Auto merge of #17586 - ShoyuVanilla:tuple-arg-macro-rest, r=Veykril

Allow macro expansions into `RestPat` in tuple args work as ellipsis like plain `RestPat`

Fixes #17292

Currently, Rust Analyzer lowers `ast::Pat::RestPat` into `Pat::Missing` in general cases on the following lines;

ffbc5ad993/crates/hir-def/src/body/lower.rs (L1359-L1367)

And in some proper positions such as `TupleStruct(..)`, it is specially handed on the following lines;

ffbc5ad993/crates/hir-def/src/body/lower.rs (L1429-L1437)

This behavior is reasonable because rustc does similar things in
62c068feea/compiler/rustc_ast_lowering/src/pat.rs (L108-L111)
and
62c068feea/compiler/rustc_ast_lowering/src/pat.rs (L123-L142)

But this sometimes works differently because Rust Analyzer expands macros while ast lowering;

ffbc5ad993/crates/hir-def/src/body/lower.rs (L1386-L1398)
ffbc5ad993/crates/hir-def/src/body/lower.rs (L941-L963)
but rustc uses expanded ast in the corresponding tuple-handling process, so it does not have macro patterns there.
62c068feea/compiler/rustc_ast_lowering/src/pat.rs (L114)

So, if a macro expansion in a tuple arg results in `..`, rustc permits it like plain `..` pattern, but Rust Analyzer rejects it.
This is the root cause of #17292 and this PR allows macros expanded into `..` in a tuple arg position work as ellipsis like that.

crates/hir-def/src/body/lower.rs

@@ -4,6 +4,7 @@
 use std::mem;
 
 use base_db::CrateId;
+use either::Either;
 use hir_expand::{
     name::{AsName, Name},
     ExpandError, InFile,
@@ -1437,14 +1438,12 @@ impl ExprCollector<'_> {
         has_leading_comma: bool,
         binding_list: &mut BindingList,
     ) -> (Box<[PatId]>, Option<usize>) {
+        let args: Vec<_> = args.map(|p| self.collect_pat_possibly_rest(p, binding_list)).collect();
         // Find the location of the `..`, if there is one. Note that we do not
         // consider the possibility of there being multiple `..` here.
-        let ellipsis = args.clone().position(|p| matches!(p, ast::Pat::RestPat(_)));
+        let ellipsis = args.iter().position(|p| p.is_right());
         // We want to skip the `..` pattern here, since we account for it above.
-        let mut args: Vec<_> = args
-            .filter(|p| !matches!(p, ast::Pat::RestPat(_)))
-            .map(|p| self.collect_pat(p, binding_list))
-            .collect();
+        let mut args: Vec<_> = args.into_iter().filter_map(Either::left).collect();
         // if there is a leading comma, the user is most likely to type out a leading pattern
         // so we insert a missing pattern at the beginning for IDE features
         if has_leading_comma {
@@ -1454,6 +1453,41 @@ impl ExprCollector<'_> {
         (args.into_boxed_slice(), ellipsis)
     }
 
+    // `collect_pat` rejects `ast::Pat::RestPat`, but it should be handled in some cases that
+    // it is the macro expansion result of an arg sub-pattern in a slice or tuple pattern.
+    fn collect_pat_possibly_rest(
+        &mut self,
+        pat: ast::Pat,
+        binding_list: &mut BindingList,
+    ) -> Either<PatId, ()> {
+        match &pat {
+            ast::Pat::RestPat(_) => Either::Right(()),
+            ast::Pat::MacroPat(mac) => match mac.macro_call() {
+                Some(call) => {
+                    let macro_ptr = AstPtr::new(&call);
+                    let src = self.expander.in_file(AstPtr::new(&pat));
+                    let pat =
+                        self.collect_macro_call(call, macro_ptr, true, |this, expanded_pat| {
+                            if let Some(expanded_pat) = expanded_pat {
+                                this.collect_pat_possibly_rest(expanded_pat, binding_list)
+                            } else {
+                                Either::Left(this.missing_pat())
+                            }
+                        });
+                    if let Some(pat) = pat.left() {
+                        self.source_map.pat_map.insert(src, pat);
+                    }
+                    pat
+                }
+                None => {
+                    let ptr = AstPtr::new(&pat);
+                    Either::Left(self.alloc_pat(Pat::Missing, ptr))
+                }
+            },
+            _ => Either::Left(self.collect_pat(pat, binding_list)),
+        }
+    }
+
     // endregion: patterns
 
     /// Returns `None` (and emits diagnostics) when `owner` if `#[cfg]`d out, and `Some(())` when

crates/ide-diagnostics/src/handlers/mismatched_arg_count.rs

@@ -256,6 +256,75 @@ impl Foo {
         );
     }
 
+    #[test]
+    fn rest_pat_in_macro_expansion() {
+        check_diagnostics(
+            r#"
+// issue #17292
+#![allow(dead_code)]
+
+macro_rules! replace_with_2_dots {
+    ( $( $input:tt )* ) => {
+        ..
+    };
+}
+
+macro_rules! enum_str {
+    (
+        $(
+            $variant:ident (
+                $( $tfield:ty ),*
+            )
+        )
+        ,
+        *
+    ) => {
+        enum Foo {
+            $(
+                $variant ( $( $tfield ),* ),
+            )*
+        }
+
+        impl Foo {
+            fn variant_name_as_str(&self) -> &str {
+                match self {
+                    $(
+                        Self::$variant ( replace_with_2_dots!( $( $tfield ),* ) )
+                          => "",
+                    )*
+                }
+            }
+        }
+    };
+}
+
+enum_str! {
+    TupleVariant1(i32),
+    TupleVariant2(),
+    TupleVariant3(i8,u8,i128)
+}
+"#,
+        );
+
+        check_diagnostics(
+            r#"
+#![allow(dead_code)]
+macro_rules! two_dots1 {
+    () => { .. };
+}
+
+macro_rules! two_dots2 {
+    () => { two_dots1!() };
+}
+
+fn test() {
+    let (_, _, two_dots1!()) = ((), 42);
+    let (_, two_dots2!(), _) = (1, true, 2, false, (), (), 3);
+}
+"#,
+        );
+    }
+
     #[test]
     fn varargs() {
         check_diagnostics(