Detect more cfgd out items in resolution errors

Use a visitor to collect *all* items (including those nested) that were stripped behind a `cfg` condition.

```
error[E0425]: cannot find function `f` in this scope
  --> $DIR/nested-cfg-attrs.rs:4:13
   |
LL | fn main() { f() }
   |             ^ not found in this scope
   |
note: found an item that was configured out
  --> $DIR/nested-cfg-attrs.rs:2:4
   |
LL | fn f() {}
   |    ^
note: the item is gated here
  --> $DIR/nested-cfg-attrs.rs:1:35
   |
LL | #[cfg_attr(all(), cfg_attr(all(), cfg(FALSE)))]
   |                                   ^^^^^^^^^^
```

Author: estebank

compiler/rustc_expand/src/expand.rs

@@ -1274,25 +1274,33 @@ impl InvocationCollectorNode for P<ast::Item> {
         res
     }
     fn declared_names(&self) -> Vec<Ident> {
-        if let ItemKind::Use(ut) = &self.kind {
-            fn collect_use_tree_leaves(ut: &ast::UseTree, idents: &mut Vec<Ident>) {
-                match &ut.kind {
-                    ast::UseTreeKind::Glob => {}
-                    ast::UseTreeKind::Simple(_) => idents.push(ut.ident()),
-                    ast::UseTreeKind::Nested { items, .. } => {
-                        for (ut, _) in items {
-                            collect_use_tree_leaves(ut, idents);
+        struct ItemNameVisitor(Vec<Ident>);
+        impl Visitor<'_> for ItemNameVisitor {
+            fn visit_item(&mut self, i: &ast::Item) {
+                if let ItemKind::Use(ut) = &i.kind {
+                    fn collect_use_tree_leaves(ut: &ast::UseTree, idents: &mut Vec<Ident>) {
+                        match &ut.kind {
+                            ast::UseTreeKind::Glob => {}
+                            ast::UseTreeKind::Simple(_) => idents.push(ut.ident()),
+                            ast::UseTreeKind::Nested { items, .. } => {
+                                for (ut, _) in items {
+                                    collect_use_tree_leaves(ut, idents);
+                                }
+                            }
                         }
                     }
+
+                    collect_use_tree_leaves(ut, &mut self.0);
+                } else {
+                    self.0.push(i.ident);
                 }
+                visit::walk_item(self, i);
             }
-
-            let mut idents = Vec::new();
-            collect_use_tree_leaves(ut, &mut idents);
-            return idents;
         }
 
-        vec![self.ident]
+        let mut v = ItemNameVisitor(vec![]);
+        v.visit_item(self);
+        v.0
     }
 }
 

compiler/rustc_resolve/src/diagnostics.rs

@@ -810,11 +810,15 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     err.multipart_suggestion(msg, suggestions, applicability);
                 }
 
-                if let Some(ModuleOrUniformRoot::Module(module)) = module
-                    && let Some(module) = module.opt_def_id()
-                    && let Some(segment) = segment
-                {
-                    self.find_cfg_stripped(&mut err, &segment, module);
+                if let Some(segment) = segment {
+                    if let Some(ModuleOrUniformRoot::Module(module)) = module {
+                        let module =
+                            module.opt_def_id().unwrap_or_else(|| CRATE_DEF_ID.to_def_id());
+                        self.find_cfg_stripped(&mut err, &segment, module);
+                    } else {
+                        let module = CRATE_DEF_ID.to_def_id();
+                        self.find_cfg_stripped(&mut err, &segment, module);
+                    }
                 }
 
                 err

compiler/rustc_resolve/src/late.rs

@@ -4147,13 +4147,21 @@ impl<'a, 'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                 //
                 // And that's what happens below - we're just mixing both messages
                 // into a single one.
+                let failed_to_resolve = match parent_err.node {
+                    ResolutionError::FailedToResolve { .. } => true,
+                    _ => false,
+                };
                 let mut parent_err = this.r.into_struct_error(parent_err.span, parent_err.node);
 
                 // overwrite all properties with the parent's error message
                 err.messages = take(&mut parent_err.messages);
                 err.code = take(&mut parent_err.code);
                 swap(&mut err.span, &mut parent_err.span);
-                err.children = take(&mut parent_err.children);
+                if failed_to_resolve {
+                    err.children = take(&mut parent_err.children);
+                } else {
+                    err.children.append(&mut parent_err.children);
+                }
                 err.sort_span = parent_err.sort_span;
                 err.is_lint = parent_err.is_lint.clone();
 

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -494,9 +494,8 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         }
         self.err_code_special_cases(&mut err, source, path, span);
 
-        if let Some(module) = base_error.module {
-            self.r.find_cfg_stripped(&mut err, &path.last().unwrap().ident.name, module);
-        }
+        let module = base_error.module.unwrap_or_else(|| CRATE_DEF_ID.to_def_id());
+        self.r.find_cfg_stripped(&mut err, &path.last().unwrap().ident.name, module);
 
         (err, candidates)
     }

tests/ui/cfg/diagnostics-reexport.rs

@@ -1,7 +1,7 @@
 pub mod inner {
-    #[cfg(FALSE)]
+    #[cfg(FALSE)] //~ NOTE the item is gated here
     mod gone {
-        pub fn uwu() {}
+        pub fn uwu() {} //~ NOTE found an item that was configured out
     }
 
