Recover from leading comma in tuple pat and expr

crates/ide/src/signature_help.rs

@@ -2003,7 +2003,10 @@ fn main() {
     let _: (&str, u32, u32)= ($0, 1, 3);
 }
 "#,
-            expect![""],
+            expect![[r#"
+                (&str, u32)
+                 ^^^^  ---
+            "#]],
         );
         check(
             r#"
@@ -2011,7 +2014,10 @@ fn main() {
     let _: (&str, u32, u32, u32)= ($0, 1, 3);
 }
 "#,
-            expect![""],
+            expect![[r#"
+                (&str, u32)
+                 ^^^^  ---
+            "#]],
         );
         check(
             r#"
@@ -2019,7 +2025,10 @@ fn main() {
     let _: (&str, u32, u32)= ($0, 1, 3, 5);
 }
 "#,
-            expect![""],
+            expect![[r#"
+                (&str, u32, u32)
+                 ^^^^  ---  ---
+            "#]],
         );
     }
 
@@ -2111,7 +2120,7 @@ fn main() {
         check(
             r#"
 fn main() {
-    let ($0 1, 3): (i32, i32, i32);
+    let ($0, 1, 3): (i32, i32, i32);
 }
 "#,
             // FIXME: tuple pat should be of size 3 ideally

crates/parser/src/grammar/expressions/atom.rs

@@ -184,6 +184,16 @@ fn tuple_expr(p: &mut Parser<'_>) -> CompletedMarker {
 
     let mut saw_comma = false;
     let mut saw_expr = false;
+
+    // test_err tuple_expr_leading_comma
+    // fn foo() {
+    //     (,);
+    // }
+    if p.eat(T![,]) {
+        p.error("expected expression");
+        saw_comma = true;
+    }
+
     while !p.at(EOF) && !p.at(T![')']) {
         saw_expr = true;
 

crates/parser/src/grammar/patterns.rs

@@ -413,6 +413,16 @@ fn tuple_pat(p: &mut Parser<'_>) -> CompletedMarker {
     let mut has_comma = false;
     let mut has_pat = false;
     let mut has_rest = false;
+
+    // test_err tuple_pat_leading_comma
+    // fn foo() {
+    //     let (,);
+    // }
+    if p.eat(T![,]) {
+        p.error("expected pattern");
+        has_comma = true;
+    }
+
     while !p.at(EOF) && !p.at(T![')']) {
         has_pat = true;
         if !p.at_ts(PAT_TOP_FIRST) {

crates/parser/test_data/parser/inline/err/0019_tuple_expr_leading_comma.rast

@@ -0,0 +1,24 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "foo"
+    PARAM_LIST
+      L_PAREN "("
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        WHITESPACE "\n    "
+        EXPR_STMT
+          TUPLE_EXPR
+            L_PAREN "("
+            COMMA ","
+            R_PAREN ")"
+          SEMICOLON ";"
+        WHITESPACE "\n"
+        R_CURLY "}"
+  WHITESPACE "\n"
+error 17: expected expression

crates/parser/test_data/parser/inline/err/0019_tuple_expr_leading_comma.rs

@@ -0,0 +1,3 @@
+fn foo() {
+    (,);
+}

crates/parser/test_data/parser/inline/err/0020_tuple_pat_leading_comma.rast

@@ -0,0 +1,26 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "foo"
+    PARAM_LIST
+      L_PAREN "("
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        WHITESPACE "\n    "
+        LET_STMT
+          LET_KW "let"
+          WHITESPACE " "
+          TUPLE_PAT
+            L_PAREN "("
+            COMMA ","
+            R_PAREN ")"
+          SEMICOLON ";"
+        WHITESPACE "\n"
+        R_CURLY "}"
+  WHITESPACE "\n"
+error 21: expected pattern

crates/parser/test_data/parser/inline/err/0020_tuple_pat_leading_comma.rs

@@ -0,0 +1,3 @@
+fn foo() {
+    let (,);
+}