Merge #11444

11444: feat: Fix up syntax errors in attribute macro inputs to make completion work more often r=flodiebold a=flodiebold

This implements the "fix up syntax nodes" workaround mentioned in #11014. It isn't much more than a proof of concept; I have only implemented a few cases, but it already helps quite a bit.

Some notes:
 - I'm not super happy about how much the fixup procedure needs to interact with the syntax node -> token tree conversion code (e.g. needing to share the token map). This could maybe be simplified with some refactoring of that code.
 - It would maybe be nice to have the fixup procedure reuse or share information with the parser, though I'm not really sure how much that would actually help.

Co-authored-by: Florian Diebold <flodiebold@gmail.com>

14 files changed, 646 insertions, 95 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 77d856cb30f..5ccc61a83c1 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -539,6 +539,7 @@ dependencies = [
  "cfg",
  "cov-mark",
  "either",
+ "expect-test",
  "hashbrown 0.12.0",
  "itertools",
  "la-arena",
diff --git a/crates/base_db/src/fixture.rs b/crates/base_db/src/fixture.rs
index 5326a5a4404..af825a2e00b 100644
--- a/crates/base_db/src/fixture.rs
+++ b/crates/base_db/src/fixture.rs
@@ -43,6 +43,17 @@ pub trait WithFixture: Default + SourceDatabaseExt + 'static {
         db
     }
 
+    fn with_files_extra_proc_macros(
+        ra_fixture: &str,
+        proc_macros: Vec<(String, ProcMacro)>,
+    ) -> Self {
+        let fixture = ChangeFixture::parse_with_proc_macros(ra_fixture, proc_macros);
+        let mut db = Self::default();
+        fixture.change.apply(&mut db);
+        assert!(fixture.file_position.is_none());
+        db
+    }
+
     fn with_position(ra_fixture: &str) -> (Self, FilePosition) {
         let (db, file_id, range_or_offset) = Self::with_range_or_offset(ra_fixture);
         let offset = range_or_offset.expect_offset();
@@ -84,7 +95,14 @@ pub struct ChangeFixture {
 
 impl ChangeFixture {
     pub fn parse(ra_fixture: &str) -> ChangeFixture {
-        let (mini_core, proc_macros, fixture) = Fixture::parse(ra_fixture);
+        Self::parse_with_proc_macros(ra_fixture, Vec::new())
+    }
+
+    pub fn parse_with_proc_macros(
+        ra_fixture: &str,
+        mut proc_macros: Vec<(String, ProcMacro)>,
+    ) -> ChangeFixture {
+        let (mini_core, proc_macro_names, fixture) = Fixture::parse(ra_fixture);
         let mut change = Change::new();
 
         let mut files = Vec::new();
@@ -222,11 +240,12 @@ impl ChangeFixture {
             }
         }
 
-        if !proc_macros.is_empty() {
+        if !proc_macro_names.is_empty() {
             let proc_lib_file = file_id;
             file_id.0 += 1;
 
-            let (proc_macro, source) = test_proc_macros(&proc_macros);
+            proc_macros.extend(default_test_proc_macros());
+            let (proc_macro, source) = filter_test_proc_macros(&proc_macro_names, proc_macros);
             let mut fs = FileSet::default();
             fs.insert(
                 proc_lib_file,
@@ -272,52 +291,84 @@ impl ChangeFixture {
     }
 }
 
-fn test_proc_macros(proc_macros: &[String]) -> (Vec<ProcMacro>, String) {
-    // The source here is only required so that paths to the macros exist and are resolvable.
-    let source = r#"
+fn default_test_proc_macros() -> [(String, ProcMacro); 4] {
+    [
+        (
+            r#"
 #[proc_macro_attribute]
 pub fn identity(_attr: TokenStream, item: TokenStream) -> TokenStream {
     item
 }
+"#
+            .into(),
+            ProcMacro {
+                name: "identity".into(),
+                kind: crate::ProcMacroKind::Attr,
+                expander: Arc::new(IdentityProcMacroExpander),
+            },
+        ),
+        (
+            r#"
 #[proc_macro_derive(DeriveIdentity)]
 pub fn derive_identity(item: TokenStream) -> TokenStream {
     item
 }
+"#
+            .into(),
+            ProcMacro {
+                name: "DeriveIdentity".into(),
+                kind: crate::ProcMacroKind::CustomDerive,
+                expander: Arc::new(IdentityProcMacroExpander),
+            },
+        ),
+        (
+            r#"
 #[proc_macro_attribute]
 pub fn input_replace(attr: TokenStream, _item: TokenStream) -> TokenStream {
     attr
 }
+"#
+            .into(),
+            ProcMacro {
+                name: "input_replace".into(),
+                kind: crate::ProcMacroKind::Attr,
+                expander: Arc::new(AttributeInputReplaceProcMacroExpander),
+            },
+        ),
+        (
+            r#"
 #[proc_macro]
 pub fn mirror(input: TokenStream) -> TokenStream {
     input
 }
-"#;
-    let proc_macros = [
-        ProcMacro {
-            name: "identity".into(),
-            kind: crate::ProcMacroKind::Attr,
-            expander: Arc::new(IdentityProcMacroExpander),
-        },
-        ProcMacro {
-            name: "DeriveIdentity".into(),
-            kind: crate::ProcMacroKind::CustomDerive,
-            expander: Arc::new(IdentityProcMacroExpander),
-        },
-        ProcMacro {
-            name: "input_replace".into(),
-            kind: crate::ProcMacroKind::Attr,
-            expander: Arc::new(AttributeInputReplaceProcMacroExpander),
-        },
-        ProcMacro {
-            name: "mirror".into(),
-            kind: crate::ProcMacroKind::FuncLike,
-            expander: Arc::new(MirrorProcMacroExpander),
-        },
+"#
+            .into(),
+            ProcMacro {
+                name: "mirror".into(),
+                kind: crate::ProcMacroKind::FuncLike,
+                expander: Arc::new(MirrorProcMacroExpander),
+            },
+        ),
     ]
-    .into_iter()
-    .filter(|pm| proc_macros.iter().any(|name| name == &stdx::to_lower_snake_case(&pm.name)))
-    .collect();
-    (proc_macros, source.into())
+}
+
+fn filter_test_proc_macros(
+    proc_macro_names: &[String],
+    proc_macro_defs: Vec<(String, ProcMacro)>,
+) -> (Vec<ProcMacro>, String) {
+    // The source here is only required so that paths to the macros exist and are resolvable.
+    let mut source = String::new();
+    let mut proc_macros = Vec::new();
+
+    for (c, p) in proc_macro_defs {
+        if !proc_macro_names.iter().any(|name| name == &stdx::to_lower_snake_case(&p.name)) {
+            continue;
+        }
+        proc_macros.push(p);
+        source += &c;
+    }
+
+    (proc_macros, source)
 }
 
