Allow unused variables with todo!

compiler/rustc_passes/src/liveness.rs

@@ -84,14 +84,14 @@
 use self::LiveNodeKind::*;
 use self::VarKind::*;
 
-use rustc_ast::InlineAsmOptions;
+use rustc_ast::{InlineAsmOptions, LitKind, StrStyle};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::def::*;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
-use rustc_hir::{Expr, HirId, HirIdMap, HirIdSet};
+use rustc_hir::{Expr, ExprKind, HirId, HirIdMap, HirIdSet, QPath};
 use rustc_index::vec::IndexVec;
 use rustc_middle::hir::map::Map;
 use rustc_middle::ty::query::Providers;
@@ -329,6 +329,48 @@ impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
             }
         }
 
+        // Allow todo! macro
+        /*
+            Skips checking for unused variables when the trailing expression
+            of the body is a panic with a message that contains "not yet implemented".
+
+            # Example
+
+            fn foo(x: i32) {
+                // arbitrary code
+                todo!()
+            }
+        */
+        if let ExprKind::Block(block, _) = &body.value.kind {
+            if let Some(expr) = block.expr {
+                if let ExprKind::Call(call, [arg]) = expr.kind {
+                    if let ExprKind::Path(QPath::Resolved(_, path)) = call.kind {
+                        if let Res::Def(DefKind::Fn, path_def_id) = &path.res {
+                            let panic_fn = self.tcx.lang_items().panic_fn();
+                            // Note: there is no function for panic_fmt, so we have to extract it from the debug output :(
+                            // builder doesn't like this being called without panic `self.tcx.def_path_str(*path_def_id);`
+                            let path_str = format!("{:?}", path_def_id);
+                            if Some(*path_def_id) == panic_fn
+                                || ((path_str.contains("std[") || path_str.contains("core["))
+                                    && path_str.contains("panicking::panic"))
+                            {
+                                if let ExprKind::Lit(spanned) = &arg.kind {
+                                    if let LitKind::Str(symbol, StrStyle::Cooked) = spanned.node {
+                                        if symbol.as_str().starts_with("not yet implemented") {
+                                            return;
+                                        }
+                                    }
+                                } else if format!("{:?}", &arg.kind).contains("not yet implemented")
+                                {
+                                    return;
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
         if let Some(captures) = maps.tcx.typeck(local_def_id).closure_captures.get(&def_id) {
             for &var_hir_id in captures.keys() {
                 let var_name = maps.tcx.hir().name(var_hir_id);

src/test/ui/unused/allow-unused-variables-with-todo.rs

@@ -0,0 +1,30 @@
+// check-pass
+
+#[deny(unused_variables)]
+fn plain(x: i32, y: i32) -> i32 {
+    todo!()
+}
+
+#[deny(unused_variables)]
+fn message(x: i32, y: i32) -> i32 {
+    todo!("message")
+}
+
+#[deny(unused_variables)]
+fn statement(x: i32, y: i32) -> i32 {
+    let z = x + y;
+    todo!()
+}
+
+#[deny(unused_variables)]
+fn statement_message(x: i32, y: i32) -> i32 {
+    let z = x + y;
+    todo!("message")
+}
+
+fn main() {
+    plain(0, 1);
+    message(0, 1);
+    statement(0, 1);
+    statement_message(0, 1);
+}