Rollup merge of rust-lang#57734 - oli-obk:fixes_and_cleanups, r=pnkfelix

Fix evaluating trivial drop glue in constants

```rust
struct A;
impl Drop for A {
    fn drop(&mut self) {}
}

const FOO: Option<A> = None;

const BAR: () = (FOO, ()).1;
```

was erroring with

```
error: any use of this value will cause an error
 --> src/lib.rs:9:1
  |
9 | const BAR: () = (FOO, ()).1;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^-^
  |                           |
  |                           calling non-const function `std::ptr::real_drop_in_place::<(std::option::Option<A>, ())> - shim(Some((std::option::Option<A>, ())))`
  |
  = note: #[deny(const_err)] on by default

error: aborting due to previous error
```

before this PR. According to godbolt this last compiled successfully in 1.27

src/librustc/ty/mod.rs

@@ -2937,16 +2937,6 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
-    /// Given the DefId of an item, returns its MIR, borrowed immutably.
-    /// Returns None if there is no MIR for the DefId
-    pub fn maybe_optimized_mir(self, did: DefId) -> Option<&'gcx Mir<'gcx>> {
-        if self.is_mir_available(did) {
-            Some(self.optimized_mir(did))
-        } else {
-            None
-        }
-    }
-
     /// Get the attributes of a definition.
     pub fn get_attrs(self, did: DefId) -> Attributes<'gcx> {
         if let Some(id) = self.hir().as_local_node_id(did) {

src/librustc_mir/const_eval.rs

@@ -340,19 +340,22 @@ impl<'a, 'mir, 'tcx> interpret::Machine<'a, 'mir, 'tcx>
         ret: Option<mir::BasicBlock>,
     ) -> EvalResult<'tcx, Option<&'mir mir::Mir<'tcx>>> {
         debug!("eval_fn_call: {:?}", instance);
-        // Execution might have wandered off into other crates, so we cannot to a stability-
-        // sensitive check here.  But we can at least rule out functions that are not const
-        // at all.
-        if !ecx.tcx.is_const_fn_raw(instance.def_id()) {
-            // Some functions we support even if they are non-const -- but avoid testing
-            // that for const fn!  We certainly do *not* want to actually call the fn
-            // though, so be sure we return here.
-            return if ecx.hook_fn(instance, args, dest)? {
-                ecx.goto_block(ret)?; // fully evaluated and done
-                Ok(None)
-            } else {
-                err!(MachineError(format!("calling non-const function `{}`", instance)))
-            };
+        // Only check non-glue functions
+        if let ty::InstanceDef::Item(def_id) = instance.def {
+            // Execution might have wandered off into other crates, so we cannot to a stability-
+            // sensitive check here.  But we can at least rule out functions that are not const
+            // at all.
+            if !ecx.tcx.is_const_fn_raw(def_id) {
+                // Some functions we support even if they are non-const -- but avoid testing
+                // that for const fn!  We certainly do *not* want to actually call the fn
+                // though, so be sure we return here.
+                return if ecx.hook_fn(instance, args, dest)? {
+                    ecx.goto_block(ret)?; // fully evaluated and done
+                    Ok(None)
+                } else {
+                    err!(MachineError(format!("calling non-const function `{}`", instance)))
+                };
+            }
         }
         // This is a const fn. Call it.
         Ok(Some(match ecx.load_mir(instance.def) {

src/librustc_mir/interpret/eval_context.rs

@@ -273,11 +273,11 @@ impl<'a, 'mir, 'tcx: 'mir, M: Machine<'a, 'mir, 'tcx>> EvalContext<'a, 'mir, 'tc
         }
         trace!("load mir {:?}", instance);
         match instance {
-            ty::InstanceDef::Item(def_id) => {
-                self.tcx.maybe_optimized_mir(def_id).ok_or_else(||
-                    EvalErrorKind::NoMirFor(self.tcx.item_path_str(def_id)).into()
-                )
-            }
+            ty::InstanceDef::Item(def_id) => if self.tcx.is_mir_available(did) {
+                Ok(self.tcx.optimized_mir(did))
+            } else {
+                err!(NoMirFor(self.tcx.item_path_str(def_id)))
+            },
             _ => Ok(self.tcx.instance_mir(instance)),
         }
     }

src/test/ui/consts/drop_none.rs

@@ -0,0 +1,13 @@
+// compile-pass
+#![allow(dead_code)]
+struct A;
+impl Drop for A {
+    fn drop(&mut self) {}
+}
+
+const FOO: Option<A> = None;
+
+const BAR: () = (FOO, ()).1;
+
+
+fn main() {}

src/test/ui/static/static-drop-scope.nll.stderr

@@ -54,7 +54,19 @@ error[E0493]: destructors cannot be evaluated at compile-time
 LL |     (x, ()).1
    |     ^^^^^^^ constant functions cannot evaluate destructors
 
-error: aborting due to 8 previous errors
+error[E0493]: destructors cannot be evaluated at compile-time
+  --> $DIR/static-drop-scope.rs:31:34
+   |
+LL | const EARLY_DROP_C_OPTION: i32 = (Some(WithDtor), 0).1;
+   |                                  ^^^^^^^^^^^^^^^^^^^ constants cannot evaluate destructors
+
+error[E0493]: destructors cannot be evaluated at compile-time
+  --> $DIR/static-drop-scope.rs:36:43
+   |
+LL | const EARLY_DROP_C_OPTION_CONSTANT: i32 = (HELPER, 0).1;
+   |                                           ^^^^^^^^^^^ constants cannot evaluate destructors
+
+error: aborting due to 10 previous errors
 
 Some errors occurred: E0493, E0716.
 For more information about an error, try `rustc --explain E0493`.

src/test/ui/static/static-drop-scope.rs

@@ -28,4 +28,12 @@ const fn const_drop2<T>(x: T) {
     //~^ ERROR destructors cannot be evaluated at compile-time
 }
 
+const EARLY_DROP_C_OPTION: i32 = (Some(WithDtor), 0).1;
+//~^ ERROR destructors cannot be evaluated at compile-time
+
+const HELPER: Option<WithDtor> = Some(WithDtor);
+
+const EARLY_DROP_C_OPTION_CONSTANT: i32 = (HELPER, 0).1;
+//~^ ERROR destructors cannot be evaluated at compile-time
+
 fn main () {}

src/test/ui/static/static-drop-scope.stderr

@@ -54,7 +54,19 @@ error[E0493]: destructors cannot be evaluated at compile-time
 LL |     (x, ()).1
    |     ^^^^^^^ constant functions cannot evaluate destructors
 
-error: aborting due to 8 previous errors
+error[E0493]: destructors cannot be evaluated at compile-time
+  --> $DIR/static-drop-scope.rs:31:34
+   |
+LL | const EARLY_DROP_C_OPTION: i32 = (Some(WithDtor), 0).1;
+   |                                  ^^^^^^^^^^^^^^^^^^^ constants cannot evaluate destructors
+
+error[E0493]: destructors cannot be evaluated at compile-time
+  --> $DIR/static-drop-scope.rs:36:43
+   |
+LL | const EARLY_DROP_C_OPTION_CONSTANT: i32 = (HELPER, 0).1;
+   |                                           ^^^^^^^^^^^ constants cannot evaluate destructors
+
+error: aborting due to 10 previous errors
 
 Some errors occurred: E0493, E0597.
 For more information about an error, try `rustc --explain E0493`.