Generate error delegation body when delegation is not resolved

compiler/rustc_ast_lowering/src/delegation.rs

@@ -96,7 +96,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 let generics = self.lower_delegation_generics(span);
                 DelegationResults { body_id, sig, ident, generics }
             }
-            Err(err) => self.generate_delegation_error(err, span),
+            Err(err) => self.generate_delegation_error(err, span, delegation),
         }
     }
 
@@ -404,6 +404,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         &mut self,
         err: ErrorGuaranteed,
         span: Span,
+        delegation: &Delegation,
     ) -> DelegationResults<'hir> {
         let generics = self.lower_delegation_generics(span);
 
@@ -418,8 +419,41 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let header = self.generate_header_error();
         let sig = hir::FnSig { decl, header, span };
 
-        let ident = Ident::dummy();
-        let body_id = self.lower_body(|this| (&[], this.mk_expr(hir::ExprKind::Err(err), span)));
+        let ident = self.lower_ident(delegation.ident);
+
+        let body_id = self.lower_body(|this| {
+            let body_expr = match delegation.body.as_ref() {
+                Some(box block) => {
+                    // Generates a block when we failed to resolve delegation, where a target expression is its only statement,
+                    // thus there will be no ICEs on further stages of analysis (see #144594)
+
+                    // As we generate a void function we want to convert target expression to statement to avoid additional
+                    // errors, such as mismatched return type
+                    let stmts = this.arena.alloc_from_iter([hir::Stmt {
+                        hir_id: this.next_id(),
+                        kind: rustc_hir::StmtKind::Semi(
+                            this.arena.alloc(this.lower_target_expr(block)),
+                        ),
+                        span,
+                    }]);
+
+                    let block = this.arena.alloc(hir::Block {
+                        stmts,
+                        expr: None,
+                        hir_id: this.next_id(),
+                        rules: hir::BlockCheckMode::DefaultBlock,
+                        span,
+                        targeted_by_break: false,
+                    });
+
+                    hir::ExprKind::Block(block, None)
+                }
+                None => hir::ExprKind::Err(err),
+            };
+
+            (&[], this.mk_expr(body_expr, span))
+        });
+
         DelegationResults { ident, generics, body_id, sig }
     }
 

tests/ui/delegation/ice-line-bounds-issue-148732.rs

@@ -3,6 +3,7 @@ reuse a as b {
     //~| ERROR functions delegation is not yet fully implemented
     dbg!(b);
     //~^ ERROR missing lifetime specifier
+    //~| ERROR `fn() {b}` doesn't implement `Debug`
 }
 
 fn main() {}

tests/ui/delegation/ice-line-bounds-issue-148732.stderr

@@ -20,7 +20,7 @@ LL | / reuse a as b {
 LL | |
 LL | |
 LL | |     dbg!(b);
-LL | |
+...  |
 LL | | }
    | |_^
    |
@@ -28,7 +28,19 @@ LL | | }
    = help: add `#![feature(fn_delegation)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 3 previous errors
+error[E0277]: `fn() {b}` doesn't implement `Debug`
+  --> $DIR/ice-line-bounds-issue-148732.rs:4:5
+   |
+LL | reuse a as b {
+   |            - consider calling this function
+...
+LL |     dbg!(b);
+   |     ^^^^^^^ the trait `Debug` is not implemented for fn item `fn() {b}`
+   |
+   = help: use parentheses to call this function: `b()`
+   = note: this error originates in the macro `dbg` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-Some errors have detailed explanations: E0106, E0425, E0658.
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0106, E0277, E0425, E0658.
 For more information about an error, try `rustc --explain E0106`.

tests/ui/delegation/unused-import-ice-144594.rs

@@ -0,0 +1,13 @@
+#![allow(incomplete_features)]
+#![feature(fn_delegation)]
+
+reuse a as b {
+    //~^ ERROR cannot find function `a` in this scope [E0425]
+    || {
+        use std::ops::Add;
+        x.add
+        //~^ ERROR cannot find value `x` in this scope [E0425]
+    }
+}
+
+fn main() {}

tests/ui/delegation/unused-import-ice-144594.stderr

@@ -0,0 +1,15 @@
+error[E0425]: cannot find function `a` in this scope
+  --> $DIR/unused-import-ice-144594.rs:4:7
+   |
+LL | reuse a as b {
+   |       ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/unused-import-ice-144594.rs:8:9
+   |
+LL |         x.add
+   |         ^ not found in this scope
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0425`.