[move][move-compiler] Allow braced module definitions after labeld module definitions (#24350)

## Description 

Enhances the Move compiler's handling around multiple modules in a
single file, plus improves diagnostics around that. This better-supports
`extend module` definitions within the same file as the module itself,
etc.

## Test plan 

Added new tests for valid and invalid uses of `extend module`, including
cases with multiple modules and extensions, and updated expected outputs
to match the improved diagnostics.

---

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Author: cgswords

external-crates/move/crates/move-compiler/src/parser/syntax.rs

@@ -4547,10 +4547,12 @@ fn parse_module(
     let mut stop_parsing = false;
     while context.tokens.peek() != Tok::RBrace {
         // If we are in semicolon mode, EOF is a fine place to stop.
-        // If we see the `module` keyword, this is most-likely someone defining a second module
-        // (erroneously), so we also bail in that case.
+        // If we see the `module` keyword, we are defining a second module, so bail
         if matches!(definition_mode, ModuleDefinitionMode::Semicolon)
-            && (context.tokens.peek() == Tok::EOF || context.tokens.peek() == Tok::Module)
+            && (context.tokens.peek() == Tok::EOF
+                || context.tokens.peek() == Tok::Module
+                || (context.tokens.peek() == Tok::Identifier
+                    && context.tokens.content() == EXTEND_MODIFIER))
         {
             stop_parsing = true;
             break;
@@ -4671,6 +4673,11 @@ fn parse_module_member(context: &mut Context) -> Result<ModuleMember, ErrCase> {
                 }
             }
         }
+        // If we find `extend` or `module`, bail out.
+        Tok::Module => Err(ErrCase::ContinueToModule(attributes)),
+        Tok::Identifier if context.tokens.content() == EXTEND_MODIFIER => {
+            Err(ErrCase::ContinueToModule(attributes))
+        }
         // Regular move constructs
         Tok::Friend => Ok(ModuleMember::Friend(parse_friend_decl(
             attributes, context,
@@ -4755,14 +4762,7 @@ fn parse_module_member(context: &mut Context) -> Result<ModuleMember, ErrCase> {
                             ),
                         )
                     };
-                    if tok == Tok::Module
-                        || (tok == Tok::Identifier && context.tokens.content() == EXTEND_MODIFIER)
-                    {
-                        context.add_diag(*diag);
-                        Err(ErrCase::ContinueToModule(attributes))
-                    } else {
-                        Err(ErrCase::Unknown(diag))
-                    }
+                    Err(ErrCase::Unknown(diag))
                 }
             }
         }
@@ -4860,12 +4860,12 @@ fn parse_file_def(
             if matches!(module.definition_mode, ModuleDefinitionMode::Semicolon)
                 && let Some(prev) = defs.last()
             {
-                let msg = "Cannot define a 'module' label form in a file with multiple modules";
+                let msg = "Cannot define an additional 'module' via label in a file with multiple modules";
                 let mut diag = diag!(Declarations::InvalidModule, (module.name.loc(), msg));
-                diag.add_secondary_label((prev.name_loc(), "Previous definition here"));
+                diag.add_secondary_label((prev.name_loc(), "Previous module defined here"));
                 diag.add_note(
-                    "Either move each 'module' label and definitions into its own file or \
-                            define each as 'module <name> { contents }'",
+                    "Either move each 'module' into its own file or \
+                            define each addiitional module as 'module <name> { contents }'",
                 );
                 context.add_diag(diag);
             }

external-crates/move/crates/move-compiler/tests/move_2024/extensions/empty_extension.snap

@@ -5,11 +5,4 @@ info:
   edition: 2024.alpha
   lint: false
 ---
-error[E01002]: unexpected token
-  ┌─ tests/move_2024/extensions/empty_extension.move:4:1
-  │
-4 │ extend module a::test_extend_module {}
-  │ ^^^^^^
-  │ │
-  │ Unexpected 'extend'
-  │ Expected a module member: 'use', 'friend', 'const', 'fun', 'struct', or 'enum'
+

external-crates/move/crates/move-compiler/tests/move_2024/parser/extend_module_internal.move

@@ -0,0 +1,8 @@
+module a::m;
+
+public struct S { x: u64 }
+
+#[mode(test)]
+extend module a::m {
+    fun get_x(s: &S): u64 { s.x }
+}

external-crates/move/crates/move-compiler/tests/move_2024/parser/extend_module_internal.snap

@@ -0,0 +1,8 @@
+---
+source: crates/move-compiler/tests/move_check_testsuite.rs
+info:
+  flavor: core
+  edition: 2024.alpha
+  lint: false
+---
+

external-crates/move/crates/move-compiler/tests/move_2024/parser/extend_module_invalid_semicolon.move

@@ -0,0 +1,8 @@
+module a::m;
+
+public struct S { x: u64 }
+
+#[mode(test)]
+extend module a::m;
+
+fun get_x(s: &S): u64 { s.x }

external-crates/move/crates/move-compiler/tests/move_2024/parser/extend_module_invalid_semicolon.snap

@@ -0,0 +1,17 @@
+---
+source: crates/move-compiler/tests/move_check_testsuite.rs
+info:
+  flavor: core
+  edition: 2024.alpha
+  lint: false
+---
+error[E02004]: invalid 'module' declaration
+  ┌─ tests/move_2024/parser/extend_module_invalid_semicolon.move:6:18
+  │
+1 │ module a::m;
+  │           - Previous module defined here
+  ·
+6 │ extend module a::m;
+  │                  ^ Cannot define an additional 'module' via label in a file with multiple modules
+  │
+  = Either move each 'module' into its own file or define each addiitional module as 'module <name> { contents }'

external-crates/move/crates/move-compiler/tests/move_2024/parser/multi_module_invalid_0.move

@@ -0,0 +1,3 @@
+module a::m { }
+
+module a::n;

external-crates/move/crates/move-compiler/tests/move_2024/parser/multi_module_invalid_0.snap

@@ -0,0 +1,17 @@
+---
+source: crates/move-compiler/tests/move_check_testsuite.rs
+info:
+  flavor: core
+  edition: 2024.alpha
+  lint: false
+---
+error[E02004]: invalid 'module' declaration
+  ┌─ tests/move_2024/parser/multi_module_invalid_0.move:3:11
+  │
+1 │ module a::m { }
+  │           - Previous module defined here
+2 │ 
+3 │ module a::n;
+  │           ^ Cannot define an additional 'module' via label in a file with multiple modules
+  │
+  = Either move each 'module' into its own file or define each addiitional module as 'module <name> { contents }'

external-crates/move/crates/move-compiler/tests/move_2024/parser/multi_module_invalid_1.move

@@ -0,0 +1,7 @@
+module a::m;
+
+#[mode(test)]
+extend module a::n { }
+
+#[mode(test)]
+extend module a::p;

external-crates/move/crates/move-compiler/tests/move_2024/parser/multi_module_invalid_1.snap

@@ -0,0 +1,17 @@
+---
+source: crates/move-compiler/tests/move_check_testsuite.rs
+info:
+  flavor: core
+  edition: 2024.alpha
+  lint: false
+---
+error[E02004]: invalid 'module' declaration
+  ┌─ tests/move_2024/parser/multi_module_invalid_1.move:7:18
+  │
+4 │ extend module a::n { }
+  │                  - Previous module defined here
+  ·
+7 │ extend module a::p;
+  │                  ^ Cannot define an additional 'module' via label in a file with multiple modules
+  │
+  = Either move each 'module' into its own file or define each addiitional module as 'module <name> { contents }'