 #[derive(Debug, Clone, Copy)]
diff --git a/crates/hir_def/src/macro_expansion_tests.rs b/crates/hir_def/src/macro_expansion_tests.rs
index c3116edc88e..1a7d9aa8411 100644
--- a/crates/hir_def/src/macro_expansion_tests.rs
+++ b/crates/hir_def/src/macro_expansion_tests.rs
@@ -14,10 +14,10 @@ mod builtin_fn_macro;
 mod builtin_derive_macro;
 mod proc_macros;
 
-use std::{iter, ops::Range};
+use std::{iter, ops::Range, sync::Arc};
 
 use ::mbe::TokenMap;
-use base_db::{fixture::WithFixture, SourceDatabase};
+use base_db::{fixture::WithFixture, ProcMacro, SourceDatabase};
 use expect_test::Expect;
 use hir_expand::{
     db::{AstDatabase, TokenExpander},
@@ -39,7 +39,21 @@ use crate::{
 
 #[track_caller]
 fn check(ra_fixture: &str, mut expect: Expect) {
-    let db = TestDB::with_files(ra_fixture);
+    let extra_proc_macros = vec![(
+        r#"
+#[proc_macro_attribute]
+pub fn identity_when_valid(_attr: TokenStream, item: TokenStream) -> TokenStream {
+    item
+}
+"#
+        .into(),
+        ProcMacro {
+            name: "identity_when_valid".into(),
+            kind: base_db::ProcMacroKind::Attr,
+            expander: Arc::new(IdentityWhenValidProcMacroExpander),
+        },
+    )];
+    let db = TestDB::with_files_extra_proc_macros(ra_fixture, extra_proc_macros);
     let krate = db.crate_graph().iter().next().unwrap();
     let def_map = db.crate_def_map(krate);
     let local_id = def_map.root();
@@ -172,7 +186,7 @@ fn check(ra_fixture: &str, mut expect: Expect) {
         let range: Range<usize> = range.into();
 
         if show_token_ids {
-            if let Some((tree, map)) = arg.as_deref() {
+            if let Some((tree, map, _)) = arg.as_deref() {
                 let tt_range = call.token_tree().unwrap().syntax().text_range();
                 let mut ranges = Vec::new();
                 extract_id_ranges(&mut ranges, &map, &tree);
@@ -201,10 +215,19 @@ fn check(ra_fixture: &str, mut expect: Expect) {
     }
 
     for decl_id in def_map[local_id].scope.declarations() {
-        if let ModuleDefId::AdtId(AdtId::StructId(struct_id)) = decl_id {
-            let src = struct_id.lookup(&db).source(&db);
+        // FIXME: I'm sure there's already better way to do this
+        let src = match decl_id {
+            ModuleDefId::AdtId(AdtId::StructId(struct_id)) => {
+                Some(struct_id.lookup(&db).source(&db).syntax().cloned())
+            }
+            ModuleDefId::FunctionId(function_id) => {
+                Some(function_id.lookup(&db).source(&db).syntax().cloned())
+            }
+            _ => None,
+        };
+        if let Some(src) = src {
             if src.file_id.is_attr_macro(&db) || src.file_id.is_custom_derive(&db) {
-                let pp = pretty_print_macro_expansion(src.value.syntax().clone(), None);
+                let pp = pretty_print_macro_expansion(src.value, None);
                 format_to!(expanded_text, "\n{}", pp)
             }
         }
@@ -304,3 +327,25 @@ fn pretty_print_macro_expansion(expn: SyntaxNode, map: Option<&TokenMap>) -> Str
     }
     res
 }
+
+// Identity mapping, but only works when the input is syntactically valid. This
+// simulates common proc macros that unnecessarily parse their input and return
+// compile errors.
+#[derive(Debug)]
+struct IdentityWhenValidProcMacroExpander;
+impl base_db::ProcMacroExpander for IdentityWhenValidProcMacroExpander {
+    fn expand(
+        &self,
+        subtree: &Subtree,
+        _: Option<&Subtree>,
+        _: &base_db::Env,
+    ) -> Result<Subtree, base_db::ProcMacroExpansionError> {
+        let (parse, _) =
+            ::mbe::token_tree_to_syntax_node(subtree, ::mbe::TopEntryPoint::MacroItems);
+        if parse.errors().is_empty() {
+            Ok(subtree.clone())
+        } else {
+            panic!("got invalid macro input: {:?}", parse.errors());
+        }
+    }
+}
diff --git a/crates/hir_def/src/macro_expansion_tests/proc_macros.rs b/crates/hir_def/src/macro_expansion_tests/proc_macros.rs
index 901872eddab..0ca30fb7992 100644
--- a/crates/hir_def/src/macro_expansion_tests/proc_macros.rs
+++ b/crates/hir_def/src/macro_expansion_tests/proc_macros.rs
@@ -52,3 +52,43 @@ struct S;
 #[attr2] struct S;"##]],
     );
 }
+
+#[test]
+fn attribute_macro_syntax_completion_1() {
+    // this is just the case where the input is actually valid
+    check(
+        r#"
+//- proc_macros: identity_when_valid
+#[proc_macros::identity_when_valid]
+fn foo() { bar.baz(); blub }
+"#,
+        expect![[r##"
+#[proc_macros::identity_when_valid]
+fn foo() { bar.baz(); blub }
+
+fn foo() {
+    bar.baz();
+    blub
+}"##]],
+    );
+}
+
+#[test]
+fn attribute_macro_syntax_completion_2() {
+    // common case of dot completion while typing
+    check(
+        r#"
+//- proc_macros: identity_when_valid
+#[proc_macros::identity_when_valid]
+fn foo() { bar.; blub }
+"#,
+        expect![[r##"
+#[proc_macros::identity_when_valid]
+fn foo() { bar.; blub }
+
+fn foo() {
+    bar. ;
+    blub
+}"##]],
+    );
+}
diff --git a/crates/hir_expand/Cargo.toml b/crates/hir_expand/Cargo.toml
index e5455660d3a..d7b4cbf82e2 100644
--- a/crates/hir_expand/Cargo.toml
+++ b/crates/hir_expand/Cargo.toml
@@ -27,3 +27,6 @@ profile = { path = "../profile", version = "0.0.0" }
 tt = { path = "../tt", version = "0.0.0" }
 mbe = { path = "../mbe", version = "0.0.0" }
 limit = { path = "../limit", version = "0.0.0" }
+
+[dev-dependencies]
+expect-test = "1.2.0-pre.1"
diff --git a/crates/hir_expand/src/db.rs b/crates/hir_expand/src/db.rs
index d9bfdd6fd4f..75766a54a74 100644
--- a/crates/hir_expand/src/db.rs
+++ b/crates/hir_expand/src/db.rs
@@ -14,7 +14,7 @@ use syntax::{
 };
 
