reduce unreachable-code churn after todo!()

compiler/rustc_hir_typeck/src/expr.rs

@@ -322,7 +322,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if self.try_structurally_resolve_type(expr.span, ty).is_never()
             && self.expr_guaranteed_to_constitute_read_for_never(expr)
         {
-            self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
+            let diverges = if self.tcx.is_expanded_by(expr.span, sym::todo_macro) {
+                // We don't warn for `todo!()` that diverges to avoid flooding the
+                // user with warnings while they are still working on their code.
+                Diverges::WarnedAlways
+            } else {
+                Diverges::always(expr.span)
+            };
+            self.diverges.set(self.diverges.get() | diverges);
         }
 
         // Record the type, which applies it effects.

compiler/rustc_middle/src/ty/context.rs

@@ -2131,6 +2131,21 @@ impl<'tcx> TyCtxt<'tcx> {
             None => Err(VarError::NotPresent),
         }
     }
+
+    /// Returns whether this context was expanded by the macro with the given name.
+    pub fn is_expanded_by(self, span: Span, mac: Symbol) -> bool {
+        let mut ctxt = span.ctxt();
+        while !ctxt.is_root() {
+            let data = ctxt.outer_expn_data();
+            if let Some(def_id) = data.macro_def_id
+                && self.is_diagnostic_item(mac, def_id)
+            {
+                return true;
+            }
+            ctxt = data.call_site.ctxt();
+        }
+        false
+    }
 }
 
 impl<'tcx> TyCtxtAt<'tcx> {

library/core/src/macros/mod.rs

@@ -812,7 +812,10 @@ macro_rules! unimplemented {
 /// an intent of implementing the functionality later and the message is "not yet
 /// implemented", `unimplemented!` makes no such claims. Its message is "not implemented".
 ///
-/// Also, some IDEs will mark `todo!`s.
+/// Also, some IDEs will mark `todo!`s. Furthermore, the `unreachable_code` lint will
+/// not warn on code that is unreachable because of a `todo!()` to reduce unhelpful
+/// messages. The code will however still be marked as unreachable, which may have an
+/// effect on type and lifetime checks.
 ///
 /// # Panics
 ///

tests/ui/reachable/todo.rs

@@ -0,0 +1,41 @@
+//@ check-pass
+//@ edition: 2018
+
+#![allow(unused)]
+#![warn(unreachable_code)]
+
+macro_rules! later {
+    () => { todo!() };
+}
+
+fn foo() {
+    todo!();
+    let this_is_unreachable = 1;
+}
+
+fn bar() {
+    panic!("This is really unreachable");
+    let really_unreachable = true;
+    //~^ WARNING: unreachable
+}
+
+fn baz() -> bool {
+    if true {
+        todo!();
+        false
+    } else if todo!() {
+        true
+    } else {
+        later!();
+        false
+    }
+}
+
+fn main() {
+    foo();
+    bar();
+    if baz() {
+        todo!();
+    }
+    let this_is_reachable = 1;
+}

tests/ui/reachable/todo.stderr

@@ -0,0 +1,16 @@
+warning: unreachable statement
+  --> $DIR/todo.rs:18:5
+   |
+LL |     panic!("This is really unreachable");
+   |     ------------------------------------ any code following this expression is unreachable
+LL |     let really_unreachable = true;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unreachable statement
+   |
+note: the lint level is defined here
+  --> $DIR/todo.rs:5:9
+   |
+LL | #![warn(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+
+warning: 1 warning emitted
+