Add let-else to AST

10 files changed, 92 insertions, 26 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index e1ea464dedb..0632d937c4c 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -1005,13 +1005,42 @@ pub struct Local {
     pub id: NodeId,
     pub pat: P<Pat>,
     pub ty: Option<P<Ty>>,
-    /// Initializer expression to set the value, if any.
-    pub init: Option<P<Expr>>,
+    pub kind: LocalKind,
     pub span: Span,
     pub attrs: AttrVec,
     pub tokens: Option<LazyTokenStream>,
 }
 
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum LocalKind {
+    /// Local declaration.
+    /// Example: `let x;`
+    Decl,
+    /// Local declaration with an initializer.
+    /// Example: `let x = y;`
+    Init(P<Expr>),
+    /// Local declaration with an initializer and an `else` clause.
+    /// Example: `let Some(x) = y else { return };`
+    InitElse(P<Expr>, P<Block>),
+}
+
+impl LocalKind {
+    pub fn init(&self) -> Option<&Expr> {
+        match self {
+            Self::Decl => None,
+            Self::Init(i) | Self::InitElse(i, _) => Some(i),
+        }
+    }
+
+    pub fn init_else_opt(&self) -> Option<(&Expr, Option<&Block>)> {
+        match self {
+            Self::Decl => None,
+            Self::Init(init) => Some((init, None)),
+            Self::InitElse(init, els) => Some((init, Some(els))),
+        }
+    }
+}
+
 /// An arm of a 'match'.
 ///
 /// E.g., `0..=10 => { println!("match!") }` as in
diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
index fb4db6005ac..368a23e3429 100644
--- a/compiler/rustc_ast/src/mut_visit.rs
+++ b/compiler/rustc_ast/src/mut_visit.rs
@@ -571,11 +571,20 @@ pub fn noop_visit_parenthesized_parameter_data<T: MutVisitor>(
 }
 
 pub fn noop_visit_local<T: MutVisitor>(local: &mut P<Local>, vis: &mut T) {
-    let Local { id, pat, ty, init, span, attrs, tokens } = local.deref_mut();
+    let Local { id, pat, ty, kind, span, attrs, tokens } = local.deref_mut();
     vis.visit_id(id);
     vis.visit_pat(pat);
     visit_opt(ty, |ty| vis.visit_ty(ty));
-    visit_opt(init, |init| vis.visit_expr(init));
+    match kind {
+        LocalKind::Decl => {}
+        LocalKind::Init(init) => {
+            vis.visit_expr(init);
+        }
+        LocalKind::InitElse(init, els) => {
+            vis.visit_expr(init);
+            vis.visit_block(els);
+        }
+    }
     vis.visit_span(span);
     visit_thin_attrs(attrs, vis);
     visit_lazy_tts(tokens, vis);
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index dd8927496e0..c30f711b397 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -242,7 +242,10 @@ pub fn walk_local<'a, V: Visitor<'a>>(visitor: &mut V, local: &'a Local) {
     }
     visitor.visit_pat(&local.pat);
     walk_list!(visitor, visit_ty, &local.ty);
-    walk_list!(visitor, visit_expr, &local.init);
+    if let Some((init, els)) = local.kind.init_else_opt() {
+        visitor.visit_expr(init);
+        walk_list!(visitor, visit_block, els);
+    }
 }
 