     #[cfg(FALSE)] //~ NOTE the item is gated here
@@ -34,7 +34,7 @@ mod b {
 }
 
 fn main() {
-    // There is no uwu at this path - no diagnostic.
+    // There is no uwu at this path, but there's one in a cgfd out sub-module, so we mention it.
     inner::uwu(); //~ ERROR cannot find function
     //~^ NOTE not found in `inner`
 }

tests/ui/cfg/diagnostics-reexport.stderr

@@ -50,6 +50,16 @@ error[E0425]: cannot find function `uwu` in module `inner`
 LL |     inner::uwu();
    |            ^^^ not found in `inner`
    |
+note: found an item that was configured out
+  --> $DIR/diagnostics-reexport.rs:4:16
+   |
+LL |         pub fn uwu() {}
+   |                ^^^
+note: the item is gated here
+  --> $DIR/diagnostics-reexport.rs:2:5
+   |
+LL |     #[cfg(FALSE)]
+   |     ^^^^^^^^^^^^^
 note: found an item that was configured out
   --> $DIR/diagnostics-reexport.rs:8:20
    |

tests/ui/cfg/diagnostics-same-crate.rs

@@ -37,8 +37,8 @@ mod placeholder {
     //~| NOTE could not find `doesnt_exist` in `inner`
 }
 
-#[cfg(i_dont_exist_and_you_can_do_nothing_about_it)]
-pub fn vanished() {}
+#[cfg(i_dont_exist_and_you_can_do_nothing_about_it)] //~ NOTE the item is gated here
+pub fn vanished() {} //~ NOTE found an item that was configured out
 
 fn main() {
     // There is no uwu at this path - no diagnostic.
@@ -49,18 +49,16 @@ fn main() {
     inner::uwu(); //~ ERROR cannot find function
     //~| NOTE not found in `inner`
 
-    // The module isn't found - we would like to get a diagnostic, but currently don't due to
-    // the awkward way the resolver diagnostics are currently implemented.
+    // The module isn't found - we get a diagnostic.
     inner::doesnt_exist::hello(); //~ ERROR failed to resolve
     //~| NOTE could not find `doesnt_exist` in `inner`
 
     // It should find the one in the right module, not the wrong one.
     inner::right::meow(); //~ ERROR cannot find function
     //~| NOTE not found in `inner::right
 
-    // Exists in the crate root - we would generally want a diagnostic,
-    // but currently don't have one.
-    // Not that it matters much though, this is highly unlikely to confuse anyone.
+    // Exists in the crate root - we show a diagnostic because we treat "no module DefId" as "crate
+    // root DefId".
     vanished(); //~ ERROR cannot find function
     //~^ NOTE not found in this scope
 }

tests/ui/cfg/diagnostics-same-crate.stderr

@@ -33,7 +33,7 @@ LL |     #[cfg(FALSE)]
    |     ^^^^^^^^^^^^^
 
 error[E0433]: failed to resolve: could not find `doesnt_exist` in `inner`
-  --> $DIR/diagnostics-same-crate.rs:54:12
+  --> $DIR/diagnostics-same-crate.rs:53:12
    |
 LL |     inner::doesnt_exist::hello();
    |            ^^^^^^^^^^^^ could not find `doesnt_exist` in `inner`
@@ -67,7 +67,7 @@ LL |     #[cfg(FALSE)]
    |     ^^^^^^^^^^^^^
 
 error[E0425]: cannot find function `meow` in module `inner::right`
-  --> $DIR/diagnostics-same-crate.rs:58:19
+  --> $DIR/diagnostics-same-crate.rs:57:19
    |
 LL |     inner::right::meow();
    |                   ^^^^ not found in `inner::right`
@@ -90,10 +90,21 @@ LL |     uwu();
    |     ^^^ not found in this scope
 
 error[E0425]: cannot find function `vanished` in this scope
-  --> $DIR/diagnostics-same-crate.rs:64:5
+  --> $DIR/diagnostics-same-crate.rs:62:5
    |
 LL |     vanished();
    |     ^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/diagnostics-same-crate.rs:41:8
+   |
+LL | pub fn vanished() {}
+   |        ^^^^^^^^
+note: the item is gated here
+  --> $DIR/diagnostics-same-crate.rs:40:1
+   |
+LL | #[cfg(i_dont_exist_and_you_can_do_nothing_about_it)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 7 previous errors
 

tests/ui/conditional-compilation/test-cfg.rs

@@ -1,8 +1,10 @@
 //@ compile-flags: --cfg foo --check-cfg=cfg(foo,bar)
 
 #[cfg(all(foo, bar))] // foo AND bar
-fn foo() {}
+//~^ NOTE the item is gated here
+fn foo() {} //~ NOTE found an item that was configured out
 
 fn main() {
     foo(); //~ ERROR cannot find function `foo` in this scope
+    //~^ NOTE not found in this scope
 }

tests/ui/conditional-compilation/test-cfg.stderr

@@ -1,8 +1,19 @@
 error[E0425]: cannot find function `foo` in this scope
-  --> $DIR/test-cfg.rs:7:5
+  --> $DIR/test-cfg.rs:8:5
    |
 LL |     foo();
    |     ^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/test-cfg.rs:5:4
+   |
+LL | fn foo() {}
+   |    ^^^
+note: the item is gated here
+  --> $DIR/test-cfg.rs:3:1
+   |
+LL | #[cfg(all(foo, bar))] // foo AND bar
+   | ^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 1 previous error