Auto merge of #36482 - jseyfried:dont_load_unconfigured_noninline_modules, r=nrc

Avoid loading and parsing unconfigured non-inline modules.

For example, `#[cfg(any())] mod foo;` will always compile after this PR, even if `foo.rs` and `foo/mod.rs` do not exist or do not contain valid Rust.

Fixes #36478 and fixes #27873.

r? @nrc

src/libsyntax/config.rs

@@ -126,7 +126,7 @@ impl<'a> StripUnconfigured<'a> {
     }
 
     // Determine if a node with the given attributes should be included in this configuation.
-    fn in_cfg(&mut self, attrs: &[ast::Attribute]) -> bool {
+    pub fn in_cfg(&mut self, attrs: &[ast::Attribute]) -> bool {
         attrs.iter().all(|attr| {
             // When not compiling with --test we should not compile the #[test] functions
             if !self.should_test && is_test_or_bench(attr) {

src/libsyntax/parse/parser.rs

@@ -5260,20 +5260,29 @@ impl<'a> Parser<'a> {
 
     /// Parse a `mod <foo> { ... }` or `mod <foo>;` item
     fn parse_item_mod(&mut self, outer_attrs: &[Attribute]) -> PResult<'a, ItemInfo> {
-        let outer_attrs = ::config::StripUnconfigured {
+        let (in_cfg, outer_attrs) = {
+            let mut strip_unconfigured = ::config::StripUnconfigured {
                 config: &self.cfg,
                 sess: self.sess,
                 should_test: false, // irrelevant
                 features: None, // don't perform gated feature checking
-        }.process_cfg_attrs(outer_attrs.to_owned());
+            };
+            let outer_attrs = strip_unconfigured.process_cfg_attrs(outer_attrs.to_owned());
+            (strip_unconfigured.in_cfg(&outer_attrs), outer_attrs)
+        };
 
         let id_span = self.span;
         let id = self.parse_ident()?;
         if self.check(&token::Semi) {
             self.bump();
+            if in_cfg {
                 // This mod is in an external file. Let's go get it!
                 let (m, attrs) = self.eval_src_mod(id, &outer_attrs, id_span)?;
                 Ok((id, m, Some(attrs)))
+            } else {
+                let placeholder = ast::Mod { inner: syntax_pos::DUMMY_SP, items: Vec::new() };
+                Ok((id, ItemKind::Mod(placeholder), None))
+            }
         } else {
             let directory = self.directory.clone();
             self.push_directory(id, &outer_attrs);

src/test/run-pass/conditional-compile.rs

@@ -148,3 +148,6 @@ mod test_methods {
         fn the(&self);
     }
 }
+
+#[cfg(any())]
+mod nonexistent_file; // Check that unconfigured non-inline modules are not loaded or parsed.