 pub fn walk_label<'a, V: Visitor<'a>>(visitor: &mut V, label: &'a Label) {
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index b71fcb7a349..8efb1680d85 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -1798,7 +1798,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             .ty
             .as_ref()
             .map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Binding)));
-        let init = l.init.as_ref().map(|e| self.lower_expr(e));
+        let init = l.kind.init().map(|init| self.lower_expr(init));
         let hir_id = self.lower_node_id(l.id);
         self.lower_attrs(hir_id, &l.attrs);
         hir::Local {
diff --git a/compiler/rustc_ast_pretty/src/pprust/state.rs b/compiler/rustc_ast_pretty/src/pprust/state.rs
index 949e7a1fbb8..3cf04be160c 100644
--- a/compiler/rustc_ast_pretty/src/pprust/state.rs
+++ b/compiler/rustc_ast_pretty/src/pprust/state.rs
@@ -1518,13 +1518,19 @@ impl<'a> State<'a> {
                 self.ibox(INDENT_UNIT);
                 self.print_local_decl(loc);
                 self.end();
-                if let Some(ref init) = loc.init {
+                if let Some((init, els)) = loc.kind.init_else_opt() {
                     self.nbsp();
                     self.word_space("=");
                     self.print_expr(init);
+                    if let Some(els) = els {
+                        self.cbox(INDENT_UNIT);
+                        self.ibox(INDENT_UNIT);
+                        self.s.word(" else ");
+                        self.print_block(els);
+                    }
                 }
                 self.s.word(";");
-                self.end();
+                self.end(); // `let` ibox
             }
             ast::StmtKind::Item(ref item) => self.print_item(item),
             ast::StmtKind::Expr(ref expr) => {
diff --git a/compiler/rustc_builtin_macros/src/deriving/debug.rs b/compiler/rustc_builtin_macros/src/deriving/debug.rs
index cc6dac52d76..14506f296bf 100644
--- a/compiler/rustc_builtin_macros/src/deriving/debug.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/debug.rs
@@ -3,7 +3,7 @@ use crate::deriving::generic::*;
 use crate::deriving::path_std;
 
 use rustc_ast::ptr::P;
-use rustc_ast::{self as ast, Expr, MetaItem};
+use rustc_ast::{self as ast, Expr, LocalKind, MetaItem};
 use rustc_expand::base::{Annotatable, ExtCtxt};
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::{Span, DUMMY_SP};
@@ -135,8 +135,8 @@ fn stmt_let_underscore(cx: &mut ExtCtxt<'_>, sp: Span, expr: P<ast::Expr>) -> as
     let local = P(ast::Local {
         pat: cx.pat_wild(sp),
         ty: None,
-        init: Some(expr),
         id: ast::DUMMY_NODE_ID,
+        kind: LocalKind::Init(expr),
         span: sp,
         attrs: ast::AttrVec::new(),
         tokens: None,
diff --git a/compiler/rustc_expand/src/build.rs b/compiler/rustc_expand/src/build.rs
index 824df2757ea..1d83ecbfd40 100644
--- a/compiler/rustc_expand/src/build.rs
+++ b/compiler/rustc_expand/src/build.rs
@@ -2,7 +2,7 @@ use crate::base::ExtCtxt;
 
