ast: Avoid aborts on fatal errors thrown from mutable AST visitor

compiler/rustc_ast/src/mut_visit.rs

@@ -14,13 +14,14 @@ use crate::tokenstream::*;
 
 use rustc_data_structures::map_in_place::MapInPlace;
 use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::thin_vec::ThinVec;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::Ident;
 use rustc_span::Span;
 
 use smallvec::{smallvec, Array, SmallVec};
 use std::ops::DerefMut;
-use std::{panic, process, ptr};
+use std::{panic, ptr};
 
 pub trait ExpectOne<A: Array> {
     fn expect_one(self, err: &'static str) -> A::Item;
@@ -283,23 +284,21 @@ pub trait MutVisitor: Sized {
 
 /// Use a map-style function (`FnOnce(T) -> T`) to overwrite a `&mut T`. Useful
 /// when using a `flat_map_*` or `filter_map_*` method within a `visit_`
-/// method. Abort the program if the closure panics.
-///
-/// FIXME: Abort on panic means that any fatal error inside `visit_clobber` will abort the compiler.
-/// Instead of aborting on catching a panic we need to reset the visited node to some valid but
-/// possibly meaningless value and rethrow the panic.
+/// method.
 //
 // No `noop_` prefix because there isn't a corresponding method in `MutVisitor`.
-pub fn visit_clobber<T, F>(t: &mut T, f: F)
-where
-    F: FnOnce(T) -> T,
-{
+pub fn visit_clobber<T: DummyAstNode>(t: &mut T, f: impl FnOnce(T) -> T) {
     unsafe {
         // Safe because `t` is used in a read-only fashion by `read()` before
         // being overwritten by `write()`.
         let old_t = ptr::read(t);
-        let new_t = panic::catch_unwind(panic::AssertUnwindSafe(|| f(old_t)))
-            .unwrap_or_else(|_| process::abort());
+        let new_t =
+            panic::catch_unwind(panic::AssertUnwindSafe(|| f(old_t))).unwrap_or_else(|err| {
+                // Set `t` to some valid but possible meaningless value,
+                // and pass the fatal error further.
+                ptr::write(t, T::dummy());
+                panic::resume_unwind(err);
+            });
         ptr::write(t, new_t);
     }
 }
@@ -1454,3 +1453,108 @@ pub fn noop_visit_vis<T: MutVisitor>(visibility: &mut Visibility, vis: &mut T) {
     }
     vis.visit_span(&mut visibility.span);
 }
+
+/// Some value for the AST node that is valid but possibly meaningless.
+pub trait DummyAstNode {
+    fn dummy() -> Self;
+}
+
+impl<T> DummyAstNode for Option<T> {
+    fn dummy() -> Self {
+        Default::default()
+    }
+}
+
+impl<T: DummyAstNode + 'static> DummyAstNode for P<T> {
+    fn dummy() -> Self {
+        P(DummyAstNode::dummy())
+    }
+}
+
+impl<T> DummyAstNode for ThinVec<T> {
+    fn dummy() -> Self {
+        Default::default()
+    }
+}
+
+impl DummyAstNode for Item {
+    fn dummy() -> Self {
+        Item {
+            attrs: Default::default(),
+            id: DUMMY_NODE_ID,
+            span: Default::default(),
+            vis: Visibility {
+                kind: VisibilityKind::Public,
+                span: Default::default(),
+                tokens: Default::default(),
+            },
+            ident: Ident::empty(),
+            kind: ItemKind::ExternCrate(None),
+            tokens: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Expr {
+    fn dummy() -> Self {
+        Expr {
+            id: DUMMY_NODE_ID,
+            kind: ExprKind::Err,
+            span: Default::default(),
+            attrs: Default::default(),
+            tokens: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Ty {
+    fn dummy() -> Self {
+        Ty {
+            id: DUMMY_NODE_ID,
+            kind: TyKind::Err,
+            span: Default::default(),
+            tokens: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Pat {
+    fn dummy() -> Self {
+        Pat {
+            id: DUMMY_NODE_ID,
+            kind: PatKind::Wild,
+            span: Default::default(),
+            tokens: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Stmt {
+    fn dummy() -> Self {
+        Stmt { id: DUMMY_NODE_ID, kind: StmtKind::Empty, span: Default::default() }
+    }
+}
+
+impl DummyAstNode for Block {
+    fn dummy() -> Self {
+        Block {
+            stmts: Default::default(),
+            id: DUMMY_NODE_ID,
+            rules: BlockCheckMode::Default,
+            span: Default::default(),
+            tokens: Default::default(),
+            could_be_bare_literal: Default::default(),
+        }
+    }
+}
+
+impl DummyAstNode for Crate {
+    fn dummy() -> Self {
+        Crate {
+            attrs: Default::default(),
+            items: Default::default(),
+            span: Default::default(),
+            is_placeholder: Default::default(),
+        }
+    }
+}

compiler/rustc_expand/src/expand.rs

@@ -1160,13 +1160,18 @@ macro_rules! assign_id {
 
 impl<'a, 'b> MutVisitor for InvocationCollector<'a, 'b> {
     fn visit_crate(&mut self, krate: &mut ast::Crate) {
-        let span = krate.span;
-        let empty_crate =
-            || ast::Crate { attrs: Vec::new(), items: Vec::new(), span, is_placeholder: None };
-        let mut fold_crate = |krate: ast::Crate| {
+        visit_clobber(krate, |krate| {
+            let span = krate.span;
             let mut krate = match self.configure(krate) {
                 Some(krate) => krate,
-                None => return empty_crate(),
+                None => {
+                    return ast::Crate {
+                        attrs: Vec::new(),
+                        items: Vec::new(),
+                        span,
+                        is_placeholder: None,
+                    };
+                }
             };
 
             if let Some(attr) = self.take_first_attr(&mut krate) {
@@ -1177,10 +1182,7 @@ impl<'a, 'b> MutVisitor for InvocationCollector<'a, 'b> {
 
             noop_visit_crate(&mut krate, self);
             krate
-        };
-
-        // Cannot use `visit_clobber` here, see the FIXME on it.
-        *krate = fold_crate(mem::replace(krate, empty_crate()));
+        })
     }
 
     fn visit_expr(&mut self, expr: &mut P<ast::Expr>) {