Some diagnostics improvements of gen blocks

compiler/rustc_middle/messages.ftl

@@ -12,6 +12,8 @@ middle_assert_coroutine_resume_after_return = coroutine resumed after completion
 middle_assert_divide_by_zero =
     attempt to divide `{$val}` by zero
 
+middle_assert_gen_resume_after_panic = `gen` fn or block cannot be further iterated on after it panicked
+
 middle_assert_misaligned_ptr_deref =
     misaligned pointer dereference: address must be a multiple of {$required} but is {$found}
 

compiler/rustc_middle/src/mir/terminator.rs

@@ -250,8 +250,7 @@ impl<O> AssertKind<O> {
                 middle_assert_coroutine_resume_after_return
             }
             ResumedAfterPanic(CoroutineKind::Async(_)) => middle_assert_async_resume_after_panic,
-            // FIXME(gen_blocks): custom error message for `gen` blocks
-            ResumedAfterPanic(CoroutineKind::Gen(_)) => middle_assert_async_resume_after_panic,
+            ResumedAfterPanic(CoroutineKind::Gen(_)) => middle_assert_gen_resume_after_panic,
             ResumedAfterPanic(CoroutineKind::Coroutine) => {
                 middle_assert_coroutine_resume_after_panic
             }

compiler/rustc_parse/messages.ftl

@@ -278,8 +278,8 @@ parse_found_expr_would_be_stmt = expected expression, found `{$token}`
 parse_function_body_equals_expr = function body cannot be `= expression;`
     .suggestion = surround the expression with `{"{"}` and `{"}"}` instead of `=` and `;`
 
-parse_gen_block = `gen` blocks are not yet implemented
-    .help = only the keyword is reserved for now
+parse_gen_fn = `gen` functions are not yet implemented
+    .help = for now you can use `gen {"{}"}` blocks and return `impl Iterator` instead
 
 parse_generic_args_in_pat_require_turbofish_syntax = generic args in patterns require the turbofish syntax
 

compiler/rustc_parse/src/errors.rs

@@ -521,9 +521,9 @@ pub(crate) struct CatchAfterTry {
 }
 
 #[derive(Diagnostic)]
-#[diag(parse_gen_block)]
+#[diag(parse_gen_fn)]
 #[help]
-pub(crate) struct GenBlock {
+pub(crate) struct GenFn {
     #[primary_span]
     pub span: Span,
 }

compiler/rustc_parse/src/parser/item.rs

@@ -2372,7 +2372,7 @@ impl<'a> Parser<'a> {
         }
 
         if let Gen::Yes { span, .. } = genness {
-            self.sess.emit_err(errors::GenBlock { span });
+            self.sess.emit_err(errors::GenFn { span });
         }
 
         if !self.eat_keyword_case(kw::Fn, case) {

tests/ui/coroutine/gen_block_panic.rs

@@ -0,0 +1,26 @@
+//compile-flags: --edition 2024 -Zunstable-options
+// run-pass
+// needs-unwind
+#![feature(gen_blocks)]
+
+fn main() {
+    let mut iter = gen {
+        yield 42;
+        panic!("foo");
+        yield 69; //~ WARN: unreachable statement
+    };
+    assert_eq!(iter.next(), Some(42));
+    let mut tmp = std::panic::AssertUnwindSafe(&mut iter);
+    match std::panic::catch_unwind(move || tmp.next()) {
+        Ok(_) => unreachable!(),
+        Err(err) => assert_eq!(*err.downcast::<&'static str>().unwrap(), "foo"),
+    }
+
+    match std::panic::catch_unwind(move || iter.next()) {
+        Ok(_) => unreachable!(),
+        Err(err) => assert_eq!(
+            *err.downcast::<&'static str>().unwrap(),
+            "`gen fn` should just keep returning `None` after panicking",
+        ),
+    }
+}

tests/ui/coroutine/gen_block_panic.stderr

@@ -0,0 +1,12 @@
+warning: unreachable statement
+  --> $DIR/gen_block_panic.rs:10:9
+   |
+LL |         panic!("foo");
+   |         ------------- any code following this expression is unreachable
+LL |         yield 69;
+   |         ^^^^^^^^^ unreachable statement
+   |
+   = note: `#[warn(unreachable_code)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/coroutine/gen_fn.e2024.stderr

@@ -1,10 +1,10 @@
-error: `gen` blocks are not yet implemented
+error: `gen` functions are not yet implemented
   --> $DIR/gen_fn.rs:4:1
    |
 LL | gen fn foo() {}
    | ^^^
    |
-   = help: only the keyword is reserved for now
+   = help: for now you can use `gen {}` blocks and return `impl Iterator` instead
 
 error: aborting due to previous error
 

tests/ui/coroutine/gen_fn.rs

@@ -3,6 +3,6 @@
 
 gen fn foo() {}
 //[none]~^ ERROR: expected one of `#`, `async`, `const`, `default`, `extern`, `fn`, `pub`, `unsafe`, or `use`, found `gen`
-//[e2024]~^^ ERROR: `gen` blocks are not yet implemented
+//[e2024]~^^ ERROR: `gen` functions are not yet implemented
 
 fn main() {}