Merge #3960

3960: ellipsis in tuple patterns r=JoshMcguigan a=JoshMcguigan

This PR lowers ellipsis in tuple patterns. It fixes a bug in the way ellipsis were previously lowered (by replacing the ellipsis with a single `Pat::Wild` no matter how many items the `..` was taking the place of).

It also uses this new information to properly handle `..` in tuple struct patterns when perform match statement exhaustiveness checks.

While this PR provides the building blocks for match statement exhaustiveness checks for tuples, there are some additional challenges there, so that is still unimplemented (unlike tuple structs).

Co-authored-by: Josh Mcguigan <[email protected]>

2 files changed, 25 insertions, 8 deletions

diff --git a/crates/ra_hir_def/src/body/lower.rs b/crates/ra_hir_def/src/body/lower.rs
index 6caa87db4..79abe55ce 100644
--- a/crates/ra_hir_def/src/body/lower.rs
+++ b/crates/ra_hir_def/src/body/lower.rs
@@ -33,6 +33,7 @@ use crate::{
 };
 
 use super::{ExprSource, PatSource};
+use ast::AstChildren;
 
 pub(super) fn lower(
     db: &dyn DefDatabase,
@@ -598,8 +599,8 @@ impl ExprCollector<'_> {
             }
             ast::Pat::TupleStructPat(p) => {
                 let path = p.path().and_then(|path| self.expander.parse_path(path));
-                let args = p.args().map(|p| self.collect_pat(p)).collect();
-                Pat::TupleStruct { path, args }
+                let (args, ellipsis) = self.collect_tuple_pat(p.args());
+                Pat::TupleStruct { path, args, ellipsis }
             }
             ast::Pat::RefPat(p) => {
                 let pat = self.collect_pat_opt(p.pat());
@@ -616,10 +617,10 @@ impl ExprCollector<'_> {
             }
             ast::Pat::ParenPat(p) => return self.collect_pat_opt(p.pat()),
             ast::Pat::TuplePat(p) => {
-                let args = p.args().map(|p| self.collect_pat(p)).collect();
-                Pat::Tuple(args)
+                let (args, ellipsis) = self.collect_tuple_pat(p.args());
+                Pat::Tuple { args, ellipsis }
             }
-            ast::Pat::PlaceholderPat(_) | ast::Pat::DotDotPat(_) => Pat::Wild,
+            ast::Pat::PlaceholderPat(_) => Pat::Wild,
             ast::Pat::RecordPat(p) => {
                 let path = p.path().and_then(|path| self.expander.parse_path(path));
                 let record_field_pat_list =
@@ -665,6 +666,9 @@ impl ExprCollector<'_> {
                     Pat::Missing
                 }
             }
+            ast::Pat::DotDotPat(_) => unreachable!(
+                "`DotDotPat` requires special handling and should not be mapped to a Pat."
+            ),
             // FIXME: implement
             ast::Pat::BoxPat(_) | ast::Pat::RangePat(_) | ast::Pat::MacroPat(_) => Pat::Missing,
         };
@@ -679,6 +683,19 @@ impl ExprCollector<'_> {
             self.missing_pat()
         }
     }
+
+    fn collect_tuple_pat(&mut self, args: AstChildren<ast::Pat>) -> (Vec<PatId>, Option<usize>) {
+        // Find the location of the `..`, if there is one. Note that we do not
+        // consider the possiblity of there being multiple `..` here.
+        let ellipsis = args.clone().position(|p| matches!(p, ast::Pat::DotDotPat(_)));
+        // We want to skip the `..` pattern here, since we account for it above.
+        let args = args
+            .filter(|p| !matches!(p, ast::Pat::DotDotPat(_)))
+            .map(|p| self.collect_pat(p))
+            .collect();
+
+        (args, ellipsis)
+    }
 }
 
 impl From<ast::BinOp> for BinaryOp {
diff --git a/crates/ra_hir_def/src/expr.rs b/crates/ra_hir_def/src/expr.rs
index e11bdf3ec..a0cdad529 100644
--- a/crates/ra_hir_def/src/expr.rs
+++ b/crates/ra_hir_def/src/expr.rs
@@ -374,7 +374,7 @@ pub struct RecordFieldPat {
 pub enum Pat {
     Missing,
     Wild,
-    Tuple(Vec<PatId>),
+    Tuple { args: Vec<PatId>, ellipsis: Option<usize> },
     Or(Vec<PatId>),
     Record { path: Option<Path>, args: Vec<RecordFieldPat>, ellipsis: bool },
     Range { start: ExprId, end: ExprId },
@@ -382,7 +382,7 @@ pub enum Pat {
     Path(Path),
     Lit(ExprId),
     Bind { mode: BindingAnnotation, name: Name, subpat: Option<PatId> },
-    TupleStruct { path: Option<Path>, args: Vec<PatId> },
+    TupleStruct { path: Option<Path>, args: Vec<PatId>, ellipsis: Option<usize> },
     Ref { pat: PatId, mutability: Mutability },
 }
 
@@ -393,7 +393,7 @@ impl Pat {
             Pat::Bind { subpat, .. } => {
                 subpat.iter().copied().for_each(f);
             }
-            Pat::Or(args) | Pat::Tuple(args) | Pat::TupleStruct { args, .. } => {
+            Pat::Or(args) | Pat::Tuple { args, .. } | Pat::TupleStruct { args, .. } => {
                 args.iter().copied().for_each(f);
             }
             Pat::Ref { pat, .. } => f(*pat),