From c930bba2836c1daa4ffa72ce3ec4d3e570ce188e Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Thu, 31 Oct 2024 16:05:42 +0000
Subject: [PATCH] And locals too

---
 compiler/rustc_hir_typeck/src/errors.rs             |  4 ++++
 compiler/rustc_hir_typeck/src/fallback.rs           | 13 +++++++++++++
 .../never-type-fallback-breaking.e2021.stderr       |  4 ++--
 .../defaulted-never-note.nofallback.stderr          |  4 ++++
 ...iverging-fallback-control-flow.nofallback.stderr |  8 ++++----
 ...-fallback-unconstrained-return.nofallback.stderr |  4 ++++
 6 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/compiler/rustc_hir_typeck/src/errors.rs b/compiler/rustc_hir_typeck/src/errors.rs
index 2f09e5e163c15..fdeeed92565c3 100644
--- a/compiler/rustc_hir_typeck/src/errors.rs
+++ b/compiler/rustc_hir_typeck/src/errors.rs
@@ -214,6 +214,7 @@ pub(crate) struct DependencyOnUnitNeverTypeFallback<'tcx> {
 pub(crate) enum SuggestAnnotation {
     Unit(Span),
     Path(Span),
+    Local(Span),
 }
 
 #[derive(Clone)]
@@ -240,6 +241,9 @@ impl Subdiagnostic for SuggestAnnotations {
                     suggestions.push((span.shrink_to_lo(), "<() as ".to_string()));
                     suggestions.push((span.shrink_to_hi(), ">".to_string()));
                 }
+                SuggestAnnotation::Local(span) => {
+                    suggestions.push((span, ": ()".to_string()));
+                }
             }
         }
 
diff --git a/compiler/rustc_hir_typeck/src/fallback.rs b/compiler/rustc_hir_typeck/src/fallback.rs
index d6be237acf4d6..cefc28b400f6e 100644
--- a/compiler/rustc_hir_typeck/src/fallback.rs
+++ b/compiler/rustc_hir_typeck/src/fallback.rs
@@ -620,6 +620,19 @@ impl<'tcx> Visitor<'tcx> for VidVisitor<'_, 'tcx> {
         }
         hir::intravisit::walk_expr(self, expr)
     }
+
+    fn visit_local(&mut self, local: &'tcx hir::LetStmt<'tcx>) -> Self::Result {
+        if let None = local.ty
+            && let ty = self.fcx.typeck_results.borrow().node_type(local.hir_id)
+            && let Some(vid) = self.fcx.root_vid(ty)
+            && self.reachable_vids.contains(&vid)
+        {
+            return ControlFlow::Break(errors::SuggestAnnotation::Local(
+                local.pat.span.shrink_to_hi(),
+            ));
+        }
+        hir::intravisit::walk_local(self, local)
+    }
 }
 
 #[derive(Debug, Copy, Clone)]
diff --git a/tests/ui/editions/never-type-fallback-breaking.e2021.stderr b/tests/ui/editions/never-type-fallback-breaking.e2021.stderr
index 703f3d6266cf6..1c6685261fb3e 100644
--- a/tests/ui/editions/never-type-fallback-breaking.e2021.stderr
+++ b/tests/ui/editions/never-type-fallback-breaking.e2021.stderr
@@ -15,8 +15,8 @@ LL |         true => Default::default(),
    = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
 help: use `()` annotations to avoid fallback changes
    |
-LL |         true => <() as Default>::default(),
-   |                 ++++++        +
+LL |     let x: () = match true {
+   |          ++++
 
 warning: this function depends on never type fallback being `()`
   --> $DIR/never-type-fallback-breaking.rs:27:1
diff --git a/tests/ui/never_type/defaulted-never-note.nofallback.stderr b/tests/ui/never_type/defaulted-never-note.nofallback.stderr
index d88615186dd63..6bc4501b6a375 100644
--- a/tests/ui/never_type/defaulted-never-note.nofallback.stderr
+++ b/tests/ui/never_type/defaulted-never-note.nofallback.stderr
@@ -13,6 +13,10 @@ note: in edition 2024, the requirement `!: ImplementedForUnitButNotNever` will f
 LL |     foo(_x);
    |         ^^
    = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
+help: use `()` annotations to avoid fallback changes
+   |
+LL |     let _x: () = return;
+   |           ++++
 
 warning: 1 warning emitted
 
diff --git a/tests/ui/never_type/diverging-fallback-control-flow.nofallback.stderr b/tests/ui/never_type/diverging-fallback-control-flow.nofallback.stderr
index dee112e245a76..d40d1da76f9ac 100644
--- a/tests/ui/never_type/diverging-fallback-control-flow.nofallback.stderr
+++ b/tests/ui/never_type/diverging-fallback-control-flow.nofallback.stderr
@@ -15,8 +15,8 @@ LL |         x = UnitDefault::default();
    = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
 help: use `()` annotations to avoid fallback changes
    |
-LL |         x = <() as UnitDefault>::default();
-   |             ++++++            +
+LL |     let x: ();
+   |          ++++
 
 warning: this function depends on never type fallback being `()`
   --> $DIR/diverging-fallback-control-flow.rs:42:1
@@ -34,8 +34,8 @@ LL |         x = UnitDefault::default();
    |             ^^^^^^^^^^^^^^^^^^^^^^
 help: use `()` annotations to avoid fallback changes
    |
-LL |         x = <() as UnitDefault>::default();
-   |             ++++++            +
+LL |     let x: ();
+   |          ++++
 
 warning: 2 warnings emitted
 
diff --git a/tests/ui/never_type/diverging-fallback-unconstrained-return.nofallback.stderr b/tests/ui/never_type/diverging-fallback-unconstrained-return.nofallback.stderr
index b485c94df4d6f..30a5e60a75848 100644
--- a/tests/ui/never_type/diverging-fallback-unconstrained-return.nofallback.stderr
+++ b/tests/ui/never_type/diverging-fallback-unconstrained-return.nofallback.stderr
@@ -13,6 +13,10 @@ note: in edition 2024, the requirement `!: UnitReturn` will fail
 LL |     let _ = if true { unconstrained_return() } else { panic!() };
    |                       ^^^^^^^^^^^^^^^^^^^^^^
    = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default
+help: use `()` annotations to avoid fallback changes
+   |
+LL |     let _: () = if true { unconstrained_return() } else { panic!() };
+   |          ++++
 
 warning: 1 warning emitted