 use crate::{
-    ast_id_map::AstIdMap, hygiene::HygieneFrame, BuiltinAttrExpander, BuiltinDeriveExpander,
+    ast_id_map::AstIdMap, fixup, hygiene::HygieneFrame, BuiltinAttrExpander, BuiltinDeriveExpander,
     BuiltinFnLikeExpander, ExpandTo, HirFileId, HirFileIdRepr, MacroCallId, MacroCallKind,
     MacroCallLoc, MacroDefId, MacroDefKind, MacroFile, ProcMacroExpander,
 };
@@ -108,7 +108,10 @@ pub trait AstDatabase: SourceDatabase {
 
     /// Lowers syntactic macro call to a token tree representation.
     #[salsa::transparent]
-    fn macro_arg(&self, id: MacroCallId) -> Option<Arc<(tt::Subtree, mbe::TokenMap)>>;
+    fn macro_arg(
+        &self,
+        id: MacroCallId,
+    ) -> Option<Arc<(tt::Subtree, mbe::TokenMap, fixup::SyntaxFixupUndoInfo)>>;
     /// Extracts syntax node, corresponding to a macro call. That's a firewall
     /// query, only typing in the macro call itself changes the returned
     /// subtree.
@@ -146,8 +149,15 @@ pub fn expand_speculative(
 
     // Build the subtree and token mapping for the speculative args
     let censor = censor_for_macro_input(&loc, &speculative_args);
-    let (mut tt, spec_args_tmap) =
-        mbe::syntax_node_to_token_tree_censored(&speculative_args, &censor);
+    let mut fixups = fixup::fixup_syntax(&speculative_args);
+    fixups.replace.extend(censor.into_iter().map(|node| (node, Vec::new())));
+    let (mut tt, spec_args_tmap, _) = mbe::syntax_node_to_token_tree_with_modifications(
+        &speculative_args,
+        fixups.token_map,
+        fixups.next_id,
+        fixups.replace,
+        fixups.append,
+    );
 
     let (attr_arg, token_id) = match loc.kind {
         MacroCallKind::Attr { invoc_attr_index, .. } => {
@@ -194,7 +204,7 @@ pub fn expand_speculative(
 
     // Do the actual expansion, we need to directly expand the proc macro due to the attribute args
     // Otherwise the expand query will fetch the non speculative attribute args and pass those instead.
-    let speculative_expansion = if let MacroDefKind::ProcMacro(expander, ..) = loc.def.kind {
+    let mut speculative_expansion = if let MacroDefKind::ProcMacro(expander, ..) = loc.def.kind {
         tt.delimiter = None;
         expander.expand(db, loc.krate, &tt, attr_arg.as_ref())
     } else {
@@ -202,6 +212,7 @@ pub fn expand_speculative(
     };
 
     let expand_to = macro_expand_to(db, actual_macro_call);
+    fixup::reverse_fixups(&mut speculative_expansion.value, &spec_args_tmap, &fixups.undo_info);
     let (node, rev_tmap) = token_tree_to_syntax_node(&speculative_expansion.value, expand_to);
 
     let range = rev_tmap.first_range_by_token(token_id, token_to_map.kind())?;
@@ -289,20 +300,31 @@ fn parse_macro_expansion(
     }
 }
 
-fn macro_arg(db: &dyn AstDatabase, id: MacroCallId) -> Option<Arc<(tt::Subtree, mbe::TokenMap)>> {
+fn macro_arg(
+    db: &dyn AstDatabase,
+    id: MacroCallId,
+) -> Option<Arc<(tt::Subtree, mbe::TokenMap, fixup::SyntaxFixupUndoInfo)>> {
     let arg = db.macro_arg_text(id)?;
     let loc = db.lookup_intern_macro_call(id);
 
     let node = SyntaxNode::new_root(arg);
     let censor = censor_for_macro_input(&loc, &node);
-    let (mut tt, tmap) = mbe::syntax_node_to_token_tree_censored(&node, &censor);
+    let mut fixups = fixup::fixup_syntax(&node);
+    fixups.replace.extend(censor.into_iter().map(|node| (node, Vec::new())));
+    let (mut tt, tmap, _) = mbe::syntax_node_to_token_tree_with_modifications(
+        &node,
+        fixups.token_map,
+        fixups.next_id,
+        fixups.replace,
+        fixups.append,
+    );
 
     if loc.def.is_proc_macro() {
         // proc macros expect their inputs without parentheses, MBEs expect it with them included
         tt.delimiter = None;
     }
 
-    Some(Arc::new((tt, tmap)))
+    Some(Arc::new((tt, tmap, fixups.undo_info)))
 }
 
 fn censor_for_macro_input(loc: &MacroCallLoc, node: &SyntaxNode) -> FxHashSet<SyntaxNode> {
@@ -419,10 +441,9 @@ fn macro_expand(db: &dyn AstDatabase, id: MacroCallId) -> ExpandResult<Option<Ar
         // be reported at the definition site (when we construct a def map).
         Err(err) => return ExpandResult::str_err(format!("invalid macro definition: {}", err)),
     };
-    let ExpandResult { value: tt, err } = expander.expand(db, id, &macro_arg.0);
+    let ExpandResult { value: mut tt, err } = expander.expand(db, id, &macro_arg.0);
     // Set a hard limit for the expanded tt
     let count = tt.count();
-    // XXX: Make ExpandResult a real error and use .map_err instead?
     if TOKEN_LIMIT.check(count).is_err() {
         return ExpandResult::str_err(format!(
             "macro invocation exceeds token limit: produced {} tokens, limit is {}",
@@ -431,6 +452,8 @@ fn macro_expand(db: &dyn AstDatabase, id: MacroCallId) -> ExpandResult<Option<Ar
         ));
     }
 
