Improve diagnostics: update note and add help message

compiler/rustc_expand/messages.ftl

@@ -118,8 +118,9 @@ expand_module_file_not_found =
     .note = if there is a `mod {$name}` elsewhere in the crate already, import it with `use crate::...` instead
 
 expand_module_in_block =
-    cannot declare a non-inline module inside a block unless it has a path attribute
-    .note = maybe `use` the module `{$name}` instead of redeclaring it
+    cannot declare an external module inside a block unless it has a path attribute
+    .help = maybe `use` the module `{$name}` instead of redeclaring it
+    .note = external modules are usually placed outside of blocks, at the top level of the file
 
 expand_module_multiple_candidates =
     file for module `{$name}` found at both "{$default_path}" and "{$secondary_path}"

compiler/rustc_expand/src/errors.rs

@@ -265,6 +265,7 @@ pub(crate) struct ModuleCircular {
 
 #[derive(Diagnostic)]
 #[diag(expand_module_in_block)]
+#[note]
 pub(crate) struct ModuleInBlock {
     #[primary_span]
     pub span: Span,
@@ -273,7 +274,7 @@ pub(crate) struct ModuleInBlock {
 }
 
 #[derive(Subdiagnostic)]
-#[note(expand_note)]
+#[help(expand_help)]
 pub(crate) struct ModuleInBlockName {
     #[primary_span]
     pub span: Span,

tests/ui/directory_ownership/external-mod-restriction.rs

@@ -0,0 +1,5 @@
+// Test that external modules are not allowed inside blocks.
+
+fn main() {
+    mod foo; //~ ERROR cannot declare an external module inside a block unless it has a path attribute
+}

tests/ui/directory_ownership/external-mod-restriction.stderr

@@ -0,0 +1,10 @@
+error: cannot declare an external module inside a block unless it has a path attribute
+  --> $DIR/external-mod-restriction.rs:4:5
+   |
+LL |     mod foo;
+   |     ^^^^^^^^
+   |
+   = note: external modules are usually placed outside of blocks, at the top level of the file
+
+error: aborting due to 1 previous error
+

tests/ui/directory_ownership/macro-expanded-mod.rs

@@ -2,7 +2,7 @@
 
 macro_rules! mod_decl {
     ($i:ident) => {
-        mod $i; //~ ERROR cannot declare a non-inline module inside a block
+        mod $i; //~ ERROR cannot declare an external module inside a block unless it has a path attribute
     };
 }
 

tests/ui/directory_ownership/macro-expanded-mod.stderr

@@ -1,4 +1,4 @@
-error: cannot declare a non-inline module inside a block unless it has a path attribute
+error: cannot declare an external module inside a block unless it has a path attribute
   --> $DIR/macro-expanded-mod.rs:5:9
    |
 LL |         mod $i;
@@ -7,6 +7,7 @@ LL |         mod $i;
 LL |     mod_decl!(foo);
    |     -------------- in this macro invocation
    |
+   = note: external modules are usually placed outside of blocks, at the top level of the file
    = note: this error originates in the macro `mod_decl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 1 previous error