Compute array length from type for unconditional panic.

compiler/rustc_mir_transform/src/known_panics_lint.rs

@@ -600,13 +600,15 @@ fn eval_rvalue(&mut self, rvalue: &Rvalue<'tcx>, dest: &Place<'tcx>) -> Option<(
             }
 
             Len(place) => {
-                let len = match self.get_const(place)? {
-                    Value::Immediate(src) => src.len(&self.ecx).discard_err()?,
-                    Value::Aggregate { fields, .. } => fields.len() as u64,
-                    Value::Uninit => match place.ty(self.local_decls(), self.tcx).ty.kind() {
-                        ty::Array(_, n) => n.try_eval_target_usize(self.tcx, self.param_env)?,
-                        _ => return None,
-                    },
+                let len = if let ty::Array(_, n) = place.ty(self.local_decls(), self.tcx).ty.kind()
+                {
+                    n.try_eval_target_usize(self.tcx, self.param_env)?
+                } else {
+                    match self.get_const(place)? {
+                        Value::Immediate(src) => src.len(&self.ecx).discard_err()?,
+                        Value::Aggregate { fields, .. } => fields.len() as u64,
+                        Value::Uninit => return None,
+                    }
                 };
                 ImmTy::from_scalar(Scalar::from_target_usize(len, self), layout).into()
             }

tests/ui/lint/unconditional_panic_promoted.rs

@@ -0,0 +1,8 @@
+//@ build-fail
+
+fn main() {
+    // MIR encodes this as a reborrow from a promoted constant.
+    // But the array lenth can still be gotten from the type.
+    let slice = &[0, 1];
+    let _ = slice[2]; //~ ERROR: this operation will panic at runtime [unconditional_panic]
+}

tests/ui/lint/unconditional_panic_promoted.stderr

@@ -0,0 +1,10 @@
+error: this operation will panic at runtime
+  --> $DIR/unconditional_panic_promoted.rs:7:13
+   |
+LL |     let _ = slice[2];
+   |             ^^^^^^^^ index out of bounds: the length is 2 but the index is 2
+   |
+   = note: `#[deny(unconditional_panic)]` on by default
+
+error: aborting due to 1 previous error
+