Rollup merge of #125276 - dev-ardi:no-main-diag, r=fmease

Fix parsing of erroneously placed semicolons

This closes https://github.com/rust-lang/rust/issues/124935, is a continuation of https://github.com/rust-lang/rust/pull/125245 after rebasing https://github.com/rust-lang/rust/pull/125117.

Thanks ```@gurry``` for your code and sorry for making it confusing :P

r? fmease

compiler/rustc_parse/src/parser/item.rs

@@ -58,9 +58,15 @@ impl<'a> Parser<'a> {
         let attrs = self.parse_inner_attributes()?;
 
         let post_attr_lo = self.token.span;
-        let mut items = ThinVec::new();
-        while let Some(item) = self.parse_item(ForceCollect::No)? {
-            self.maybe_consume_incorrect_semicolon(Some(&item));
+        let mut items: ThinVec<P<_>> = ThinVec::new();
+
+        // There shouldn't be any stray semicolons before or after items.
+        // `parse_item` consumes the appropriate semicolons so any leftover is an error.
+        loop {
+            while self.maybe_consume_incorrect_semicolon(items.last().map(|x| &**x)) {} // Eat all bad semicolons
+            let Some(item) = self.parse_item(ForceCollect::No)? else {
+                break;
+            };
             items.push(item);
         }
 

tests/ui/parser/issues/fn-no-semicolon-issue-124935-semi-after-item.rs

@@ -0,0 +1,6 @@
+// Regression test for issue #124935
+// Tests that we do not erroneously emit an error about
+// missing main function when the mod starts with a `;`
+
+; //~ ERROR expected item, found `;`
+fn main() { }

tests/ui/parser/issues/fn-no-semicolon-issue-124935-semi-after-item.stderr

@@ -0,0 +1,8 @@
+error: expected item, found `;`
+  --> $DIR/fn-no-semicolon-issue-124935-semi-after-item.rs:5:1
+   |
+LL | ;
+   | ^ help: remove this semicolon
+
+error: aborting due to 1 previous error
+

tests/ui/parser/issues/issue-49040.rs

@@ -1,3 +1,3 @@
 #![allow(unused_variables)]; //~ ERROR expected item, found `;`
-//~^ ERROR `main` function
 fn foo() {}
+//~^ ERROR `main` function

tests/ui/parser/issues/issue-49040.stderr

@@ -5,10 +5,10 @@ LL | #![allow(unused_variables)];
    |                            ^ help: remove this semicolon
 
 error[E0601]: `main` function not found in crate `issue_49040`
-  --> $DIR/issue-49040.rs:1:29
+  --> $DIR/issue-49040.rs:2:12
    |
-LL | #![allow(unused_variables)];
-   |                             ^ consider adding a `main` function to `$DIR/issue-49040.rs`
+LL | fn foo() {}
+   |            ^ consider adding a `main` function to `$DIR/issue-49040.rs`
 
 error: aborting due to 2 previous errors
 

tests/ui/parser/issues/missing-main-issue-124935-semi-after-item.rs

@@ -0,0 +1,5 @@
+// Regression test for issue #124935
+// Tests that we still emit an error after an item.
+
+fn main() { }
+; //~ ERROR expected item, found `;`

tests/ui/parser/issues/missing-main-issue-124935-semi-after-item.stderr

@@ -0,0 +1,10 @@
+error: expected item, found `;`
+  --> $DIR/missing-main-issue-124935-semi-after-item.rs:5:1
+   |
+LL | ;
+   | ^ help: remove this semicolon
+   |
+   = help: function declarations are not followed by a semicolon
+
+error: aborting due to 1 previous error
+