generate correct completion edits for missing macro arguments

rust-analyzer used the token at the cursor after macro expansion
to decide whether to replace the token at the cursor before macro
expansion. In most cases these two are the same but in some cases these
can mismatch which can lead to incorrect replacements.

For example if an ident/expr macro argument is missing rust-analyzer
generates a "missing" identifier as a placeholder, there is only a
brace at the cursor. Therefore, rust-analyzer will incorrectly replace
the macro brace with the completion in that case leading to #14246.

Using the expanded token type was intentional. However, this doesn't
seem to ever be desirable (this is supported by the fact that there
were no tests that relied on this behavior) since the type of edit to
perform should always be determined by the token it's actually applied
to.

crates/ide-completion/src/context.rs

@@ -387,8 +387,7 @@ pub(crate) struct CompletionContext<'a> {
 impl<'a> CompletionContext<'a> {
     /// The range of the identifier that is being completed.
     pub(crate) fn source_range(&self) -> TextRange {
-        // check kind of macro-expanded token, but use range of original token
+        let kind = self.original_token.kind();
-        let kind = self.token.kind();
         match kind {
             CHAR => {
                 // assume we are completing a lifetime but the user has only typed the '

crates/ide-completion/src/render/macro_.rs

@@ -264,6 +264,65 @@ macro_rules! foo {
 fn main() {
     foo!($0)
 }
+"#,
+        );
+    }
+
+    #[test]
+    fn complete_missing_macro_arg() {
+        // Regression test for https://github.com/rust-lang/rust-analyzer/issues/14246
+        check_edit(
+            "BAR",
+            r#"
+macro_rules! foo {
+    ($val:ident,  $val2: ident) => {
+        $val $val2
+    };
+}
+
+const BAR: u32 = 9;
+fn main() {
+    foo!(BAR, $0)
+}
+"#,
+            r#"
+macro_rules! foo {
+    ($val:ident,  $val2: ident) => {
+        $val $val2
+    };
+}
+
+const BAR: u32 = 9;
+fn main() {
+    foo!(BAR, BAR)
+}
+"#,
+        );
+        check_edit(
+            "BAR",
+            r#"
+macro_rules! foo {
+    ($val:ident,  $val2: ident) => {
+        $val $val2
+    };
+}
+
+const BAR: u32 = 9;
+fn main() {
+    foo!($0)
+}
+"#,
+            r#"
+macro_rules! foo {
+    ($val:ident,  $val2: ident) => {
+        $val $val2
+    };
+}
+
+const BAR: u32 = 9;
+fn main() {
+    foo!(BAR)
+}
 "#,
         );
     }