Rollup merge of rust-lang#103276 - compiler-errors:default-on-uninit-…

…ice, r=TaKO8Ki

Erase regions before checking for `Default` in uninitialized binding error

Fixes rust-lang#103250

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -492,10 +492,17 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             let Some(default_trait) = tcx.get_diagnostic_item(sym::Default) else {
                 return false;
             };
+            // Regions are already solved, so we must use a fresh InferCtxt,
+            // but the type has region variables, so erase those.
             tcx.infer_ctxt()
                 .build()
-                .type_implements_trait(default_trait, ty, ty::List::empty(), param_env)
-                .may_apply()
+                .type_implements_trait(
+                    default_trait,
+                    tcx.erase_regions(ty),
+                    ty::List::empty(),
+                    param_env,
+                )
+                .must_apply_modulo_regions()
         };
 
         let assign_value = match ty.kind() {

src/test/ui/borrowck/issue-103250.rs

@@ -0,0 +1,37 @@
+// edition:2021
+
+type TranslateFn = Box<dyn Fn(String, String) -> String>;
+
+pub struct DeviceCluster {
+    devices: Vec<Device>,
+}
+
+impl DeviceCluster {
+    pub async fn do_something(&mut self) -> Result<String, Box<dyn std::error::Error>> {
+        let mut last_error: Box<dyn std::error::Error>;
+
+        for device in &mut self.devices {
+            match device.do_something().await {
+                Ok(info) => {
+                    return Ok(info);
+                }
+                Err(e) => {}
+            }
+        }
+
+        Err(last_error)
+        //~^ ERROR used binding `last_error` isn't initialized
+    }
+}
+
+pub struct Device {
+    translate_fn: Option<TranslateFn>,
+}
+
+impl Device {
+    pub async fn do_something(&mut self) -> Result<String, Box<dyn std::error::Error>> {
+        Ok(String::from(""))
+    }
+}
+
+fn main() {}

src/test/ui/borrowck/issue-103250.stderr

@@ -0,0 +1,17 @@
+error[E0381]: used binding `last_error` isn't initialized
+  --> $DIR/issue-103250.rs:22:13
+   |
+LL |         let mut last_error: Box<dyn std::error::Error>;
+   |             -------------- binding declared here but left uninitialized
+...
+LL |         Err(last_error)
+   |             ^^^^^^^^^^ `last_error` used here but it isn't initialized
+   |
+help: consider assigning a value
+   |
+LL |         let mut last_error: Box<dyn std::error::Error> = todo!();
+   |                                                        +++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0381`.