+    fixup::reverse_fixups(&mut tt, &macro_arg.1, &macro_arg.2);
+
     ExpandResult { value: Some(Arc::new(tt)), err }
 }
 
diff --git a/crates/hir_expand/src/fixup.rs b/crates/hir_expand/src/fixup.rs
new file mode 100644
index 00000000000..2eb3da79dc6
--- /dev/null
+++ b/crates/hir_expand/src/fixup.rs
@@ -0,0 +1,261 @@
+//! To make attribute macros work reliably when typing, we need to take care to
+//! fix up syntax errors in the code we're passing to them.
+use std::mem;
+
+use mbe::{SyntheticToken, SyntheticTokenId, TokenMap};
+use rustc_hash::FxHashMap;
+use syntax::{
+    ast::{self, AstNode},
+    match_ast, SyntaxKind, SyntaxNode, TextRange,
+};
+use tt::Subtree;
+
+/// The result of calculating fixes for a syntax node -- a bunch of changes
+/// (appending to and replacing nodes), the information that is needed to
+/// reverse those changes afterwards, and a token map.
+#[derive(Debug)]
+pub(crate) struct SyntaxFixups {
+    pub(crate) append: FxHashMap<SyntaxNode, Vec<SyntheticToken>>,
+    pub(crate) replace: FxHashMap<SyntaxNode, Vec<SyntheticToken>>,
+    pub(crate) undo_info: SyntaxFixupUndoInfo,
+    pub(crate) token_map: TokenMap,
+    pub(crate) next_id: u32,
+}
+
+/// This is the information needed to reverse the fixups.
+#[derive(Debug, PartialEq, Eq)]
+pub struct SyntaxFixupUndoInfo {
+    original: Vec<Subtree>,
+}
+
+const EMPTY_ID: SyntheticTokenId = SyntheticTokenId(!0);
+
+pub(crate) fn fixup_syntax(node: &SyntaxNode) -> SyntaxFixups {
+    let mut append = FxHashMap::default();
+    let mut replace = FxHashMap::default();
+    let mut preorder = node.preorder();
+    let mut original = Vec::new();
+    let mut token_map = TokenMap::default();
+    let mut next_id = 0;
+    while let Some(event) = preorder.next() {
+        let node = match event {
+            syntax::WalkEvent::Enter(node) => node,
+            syntax::WalkEvent::Leave(_) => continue,
+        };
+
+        if can_handle_error(&node) && has_error_to_handle(&node) {
+            // the node contains an error node, we have to completely replace it by something valid
+            let (original_tree, new_tmap, new_next_id) =
+                mbe::syntax_node_to_token_tree_with_modifications(
+                    &node,
+                    mem::take(&mut token_map),
+                    next_id,
+                    Default::default(),
+                    Default::default(),
+                );
+            token_map = new_tmap;
+            next_id = new_next_id;
+            let idx = original.len() as u32;
+            original.push(original_tree);
+            let replacement = SyntheticToken {
+                kind: SyntaxKind::IDENT,
+                text: "__ra_fixup".into(),
+                range: node.text_range(),
+                id: SyntheticTokenId(idx),
+            };
+            replace.insert(node.clone(), vec![replacement]);
+            preorder.skip_subtree();
+            continue;
+        }
+
+        // In some other situations, we can fix things by just appending some tokens.
+        let end_range = TextRange::empty(node.text_range().end());
+        match_ast! {
+            match node {
+                ast::FieldExpr(it) => {
+                    if it.name_ref().is_none() {
+                        // incomplete field access: some_expr.|
+                        append.insert(node.clone(), vec![
+                            SyntheticToken {
+                                kind: SyntaxKind::IDENT,
+                                text: "__ra_fixup".into(),
+                                range: end_range,
+                                id: EMPTY_ID,
+                            },
+                        ]);
+                    }
+                },
+                ast::ExprStmt(it) => {
+                    if it.semicolon_token().is_none() {
+                        append.insert(node.clone(), vec![
+                            SyntheticToken {
+                                kind: SyntaxKind::SEMICOLON,
+                                text: ";".into(),
+                                range: end_range,
+                                id: EMPTY_ID,
+                            },
+                        ]);
+                    }
+                },
+                _ => (),
+            }
+        }
+    }
+    SyntaxFixups {
+        append,
+        replace,
+        token_map,
+        next_id,
+        undo_info: SyntaxFixupUndoInfo { original },
+    }
+}
+
+fn has_error(node: &SyntaxNode) -> bool {
+    node.children().any(|c| c.kind() == SyntaxKind::ERROR)
+}
+
+fn can_handle_error(node: &SyntaxNode) -> bool {
+    ast::Expr::can_cast(node.kind())
+}
+
+fn has_error_to_handle(node: &SyntaxNode) -> bool {
+    has_error(node) || node.children().any(|c| !can_handle_error(&c) && has_error_to_handle(&c))
+}
+
+pub(crate) fn reverse_fixups(
+    tt: &mut Subtree,
+    token_map: &TokenMap,
+    undo_info: &SyntaxFixupUndoInfo,
+) {
+    tt.token_trees.retain(|tt| match tt {
+        tt::TokenTree::Leaf(leaf) => {
+            token_map.synthetic_token_id(leaf.id()).is_none()
+                || token_map.synthetic_token_id(leaf.id()) != Some(EMPTY_ID)
+        }
+        _ => true,
+    });
+    tt.token_trees.iter_mut().for_each(|tt| match tt {
+        tt::TokenTree::Subtree(tt) => reverse_fixups(tt, token_map, undo_info),
+        tt::TokenTree::Leaf(leaf) => {
+            if let Some(id) = token_map.synthetic_token_id(leaf.id()) {
+                let original = &undo_info.original[id.0 as usize];
+                *tt = tt::TokenTree::Subtree(original.clone());
+            }
+        }
+    });
+}
+
+#[cfg(test)]
+mod tests {
+    use expect_test::{expect, Expect};
+
+    use super::reverse_fixups;
+
+    #[track_caller]
+    fn check(ra_fixture: &str, mut expect: Expect) {
+        let parsed = syntax::SourceFile::parse(ra_fixture);
+        let fixups = super::fixup_syntax(&parsed.syntax_node());
+        let (mut tt, tmap, _) = mbe::syntax_node_to_token_tree_with_modifications(
+            &parsed.syntax_node(),
+            fixups.token_map,
+            fixups.next_id,
+            fixups.replace,
+            fixups.append,
+        );
+
+        let mut actual = tt.to_string();
+        actual.push_str("\n");
+
+        expect.indent(false);
+        expect.assert_eq(&actual);
+
+        // the fixed-up tree should be syntactically valid
+        let (parse, _) = mbe::token_tree_to_syntax_node(&tt, ::mbe::TopEntryPoint::MacroItems);
+        assert_eq!(
+            parse.errors(),
+            &[],
+            "parse has syntax errors. parse tree:\n{:#?}",
+            parse.syntax_node()
+        );
+
+        reverse_fixups(&mut tt, &tmap, &fixups.undo_info);
+
+        // the fixed-up + reversed version should be equivalent to the original input
+        // (but token IDs don't matter)
+        let (original_as_tt, _) = mbe::syntax_node_to_token_tree(&parsed.syntax_node());
+        assert_eq!(tt.to_string(), original_as_tt.to_string());
+    }
+
+    #[test]
+    fn incomplete_field_expr_1() {
+        check(
+            r#"
+fn foo() {
+    a.
+}
+"#,
+            expect![[r#"
+fn foo () {a . __ra_fixup}
+"#]],
+        )
+    }
+
+    #[test]
+    fn incomplete_field_expr_2() {
+        check(
+            r#"
+fn foo() {
+    a. ;
+}
+"#,
+            expect![[r#"
+fn foo () {a . __ra_fixup ;}
+"#]],
+        )
+    }
+
+    #[test]
+    fn incomplete_field_expr_3() {
+        check(
+            r#"
+fn foo() {
+    a. ;
+    bar();
+}
+"#,
+            expect![[r#"
+fn foo () {a . __ra_fixup ; bar () ;}
+"#]],
+        )
+    }
+
+    #[test]
+    fn field_expr_before_call() {
+        // another case that easily happens while typing
+        check(
+            r#"
+fn foo() {
+    a.b
+    bar();
+}
+"#,
+            expect![[r#"
+fn foo () {a . b ; bar () ;}
+"#]],
+        )
+    }
+
+    #[test]
+    fn extraneous_comma() {
+        check(
+            r#"
+fn foo() {
+    bar(,);
+}
+"#,
+            expect![[r#"
+fn foo () {__ra_fixup ;}
+"#]],
+        )
+    }
+}
diff --git a/crates/hir_expand/src/hygiene.rs b/crates/hir_expand/src/hygiene.rs
index d2b719bf570..d60734372c0 100644
--- a/crates/hir_expand/src/hygiene.rs
+++ b/crates/hir_expand/src/hygiene.rs
@@ -15,6 +15,7 @@ use syntax::{
 
