Auto merge of #15961 - ohno418:top-level-let-stmt, r=Veykril

Improve error handling for top-level `let` statements

This commit addresses the issue of excessive and unrelated errors generated by top-level `let` statements. Now, only a single error is produced, indicating that `let` statements are invalid at the top level.

---

Fixes https://github.com/rust-lang/rust-analyzer/issues/14963.

While I'm not really sure if handling a particular case in a special manner is appropriate, it would be good to suppress the excessive number of annoying and unrelated errors.

crates/parser/src/grammar.rs

@@ -376,6 +376,16 @@ fn error_block(p: &mut Parser<'_>, message: &str) {
     m.complete(p, ERROR);
 }
 
+// test_err top_level_let
+// let ref foo: fn() = 1 + 3;
+fn error_let_stmt(p: &mut Parser<'_>, message: &str) {
+    assert!(p.at(T![let]));
+    let m = p.start();
+    p.error(message);
+    expressions::let_stmt(p, expressions::Semicolon::Optional);
+    m.complete(p, ERROR);
+}
+
 /// The `parser` passed this is required to at least consume one token if it returns `true`.
 /// If the `parser` returns false, parsing will stop.
 fn delimited(

crates/parser/src/grammar/expressions.rs

@@ -59,7 +59,8 @@ pub(super) fn stmt(p: &mut Parser<'_>, semicolon: Semicolon) {
     attributes::outer_attrs(p);
 
     if p.at(T![let]) {
-        let_stmt(p, m, semicolon);
+        let_stmt(p, semicolon);
+        m.complete(p, LET_STMT);
         return;
     }
 
@@ -109,10 +110,11 @@ pub(super) fn stmt(p: &mut Parser<'_>, semicolon: Semicolon) {
             m.complete(p, EXPR_STMT);
         }
     }
+}
 
-    // test let_stmt
+// test let_stmt
-    // fn f() { let x: i32 = 92; }
+// fn f() { let x: i32 = 92; }
-    fn let_stmt(p: &mut Parser<'_>, m: Marker, with_semi: Semicolon) {
+pub(super) fn let_stmt(p: &mut Parser<'_>, with_semi: Semicolon) {
     p.bump(T![let]);
     patterns::pattern(p);
     if p.at(T![:]) {
@@ -156,8 +158,6 @@ pub(super) fn stmt(p: &mut Parser<'_>, semicolon: Semicolon) {
             p.expect(T![;]);
         }
     }
-        m.complete(p, LET_STMT);
-    }
 }
 
 pub(super) fn expr_block_contents(p: &mut Parser<'_>) {

crates/parser/src/grammar/items.rs

@@ -79,6 +79,7 @@ pub(super) fn item_or_macro(p: &mut Parser<'_>, stop_on_r_curly: bool) {
             e.complete(p, ERROR);
         }
         EOF | T!['}'] => p.error("expected an item"),
+        T![let] => error_let_stmt(p, "expected an item"),
         _ => p.err_and_bump("expected an item"),
     }
 }

crates/parser/test_data/parser/inline/err/0024_top_level_let.rast

@@ -0,0 +1,30 @@
+SOURCE_FILE
+  ERROR
+    LET_KW "let"
+    WHITESPACE " "
+    IDENT_PAT
+      REF_KW "ref"
+      WHITESPACE " "
+      NAME
+        IDENT "foo"
+    COLON ":"
+    WHITESPACE " "
+    FN_PTR_TYPE
+      FN_KW "fn"
+      PARAM_LIST
+        L_PAREN "("
+        R_PAREN ")"
+    WHITESPACE " "
+    EQ "="
+    WHITESPACE " "
+    BIN_EXPR
+      LITERAL
+        INT_NUMBER "1"
+      WHITESPACE " "
+      PLUS "+"
+      WHITESPACE " "
+      LITERAL
+        INT_NUMBER "3"
+    SEMICOLON ";"
+  WHITESPACE "\n"
+error 0: expected an item

crates/parser/test_data/parser/inline/err/0024_top_level_let.rs

@@ -0,0 +1 @@
+let ref foo: fn() = 1 + 3;