx, args).apply(
-                            cx.tcx,
-                            cx.infcx.typing_env(cx.param_env),
-                            cx.def_id.into(),
-                        )
-                }) && !adt_def.variant_list_has_applicable_non_exhaustive();
-                if irrefutable {
-                    None
-                } else {
+                // We treat non-exhaustive enums the same independent of the crate they are
+                // defined in, to avoid differences in the operational semantics between crates.
+                let refutable =
+                    adt_def.variants().len() > 1 || adt_def.is_variant_list_non_exhaustive();
+                if refutable {
                     Some(TestableCase::Variant { adt_def, variant_index })
+                } else {
+                    None
                 }
             }
 

src/tools/miri/tests/fail/match/all_variants_uninhabited.rs

@@ -0,0 +1,25 @@
+#![allow(deref_nullptr)]
+
+enum Never {}
+
+fn main() {
+    unsafe {
+        match *std::ptr::null::<Result<Never, Never>>() {
+        //~^ ERROR: read discriminant of an uninhabited enum variant
+            Ok(_) => {
+                lol();
+            }
+            Err(_) => {
+                wut();
+            }
+        }
+    }
+}
+
+fn lol() {
+    println!("lol");
+}
+
+fn wut() {
+    println!("wut");
+}

src/tools/miri/tests/fail/match/all_variants_uninhabited.stderr

@@ -0,0 +1,13 @@
+error: Undefined Behavior: read discriminant of an uninhabited enum variant
+  --> tests/fail/match/all_variants_uninhabited.rs:LL:CC
+   |
+LL |         match *std::ptr::null::<Result<Never, Never>>() {
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+

…/tests/fail/closures/deref-in-pattern.rs …/fail/match/closures/deref-in-pattern.rssrc/tools/miri/tests/fail/closures/deref-in-pattern.rs renamed to src/tools/miri/tests/fail/match/closures/deref-in-pattern.rs src/tools/miri/tests/fail/closures/deref-in-pattern.rs renamed to src/tools/miri/tests/fail/match/closures/deref-in-pattern.rs

@@ -1,5 +1,5 @@
 error: Undefined Behavior: constructing invalid value: encountered a dangling reference (use-after-free)
-  --> tests/fail/closures/deref-in-pattern.rs:LL:CC
+  --> tests/fail/match/closures/deref-in-pattern.rs:LL:CC
    |
 LL |       let _ = || {
    |  _____________^

…ts/fail/closures/deref-in-pattern.stderr …l/match/closures/deref-in-pattern.stderrsrc/tools/miri/tests/fail/closures/deref-in-pattern.stderr renamed to src/tools/miri/tests/fail/match/closures/deref-in-pattern.stderr src/tools/miri/tests/fail/closures/deref-in-pattern.stderr renamed to src/tools/miri/tests/fail/match/closures/deref-in-pattern.stderr

@@ -1,5 +1,5 @@
 error: Undefined Behavior: constructing invalid value: encountered a dangling reference (use-after-free)
-  --> tests/fail/closures/partial-pattern.rs:LL:CC
+  --> tests/fail/match/closures/partial-pattern.rs:LL:CC
    |
 LL |       let _ = || {
    |  _____________^

…i/tests/fail/closures/partial-pattern.rs …s/fail/match/closures/partial-pattern.rssrc/tools/miri/tests/fail/closures/partial-pattern.rs renamed to src/tools/miri/tests/fail/match/closures/partial-pattern.rs src/tools/miri/tests/fail/closures/partial-pattern.rs renamed to src/tools/miri/tests/fail/match/closures/partial-pattern.rs

@@ -1,5 +1,5 @@
 error: Undefined Behavior: read discriminant of an uninhabited enum variant
-  --> tests/fail/closures/uninhabited-variant.rs:LL:CC
+  --> tests/fail/match/closures/uninhabited-variant1.rs:LL:CC
    |
 LL |         match r {
    |               ^ Undefined Behavior occurred here
@@ -8,9 +8,9 @@ LL |         match r {
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: stack backtrace:
            0: main::{closure#0}
-               at tests/fail/closures/uninhabited-variant.rs:LL:CC
+               at tests/fail/match/closures/uninhabited-variant1.rs:LL:CC
            1: main
-               at tests/fail/closures/uninhabited-variant.rs:LL:CC
+               at tests/fail/match/closures/uninhabited-variant1.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 

…sts/fail/closures/partial-pattern.stderr …il/match/closures/partial-pattern.stderrsrc/tools/miri/tests/fail/closures/partial-pattern.stderr renamed to src/tools/miri/tests/fail/match/closures/partial-pattern.stderr src/tools/miri/tests/fail/closures/partial-pattern.stderr renamed to src/tools/miri/tests/fail/match/closures/partial-pattern.stderr

@@ -0,0 +1,32 @@
+// Motivated by rust-lang/rust#138961, this shows how invalid discriminants interact with
+// closure captures.
+//
+// Test case with only one inhabited variant, for which rustc used to not emit
+// a discriminant read in the first place. See: rust-lang/miri#4778
+#![feature(never_type)]
+
+#[repr(C)]
+#[allow(dead_code)]
+enum E {
+    V0,    // discriminant: 0
+    V1(!), // 1
+}
+
+fn main() {
+    assert_eq!(std::mem::size_of::<E>(), 4);
+
+    let val = 1u32;
+    let ptr = (&raw const val).cast::<E>();
+    let r = unsafe { &*ptr };
+    let f = || {
+        // After rust-lang/rust#138961, constructing the closure performs a reborrow of r.
+        // Nevertheless, the discriminant is only actually inspected when the closure
+        // is called.
+        match r { //~ ERROR: read discriminant of an uninhabited enum variant
+            E::V0 => {}
+            E::V1(_) => {}
+        }
+    };
+
+    f();
+}