 use crate::{
     db::{self, AstDatabase},
+    fixup,
     name::{AsName, Name},
     HirFileId, HirFileIdRepr, InFile, MacroCallKind, MacroCallLoc, MacroDefKind, MacroFile,
 };
@@ -127,7 +128,7 @@ struct HygieneInfo {
     attr_input_or_mac_def_start: Option<InFile<TextSize>>,
 
     macro_def: Arc<TokenExpander>,
-    macro_arg: Arc<(tt::Subtree, mbe::TokenMap)>,
+    macro_arg: Arc<(tt::Subtree, mbe::TokenMap, fixup::SyntaxFixupUndoInfo)>,
     macro_arg_shift: mbe::Shift,
     exp_map: Arc<mbe::TokenMap>,
 }
diff --git a/crates/hir_expand/src/lib.rs b/crates/hir_expand/src/lib.rs
index 136d62ef3b0..279fdc61dca 100644
--- a/crates/hir_expand/src/lib.rs
+++ b/crates/hir_expand/src/lib.rs
@@ -15,6 +15,7 @@ pub mod proc_macro;
 pub mod quote;
 pub mod eager;
 pub mod mod_path;
+mod fixup;
 
 pub use mbe::{ExpandError, ExpandResult, Origin};
 
@@ -426,7 +427,7 @@ pub struct ExpansionInfo {
     attr_input_or_mac_def: Option<InFile<ast::TokenTree>>,
 
     macro_def: Arc<TokenExpander>,
-    macro_arg: Arc<(tt::Subtree, mbe::TokenMap)>,
+    macro_arg: Arc<(tt::Subtree, mbe::TokenMap, fixup::SyntaxFixupUndoInfo)>,
     /// A shift built from `macro_arg`'s subtree, relevant for attributes as the item is the macro arg
     /// and as such we need to shift tokens if they are part of an attributes input instead of their item.
     macro_arg_shift: mbe::Shift,
diff --git a/crates/ide_completion/src/tests/attribute.rs b/crates/ide_completion/src/tests/attribute.rs
index 5a9c48a3278..2e643453afc 100644
--- a/crates/ide_completion/src/tests/attribute.rs
+++ b/crates/ide_completion/src/tests/attribute.rs
@@ -62,8 +62,7 @@ fn proc_macros_qualified() {
 struct Foo;
 "#,
         expect![[r#"
-            at input_replace pub macro input_replace
-            at identity      pub macro identity
+            at identity pub macro identity
         "#]],
     )
 }
diff --git a/crates/mbe/src/lib.rs b/crates/mbe/src/lib.rs
index 135282a3cb0..07b7f4d1a5e 100644
--- a/crates/mbe/src/lib.rs
+++ b/crates/mbe/src/lib.rs
@@ -30,7 +30,8 @@ pub use tt::{Delimiter, DelimiterKind, Punct};
 pub use crate::{
     syntax_bridge::{
         parse_exprs_with_sep, parse_to_token_tree, syntax_node_to_token_tree,
-        syntax_node_to_token_tree_censored, token_tree_to_syntax_node,
+        syntax_node_to_token_tree_with_modifications, token_tree_to_syntax_node, SyntheticToken,
+        SyntheticTokenId,
     },
     token_map::TokenMap,
 };
diff --git a/crates/mbe/src/syntax_bridge.rs b/crates/mbe/src/syntax_bridge.rs
index bac1d4ba982..7e12647cd86 100644
--- a/crates/mbe/src/syntax_bridge.rs
+++ b/crates/mbe/src/syntax_bridge.rs
@@ -1,7 +1,7 @@
 //! Conversions between [`SyntaxNode`] and [`tt::TokenTree`].
 
