Do not project to uninhabited variant in GVN

Author: compiler-errors

compiler/rustc_mir_transform/src/gvn.rs

@@ -577,7 +577,14 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     return None;
                 }
             }
-            ProjectionElem::Downcast(name, index) => ProjectionElem::Downcast(name, index),
+            ProjectionElem::Downcast(name, index) => {
+                if let Some(ct) = self.eval_to_const(value)
+                    && ct.layout.for_variant(&self.ecx, index).abi.is_uninhabited()
+                {
+                    return None;
+                }
+                ProjectionElem::Downcast(name, index)
+            }
             ProjectionElem::Field(f, ty) => {
                 if let Value::Aggregate(_, _, fields) = self.get(value) {
                     return Some(fields[f.as_usize()]);

tests/mir-opt/gvn_uninhabited.f.GVN.panic-abort.diff

@@ -0,0 +1,34 @@
+- // MIR for `f` before GVN
++ // MIR for `f` after GVN
+  
+  fn f() -> u32 {
+      let mut _0: u32;
+      let _1: u32;
+      let mut _2: E;
+      let mut _3: &U;
+      let _4: U;
+      scope 1 {
+          debug i => _1;
+      }
+      scope 2 {
+          let mut _5: &U;
+      }
+  
+      bb0: {
+          StorageLive(_2);
+          StorageLive(_3);
+          _5 = const _;
+          _3 = &(*_5);
+          _2 = ((*_3).1: E);
+-         StorageLive(_1);
++         nop;
+          _1 = ((_2 as A).1: u32);
+          StorageDead(_3);
+          StorageDead(_2);
+          _0 = _1;
+-         StorageDead(_1);
++         nop;
+          return;
+      }
+  }
+  

tests/mir-opt/gvn_uninhabited.f.GVN.panic-unwind.diff

@@ -0,0 +1,34 @@
+- // MIR for `f` before GVN
++ // MIR for `f` after GVN
+  
+  fn f() -> u32 {
+      let mut _0: u32;
+      let _1: u32;
+      let mut _2: E;
+      let mut _3: &U;
+      let _4: U;
+      scope 1 {
+          debug i => _1;
+      }
+      scope 2 {
+          let mut _5: &U;
+      }
+  
+      bb0: {
+          StorageLive(_2);
+          StorageLive(_3);
+          _5 = const _;
+          _3 = &(*_5);
+          _2 = ((*_3).1: E);
+-         StorageLive(_1);
++         nop;
+          _1 = ((_2 as A).1: u32);
+          StorageDead(_3);
+          StorageDead(_2);
+          _0 = _1;
+-         StorageDead(_1);
++         nop;
+          return;
+      }
+  }
+  

tests/mir-opt/gvn_uninhabited.rs

@@ -0,0 +1,24 @@
+// unit-test: GVN
+// compile-flags: -O
+// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
+// skip-filecheck
+
+#![feature(never_type)]
+
+#[derive(Copy, Clone)]
+pub enum E {
+    A(!, u32),
+}
+
+pub union U {
+    i: u32,
+    e: E,
+}
+
+// EMIT_MIR gvn_uninhabited.f.GVN.diff
+pub const fn f() -> u32 {
+    let E::A(_, i) = unsafe { (&U { i: 0 }).e };
+    i
+}
+
+fn main() {}