rustdoc doctest: detect fn main after an unexpected semicolon

The basic problem with this is that rustdoc, when hunting for `fn main`, will stop
parsing after it reaches a fatal error. This unexpected semicolon was a fatal error,
so in `src/test/rustdoc-ui/failed-doctest-extra-semicolon-on-item.rs`, it would wrap
the doctest in an implied main function, turning it into this:

    fn main() {
        struct S {};
        fn main() {
            assert_eq!(0, 1);
        }
    }

This, as it turns out, is totally valid, and it executes no assertions, so *it passes,*
even though the user wanted it to execute the assertion.

The Rust parser already has the ability to recover from these unexpected semicolons,
but to do so, it needs to use the `parse_mod` function, so this commit changes it to do that.

src/librustdoc/doctest.rs

@@ -1,4 +1,4 @@
-use rustc_ast as ast;
+use rustc_ast::{self as ast, token};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{ColorConfig, ErrorReported, FatalError};
@@ -537,7 +537,6 @@ fn drop(&mut self) {
             use rustc_errors::emitter::{Emitter, EmitterWriter};
             use rustc_errors::Handler;
             use rustc_parse::maybe_new_parser_from_source_str;
-            use rustc_parse::parser::ForceCollect;
             use rustc_session::parse::ParseSess;
             use rustc_span::source_map::FilePathMapping;
 
@@ -573,9 +572,9 @@ fn drop(&mut self) {
                 }
             };
 
-            loop {
-                match parser.parse_item(ForceCollect::No) {
-                    Ok(Some(item)) => {
+            match parser.parse_mod(&token::Eof) {
+                Ok((_attrs, items, _span)) => {
+                    for item in items {
                         if !found_main {
                             if let ast::ItemKind::Fn(..) = item.kind {
                                 if item.ident.name == sym::main {
@@ -607,11 +606,9 @@ fn drop(&mut self) {
                             break;
                         }
                     }
-                    Ok(None) => break,
-                    Err(mut e) => {
-                        e.cancel();
-                        break;
-                    }
+                }
+                Err(mut e) => {
+                    e.cancel();
                 }
             }
 

src/test/rustdoc-ui/failed-doctest-extra-semicolon-on-item.rs

@@ -0,0 +1,18 @@
+// FIXME: if/when the output of the test harness can be tested on its own, this test should be
+// adapted to use that, and that normalize line can go away
+
+// compile-flags:--test
+// normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
+// normalize-stdout-test "finished in \d+\.\d+s" -> "finished in $$TIME"
+// failure-status: 101
+
+/// <https://github.com/rust-lang/rust/issues/91014>
+///
+/// ```rust
+/// struct S {}; // unexpected semicolon after struct def
+///
+/// fn main() {
+///    assert_eq!(0, 1);
+/// }
+/// ```
+mod m {}

src/test/rustdoc-ui/failed-doctest-extra-semicolon-on-item.stdout

@@ -0,0 +1,24 @@
+
+running 1 test
+test $DIR/failed-doctest-extra-semicolon-on-item.rs - m (line 11) ... FAILED
+
+failures:
+
+---- $DIR/failed-doctest-extra-semicolon-on-item.rs - m (line 11) stdout ----
+error: expected item, found `;`
+  --> $DIR/failed-doctest-extra-semicolon-on-item.rs:12:12
+   |
+LL | struct S {}; // unexpected semicolon after struct def
+   |            ^ help: remove this semicolon
+   |
+   = help: braced struct declarations are not followed by a semicolon
+
+error: aborting due to previous error
+
+Couldn't compile the test.
+
+failures:
+    $DIR/failed-doctest-extra-semicolon-on-item.rs - m (line 11)
+
+test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+