-use rustc_hash::{FxHashMap, FxHashSet};
-use stdx::non_empty_vec::NonEmptyVec;
+use rustc_hash::FxHashMap;
+use stdx::{always, non_empty_vec::NonEmptyVec};
 use syntax::{
     ast::{self, make::tokens::doc_comment},
     AstToken, Parse, PreorderWithTokens, SmolStr, SyntaxElement, SyntaxKind,
@@ -15,20 +15,43 @@ use crate::{to_parser_input::to_parser_input, tt_iter::TtIter, TokenMap};
 /// Convert the syntax node to a `TokenTree` (what macro
 /// will consume).
 pub fn syntax_node_to_token_tree(node: &SyntaxNode) -> (tt::Subtree, TokenMap) {
-    syntax_node_to_token_tree_censored(node, &Default::default())
+    let (subtree, token_map, _) = syntax_node_to_token_tree_with_modifications(
+        node,
+        Default::default(),
+        0,
+        Default::default(),
+        Default::default(),
+    );
+    (subtree, token_map)
 }
 
 /// Convert the syntax node to a `TokenTree` (what macro will consume)
 /// with the censored range excluded.
-pub fn syntax_node_to_token_tree_censored(
+pub fn syntax_node_to_token_tree_with_modifications(
     node: &SyntaxNode,
-    censor: &FxHashSet<SyntaxNode>,
-) -> (tt::Subtree, TokenMap) {
+    existing_token_map: TokenMap,
+    next_id: u32,
+    replace: FxHashMap<SyntaxNode, Vec<SyntheticToken>>,
+    append: FxHashMap<SyntaxNode, Vec<SyntheticToken>>,
+) -> (tt::Subtree, TokenMap, u32) {
     let global_offset = node.text_range().start();
-    let mut c = Convertor::new(node, global_offset, censor);
+    let mut c = Convertor::new(node, global_offset, existing_token_map, next_id, replace, append);
     let subtree = convert_tokens(&mut c);
     c.id_alloc.map.shrink_to_fit();
-    (subtree, c.id_alloc.map)
+    always!(c.replace.is_empty(), "replace: {:?}", c.replace);
+    always!(c.append.is_empty(), "append: {:?}", c.append);
+    (subtree, c.id_alloc.map, c.id_alloc.next_id)
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub struct SyntheticTokenId(pub u32);
+
+#[derive(Debug, Clone)]
+pub struct SyntheticToken {
+    pub kind: SyntaxKind,
+    pub text: SmolStr,
+    pub range: TextRange,
+    pub id: SyntheticTokenId,
 }
 
 // The following items are what `rustc` macro can be parsed into :
@@ -147,13 +170,14 @@ fn convert_tokens<C: TokenConvertor>(conv: &mut C) -> tt::Subtree {
             Some(it) => it,
             None => break,
         };
+        let synth_id = token.synthetic_id(&conv);
 
         let kind = token.kind(&conv);
         if kind == COMMENT {
             if let Some(tokens) = conv.convert_doc_comment(&token) {
                 // FIXME: There has to be a better way to do this
                 // Add the comments token id to the converted doc string
-                let id = conv.id_alloc().alloc(range);
+                let id = conv.id_alloc().alloc(range, synth_id);
                 result.extend(tokens.into_iter().map(|mut tt| {
                     if let tt::TokenTree::Subtree(sub) = &mut tt {
                         if let Some(tt::TokenTree::Leaf(tt::Leaf::Literal(lit))) =
@@ -168,7 +192,9 @@ fn convert_tokens<C: TokenConvertor>(conv: &mut C) -> tt::Subtree {
             continue;
         }
         let tt = if kind.is_punct() && kind != UNDERSCORE {
-            assert_eq!(range.len(), TextSize::of('.'));
+            if synth_id.is_none() {
+                assert_eq!(range.len(), TextSize::of('.'));
+            }
 
             if let Some(delim) = subtree.delimiter {
                 let expected = match delim.kind {
@@ -220,11 +246,13 @@ fn convert_tokens<C: TokenConvertor>(conv: &mut C) -> tt::Subtree {
                     panic!("Token from lexer must be single char: token = {:#?}", token);
                 }
             };
-            tt::Leaf::from(tt::Punct { char, spacing, id: conv.id_alloc().alloc(range) }).into()
+            tt::Leaf::from(tt::Punct { char, spacing, id: conv.id_alloc().alloc(range, synth_id) })
+                .into()
         } else {
             macro_rules! make_leaf {
                 ($i:ident) => {
-                    tt::$i { id: conv.id_alloc().alloc(range), text: token.to_text(conv) }.into()
+                    tt::$i { id: conv.id_alloc().alloc(range, synth_id), text: token.to_text(conv) }
+                        .into()
                 };
             }
             let leaf: tt::Leaf = match kind {
@@ -239,14 +267,14 @@ fn convert_tokens<C: TokenConvertor>(conv: &mut C) -> tt::Subtree {
                     let apostrophe = tt::Leaf::from(tt::Punct {
                         char: '\'',
                         spacing: tt::Spacing::Joint,
-                        id: conv.id_alloc().alloc(r),
+                        id: conv.id_alloc().alloc(r, synth_id),
                     });
                     result.push(apostrophe.into());
 
                     let r = TextRange::at(range.start() + char_unit, range.len() - char_unit);
                     let ident = tt::Leaf::from(tt::Ident {
                         text: SmolStr::new(&token.to_text(conv)[1..]),
-                        id: conv.id_alloc().alloc(r),
+                        id: conv.id_alloc().alloc(r, synth_id),
                     });
                     result.push(ident.into());
                     continue;
@@ -267,7 +295,7 @@ fn convert_tokens<C: TokenConvertor>(conv: &mut C) -> tt::Subtree {
 
