parser: Allow mut occurrence only in path pattern

crates/hir/src/hir_def/pat.rs

@@ -1,16 +1,16 @@
 use cranelift_entity::entity_impl;
 
-use crate::{span::pat::LazyPatSpan, HirDb};
-
 use super::{Body, IdentId, LitKind, Partial, PathId};
+use crate::{span::pat::LazyPatSpan, HirDb};
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub enum Pat {
     WildCard,
     Rest,
     Lit(Partial<LitKind>),
     Tuple(Vec<PatId>),
-    Path(Partial<PathId>),
+    /// The second bool is `true` if the pat has `mut` in front of it.
+    Path(Partial<PathId>, bool),
     PathTuple(Partial<PathId>, Vec<PatId>),
     Record(Partial<PathId>, Vec<RecordPatField>),
     Or(PatId, PatId),
@@ -20,7 +20,7 @@ impl Pat {
     /// Return `true` if this pattern is a binding.
     pub fn is_bind(&self, db: &dyn HirDb) -> bool {
         match self {
-            Self::Path(Partial::Present(p)) => p.len(db) == 1,
+            Self::Path(Partial::Present(p), _) => p.len(db) == 1,
             _ => false,
         }
     }
@@ -53,3 +53,18 @@ pub struct RecordPatField {
     pub label: Partial<IdentId>,
     pub pat: PatId,
 }
+
+impl RecordPatField {
+    pub fn label(&self, db: &dyn HirDb, body: Body) -> Option<IdentId> {
+        if let Partial::Present(label) = self.label {
+            return Some(label);
+        }
+
+        match self.pat.data(db, body) {
+            Partial::Present(Pat::Path(Partial::Present(path), _)) if path.is_ident(db) => {
+                path.last_segment(db).to_opt()
+            }
+            _ => None,
+        }
+    }
+}

crates/hir/src/hir_def/path.rs

@@ -1,6 +1,5 @@
-use crate::{hir_def::Partial, HirDb};
-
 use super::{kw, IdentId};
+use crate::{hir_def::Partial, HirDb};
 
 #[salsa::interned]
 pub struct PathId {
@@ -17,6 +16,10 @@ impl PathId {
         self.segments(db).len()
     }
 
+    pub fn is_ident(self, db: &dyn HirDb) -> bool {
+        self.len(db) == 1
+    }
+
     pub fn self_ty(db: &dyn HirDb) -> Self {
         let self_ty = Partial::Present(kw::SELF_TY);
         Self::new(db, vec![self_ty])

crates/hir/src/lower/pat.rs

@@ -1,12 +1,11 @@
 use parser::ast;
 
+use super::body::BodyCtxt;
 use crate::{
     hir_def::{pat::*, IdentId, LitKind, PathId},
     span::HirOrigin,
 };
 
-use super::body::BodyCtxt;
-
 impl Pat {
     pub(super) fn lower_ast(ctxt: &mut BodyCtxt<'_, '_>, ast: ast::Pat) -> PatId {
         let pat = match &ast.kind() {
@@ -30,9 +29,9 @@ impl Pat {
                 Pat::Tuple(elems)
             }
 
-            ast::PatKind::Path(path) => {
-                let path = PathId::lower_ast_partial(ctxt.f_ctxt, path.path());
-                Pat::Path(path)
+            ast::PatKind::Path(path_ast) => {
+                let path = PathId::lower_ast_partial(ctxt.f_ctxt, path_ast.path());
+                Pat::Path(path, path_ast.mut_token().is_some())
             }
 
             ast::PatKind::PathTuple(path_tup) => {

crates/hir/src/span/pat.rs

@@ -46,6 +46,10 @@ define_lazy_span_node!(
 define_lazy_span_node!(
     LazyPathPatSpan,
     ast::PathPat,
+    @token {
+        (mut_token, mut_token),
+    }
+
     @node {
         (path, path, LazyPathSpan),
     }

crates/hir/src/visitor.rs

@@ -1133,7 +1133,7 @@ where
             }
         }
 
-        Pat::Path(path) => {
+        Pat::Path(path, _) => {
             if let Some(path) = path.to_opt() {
                 ctxt.with_new_ctxt(
                     |span| span.into_path_pat().path_moved(),

crates/parser2/src/ast/pat.rs

@@ -33,11 +33,6 @@ impl Pat {
             _ => unreachable!(),
         }
     }
-
-    /// Returns the `mut` keyword if the patter is mutable.
-    pub fn mut_token(&self) -> Option<SyntaxToken> {
-        support::token(self.syntax(), SK::MutKw)
-    }
 }
 
 ast_node! {
@@ -91,6 +86,11 @@ impl PathPat {
     pub fn path(&self) -> Option<super::Path> {
         support::child(self.syntax())
     }
+
+    /// Returns the `mut` keyword if the patter is mutable.
+    pub fn mut_token(&self) -> Option<SyntaxToken> {
+        support::token(self.syntax(), SK::MutKw)
+    }
 }
 
 ast_node! {
@@ -175,11 +175,10 @@ pub enum PatKind {
 
 #[cfg(test)]
 mod tests {
-    use crate::{lexer::Lexer, parser::Parser};
-
     use wasm_bindgen_test::wasm_bindgen_test;
 
     use super::*;
+    use crate::{lexer::Lexer, parser::Parser};
 
     fn parse_pat<T>(source: &str) -> T
     where
@@ -282,9 +281,13 @@ mod tests {
                     assert!(matches!(field.pat().unwrap().kind(), PatKind::Path(_)));
                 }
                 2 => {
+                    let PatKind::Path(pat) = field.pat().unwrap().kind() else {
+                        panic!("unexpected record pat");
+                    };
+
                     assert!(field.name().is_none());
                     assert!(matches!(field.pat().unwrap().kind(), PatKind::Path(_)));
-                    assert!(field.pat().unwrap().mut_token().is_some());
+                    assert!(pat.mut_token().is_some());
                 }
                 _ => panic!("unexpected record pat"),
             }

crates/parser2/src/parser/pat.rs

@@ -1,15 +1,38 @@
+use super::{define_scope, path::PathScope, token_stream::TokenStream, Parser};
 use crate::{
-    parser::lit::{is_lit, LitScope},
+    parser::{
+        lit::{is_lit, LitScope},
+        token_stream::LexicalToken,
+    },
     SyntaxKind,
 };
 
-use super::{define_scope, path::PathScope, token_stream::TokenStream, Parser};
-
 pub fn parse_pat<S: TokenStream>(parser: &mut Parser<S>) -> bool {
     use SyntaxKind::*;
     parser.bump_trivias();
     let checkpoint = parser.checkpoint();
-    parser.bump_if(SyntaxKind::MutKw);
+    let has_mut = parser.bump_if(SyntaxKind::MutKw);
+
+    let token = parser.current_token();
+    if has_mut {
+        match token.as_ref().map(|t| t.syntax_kind()) {
+            Some(Underscore | Dot2 | LParen) => {
+                parser.error_msg_on_current_token(&format!(
+                    "`mut` is not allowed on `{}`",
+                    token.unwrap().text()
+                ));
+            }
+
+            Some(kind) if is_lit(kind) => {
+                parser.error_msg_on_current_token(&format!(
+                    "`mut` is not allowed on `{}`",
+                    token.unwrap().text()
+                ));
+            }
+
+            _ => {}
+        }
+    }
 
     let success = match parser.current_kind() {
         Some(Underscore) => parser.parse(WildCardPatScope::default(), Some(checkpoint)),