         conv.id_alloc().close_delim(entry.idx, None);
         let leaf: tt::Leaf = tt::Punct {
-            id: conv.id_alloc().alloc(entry.open_range),
+            id: conv.id_alloc().alloc(entry.open_range, None),
             char: match entry.subtree.delimiter.unwrap().kind {
                 tt::DelimiterKind::Parenthesis => '(',
                 tt::DelimiterKind::Brace => '{',
@@ -361,11 +389,18 @@ struct TokenIdAlloc {
 }
 
 impl TokenIdAlloc {
-    fn alloc(&mut self, absolute_range: TextRange) -> tt::TokenId {
+    fn alloc(
+        &mut self,
+        absolute_range: TextRange,
+        synthetic_id: Option<SyntheticTokenId>,
+    ) -> tt::TokenId {
         let relative_range = absolute_range - self.global_offset;
         let token_id = tt::TokenId(self.next_id);
         self.next_id += 1;
         self.map.insert(token_id, relative_range);
+        if let Some(id) = synthetic_id {
+            self.map.insert_synthetic(token_id, id);
+        }
         token_id
     }
 
@@ -405,6 +440,8 @@ trait SrcToken<Ctx>: std::fmt::Debug {
     fn to_char(&self, ctx: &Ctx) -> Option<char>;
 
     fn to_text(&self, ctx: &Ctx) -> SmolStr;
+
+    fn synthetic_id(&self, ctx: &Ctx) -> Option<SyntheticTokenId>;
 }
 
 trait TokenConvertor: Sized {
@@ -431,6 +468,10 @@ impl<'a> SrcToken<RawConvertor<'a>> for usize {
     fn to_text(&self, ctx: &RawConvertor<'_>) -> SmolStr {
         ctx.lexed.text(*self).into()
     }
+
+    fn synthetic_id(&self, _ctx: &RawConvertor<'a>) -> Option<SyntheticTokenId> {
+        None
+    }
 }
 
 impl<'a> TokenConvertor for RawConvertor<'a> {
@@ -465,86 +506,130 @@ impl<'a> TokenConvertor for RawConvertor<'a> {
     }
 }
 
-struct Convertor<'c> {
+struct Convertor {
     id_alloc: TokenIdAlloc,
     current: Option<SyntaxToken>,
+    current_synthetic: Vec<SyntheticToken>,
     preorder: PreorderWithTokens,
-    censor: &'c FxHashSet<SyntaxNode>,
+    replace: FxHashMap<SyntaxNode, Vec<SyntheticToken>>,
+    append: FxHashMap<SyntaxNode, Vec<SyntheticToken>>,
     range: TextRange,
     punct_offset: Option<(SyntaxToken, TextSize)>,
 }
 
-impl<'c> Convertor<'c> {
+impl Convertor {
     fn new(
         node: &SyntaxNode,
         global_offset: TextSize,
-        censor: &'c FxHashSet<SyntaxNode>,
-    ) -> Convertor<'c> {
+        existing_token_map: TokenMap,
+        next_id: u32,
+        mut replace: FxHashMap<SyntaxNode, Vec<SyntheticToken>>,
+        mut append: FxHashMap<SyntaxNode, Vec<SyntheticToken>>,
+    ) -> Convertor {
         let range = node.text_range();
         let mut preorder = node.preorder_with_tokens();
-        let first = Self::next_token(&mut preorder, censor);
+        let (first, synthetic) = Self::next_token(&mut preorder, &mut replace, &mut append);
         Convertor {
-            id_alloc: { TokenIdAlloc { map: TokenMap::default(), global_offset, next_id: 0 } },
+            id_alloc: { TokenIdAlloc { map: existing_token_map, global_offset, next_id } },
             current: first,
+            current_synthetic: synthetic,
             preorder,
             range,
-            censor,
+            replace,
+            append,
             punct_offset: None,
         }
     }
 
     fn next_token(
         preorder: &mut PreorderWithTokens,
-        censor: &FxHashSet<SyntaxNode>,
-    ) -> Option<SyntaxToken> {
+        replace: &mut FxHashMap<SyntaxNode, Vec<SyntheticToken>>,
+        append: &mut FxHashMap<SyntaxNode, Vec<SyntheticToken>>,
+    ) -> (Option<SyntaxToken>, Vec<SyntheticToken>) {
         while let Some(ev) = preorder.next() {
             let ele = match ev {
                 WalkEvent::Enter(ele) => ele,
+                WalkEvent::Leave(SyntaxElement::Node(node)) => {
+                    if let Some(mut v) = append.remove(&node) {
+                        if !v.is_empty() {
+                            v.reverse();
+                            return (None, v);
+                        }
+                    }
+                    continue;
+                }
                 _ => continue,
             };
             match ele {
-                SyntaxElement::Token(t) => return Some(t),
-                SyntaxElement::Node(node) if censor.contains(&node) => preorder.skip_subtree(),
-                SyntaxElement::Node(_) => (),
+                SyntaxElement::Token(t) => return (Some(t), Vec::new()),
+                SyntaxElement::Node(node) => {
+                    if let Some(mut v) = replace.remove(&node) {
+                        preorder.skip_subtree();
+                        if !v.is_empty() {
+                            v.reverse();
+                            return (None, v);
+                        }
+                    }
+                }
             }
         }
-        None
+        (None, Vec::new())
     }
 }
 
 #[derive(Debug)]
 enum SynToken {
     Ordinary(SyntaxToken),
+    // FIXME is this supposed to be `Punct`?
     Punch(SyntaxToken, TextSize),
+    Synthetic(SyntheticToken),
 }
 
 impl SynToken {
-    fn token(&self) -> &SyntaxToken {
+    fn token(&self) -> Option<&SyntaxToken> {
         match self {
-            SynToken::Ordinary(it) | SynToken::Punch(it, _) => it,
+            SynToken::Ordinary(it) | SynToken::Punch(it, _) => Some(it),
+            SynToken::Synthetic(_) => None,
         }
     }
 }
 
-impl<'a> SrcToken<Convertor<'a>> for SynToken {
-    fn kind(&self, _ctx: &Convertor<'a>) -> SyntaxKind {
-        self.token().kind()
+impl SrcToken<Convertor> for SynToken {
+    fn kind(&self, _ctx: &Convertor) -> SyntaxKind {
+        match self {
+            SynToken::Ordinary(token) => token.kind(),
+            SynToken::Punch(token, _) => token.kind(),
+            SynToken::Synthetic(token) => token.kind,
+        }
     }
-    fn to_char(&self, _ctx: &Convertor<'a>) -> Option<char> {
+    fn to_char(&self, _ctx: &Convertor) -> Option<char> {
         match self {
             SynToken::Ordinary(_) => None,
             SynToken::Punch(it, i) => it.text().chars().nth((*i).into()),
+            SynToken::Synthetic(token) if token.text.len() == 1 => token.text.chars().next(),
+            SynToken::Synthetic(_) => None,
         }
     }
-    fn to_text(&self, _ctx: &Convertor<'a>) -> SmolStr {
-        self.token().text().into()
+    fn to_text(&self, _ctx: &Convertor) -> SmolStr {
+        match self {
+            SynToken::Ordinary(token) => token.text().into(),
+            SynToken::Punch(token, _) => token.text().into(),
+            SynToken::Synthetic(token) => token.text.clone(),
+        }
+    }
+
+    fn synthetic_id(&self, _ctx: &Convertor) -> Option<SyntheticTokenId> {
+        match self {
+            SynToken::Synthetic(token) => Some(token.id),
+            _ => None,
+        }
     }
 }
 
-impl TokenConvertor for Convertor<'_> {
+impl TokenConvertor for Convertor {
     type Token = SynToken;
     fn convert_doc_comment(&self, token: &Self::Token) -> Option<Vec<tt::TokenTree>> {
-        convert_doc_comment(token.token())
+        convert_doc_comment(token.token()?)
     }
 
     fn bump(&mut self) -> Option<(Self::Token, TextRange)> {
@@ -558,11 +643,25 @@ impl TokenConvertor for Convertor<'_> {
             }
         }
 
+        if let Some(synth_token) = self.current_synthetic.pop() {
+            if self.current_synthetic.is_empty() {
+                let (new_current, new_synth) =
+                    Self::next_token(&mut self.preorder, &mut self.replace, &mut self.append);
+                self.current = new_current;
+                self.current_synthetic = new_synth;
+            }
+            let range = synth_token.range;
+            return Some((SynToken::Synthetic(synth_token), range));
+        }
+
         let curr = self.current.clone()?;
         if !&self.range.contains_range(curr.text_range()) {
             return None;
         }
-        self.current = Self::next_token(&mut self.preorder, self.censor);
+        let (new_current, new_synth) =
+            Self::next_token(&mut self.preorder, &mut self.replace, &mut self.append);
+        self.current = new_current;
+        self.current_synthetic = new_synth;
         let token = if curr.kind().is_punct() {
             self.punct_offset = Some((curr.clone(), 0.into()));
             let range = curr.text_range();
@@ -585,6 +684,10 @@ impl TokenConvertor for Convertor<'_> {
             }
         }
 
+        if let Some(synth_token) = self.current_synthetic.last() {
+            return Some(SynToken::Synthetic(synth_token.clone()));
+        }
+
         let curr = self.current.clone()?;
         if !self.range.contains_range(curr.text_range()) {
             return None;
diff --git a/crates/mbe/src/token_map.rs b/crates/mbe/src/token_map.rs
index 9053526d203..c923e7a69a1 100644
--- a/crates/mbe/src/token_map.rs
+++ b/crates/mbe/src/token_map.rs
@@ -5,6 +5,8 @@ use std::hash::Hash;
 use parser::{SyntaxKind, T};
 use syntax::{TextRange, TextSize};
 
+use crate::syntax_bridge::SyntheticTokenId;
+
 #[derive(Debug, PartialEq, Eq, Clone, Copy, Hash)]
 enum TokenTextRange {
     Token(TextRange),
@@ -31,6 +33,7 @@ impl TokenTextRange {
 pub struct TokenMap {
     /// Maps `tt::TokenId` to the *relative* source range.
     entries: Vec<(tt::TokenId, TokenTextRange)>,
+    pub synthetic_entries: Vec<(tt::TokenId, SyntheticTokenId)>,
 }
 
 impl TokenMap {
@@ -57,6 +60,10 @@ impl TokenMap {
             .filter_map(move |(_, range)| range.by_kind(kind))
     }
 
+    pub fn synthetic_token_id(&self, token_id: tt::TokenId) -> Option<SyntheticTokenId> {
+        self.synthetic_entries.iter().find(|(tid, _)| *tid == token_id).map(|(_, id)| *id)
+    }
+
     pub fn first_range_by_token(
         &self,
         token_id: tt::TokenId,
@@ -67,12 +74,17 @@ impl TokenMap {
 
     pub(crate) fn shrink_to_fit(&mut self) {
         self.entries.shrink_to_fit();
+        self.synthetic_entries.shrink_to_fit();
     }
 
     pub(crate) fn insert(&mut self, token_id: tt::TokenId, relative_range: TextRange) {
         self.entries.push((token_id, TokenTextRange::Token(relative_range)));
     }
 
+    pub(crate) fn insert_synthetic(&mut self, token_id: tt::TokenId, id: SyntheticTokenId) {
+        self.synthetic_entries.push((token_id, id));
+    }
+
     pub(crate) fn insert_delim(
         &mut self,
         token_id: tt::TokenId,
diff --git a/crates/tt/src/lib.rs b/crates/tt/src/lib.rs
index 9eca970ee21..0316b15038c 100644
--- a/crates/tt/src/lib.rs
+++ b/crates/tt/src/lib.rs
@@ -87,6 +87,16 @@ pub struct Ident {
     pub id: TokenId,
 }
 
+impl Leaf {
+    pub fn id(&self) -> TokenId {
+        match self {
+            Leaf::Literal(l) => l.id,
+            Leaf::Punct(p) => p.id,
+            Leaf::Ident(i) => i.id,
+        }
+    }
+}
+
 fn print_debug_subtree(f: &mut fmt::Formatter<'_>, subtree: &Subtree, level: usize) -> fmt::Result {
     let align = "  ".repeat(level);