 use rustc_ast::attr;
 use rustc_ast::ptr::P;
-use rustc_ast::{self as ast, AttrVec, BlockCheckMode, Expr, PatKind, UnOp};
+use rustc_ast::{self as ast, AttrVec, BlockCheckMode, Expr, LocalKind, PatKind, UnOp};
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 
@@ -153,8 +153,8 @@ impl<'a> ExtCtxt<'a> {
         let local = P(ast::Local {
             pat,
             ty: None,
-            init: Some(ex),
             id: ast::DUMMY_NODE_ID,
+            kind: LocalKind::Init(ex),
             span: sp,
             attrs: AttrVec::new(),
             tokens: None,
@@ -167,8 +167,8 @@ impl<'a> ExtCtxt<'a> {
         let local = P(ast::Local {
             pat: self.pat_wild(span),
             ty: Some(ty),
-            init: None,
             id: ast::DUMMY_NODE_ID,
+            kind: LocalKind::Decl,
             span,
             attrs: AttrVec::new(),
             tokens: None,
diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
index be137884b4b..a289b379fc8 100644
--- a/compiler/rustc_lint/src/unused.rs
+++ b/compiler/rustc_lint/src/unused.rs
@@ -618,7 +618,7 @@ trait UnusedDelimLint {
     fn check_stmt(&mut self, cx: &EarlyContext<'_>, s: &ast::Stmt) {
         match s.kind {
             StmtKind::Local(ref local) if Self::LINT_EXPR_IN_PATTERN_MATCHING_CTX => {
-                if let Some(ref value) = local.init {
+                if let Some(value) = local.kind.init() {
                     self.check_unused_delims_expr(
                         cx,
                         &value,
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
index 85515bd2a63..d083c379f77 100644
--- a/compiler/rustc_parse/src/parser/stmt.rs
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -11,8 +11,9 @@ use rustc_ast as ast;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, TokenKind};
 use rustc_ast::util::classify;
-use rustc_ast::AstLike;
-use rustc_ast::{AttrStyle, AttrVec, Attribute, MacCall, MacCallStmt, MacStmtStyle};
+use rustc_ast::{
+    AstLike, AttrStyle, AttrVec, Attribute, LocalKind, MacCall, MacCallStmt, MacStmtStyle,
+};
 use rustc_ast::{Block, BlockCheckMode, Expr, ExprKind, Local, Stmt};
 use rustc_ast::{StmtKind, DUMMY_NODE_ID};
 use rustc_errors::{Applicability, PResult};
@@ -292,8 +293,19 @@ impl<'a> Parser<'a> {
                 return Err(err);
             }
         };
+        let kind = match init {
+            None => LocalKind::Decl,
+            Some(init) => {
+                if self.eat_keyword(kw::Else) {
+                    let els = self.parse_block()?;
+                    LocalKind::InitElse(init, els)
+                } else {
+                    LocalKind::Init(init)
+                }
+            }
+        };
         let hi = if self.token == token::Semi { self.token.span } else { self.prev_token.span };
-        Ok(P(ast::Local { ty, pat, init, id: DUMMY_NODE_ID, span: lo.to(hi), attrs, tokens: None }))
+        Ok(P(ast::Local { ty, pat, kind, id: DUMMY_NODE_ID, span: lo.to(hi), attrs, tokens: None }))
     }
 
     /// Parses the RHS of a local variable declaration (e.g., `= 14;`).
@@ -495,13 +507,13 @@ impl<'a> Parser<'a> {
             StmtKind::Expr(_) | StmtKind::MacCall(_) => {}
             StmtKind::Local(ref mut local) if let Err(e) = self.expect_semi() => {
                 // We might be at the `,` in `let x = foo<bar, baz>;`. Try to recover.
-                match &mut local.init {
-                    Some(ref mut expr) => {
-                        self.check_mistyped_turbofish_with_multiple_type_params(e, expr)?;
-                        // We found `foo<bar, baz>`, have we fully recovered?
-                        self.expect_semi()?;
-                    }
-                    None => return Err(e),
+                match &mut local.kind {
+                    LocalKind::Init(expr) | LocalKind::InitElse(expr, _) => {
+                            self.check_mistyped_turbofish_with_multiple_type_params(e, expr)?;
+                            // We found `foo<bar, baz>`, have we fully recovered?
+                            self.expect_semi()?;
+                        }
+                        LocalKind::Decl => return Err(e),
                 }
                 eat_semi = false;
             }
diff --git a/compiler/rustc_resolve/src/late.rs b/compiler/rustc_resolve/src/late.rs
index 0b552aa07f5..5c7b4b02822 100644
--- a/compiler/rustc_resolve/src/late.rs
+++ b/compiler/rustc_resolve/src/late.rs
@@ -454,7 +454,7 @@ impl<'a: 'ast, 'ast> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast> {
             _ => Some((
                 local.pat.span,
                 local.ty.as_ref().map(|ty| ty.span),
-                local.init.as_ref().map(|init| init.span),
+                local.kind.init().map(|init| init.span),
             )),
         };
         let original = replace(&mut self.diagnostic_metadata.current_let_binding, local_spans);
@@ -1426,7 +1426,14 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
         walk_list!(self, visit_ty, &local.ty);
 
         // Resolve the initializer.
-        walk_list!(self, visit_expr, &local.init);
+        if let Some((init, els)) = local.kind.init_else_opt() {
+            self.visit_expr(init);
+
+            // Resolve the `else` block
+            if let Some(els) = els {
+                self.visit_block(els);
+            }
+        }
 
         // Resolve the pattern.
         self.resolve_pattern_top(&local.pat, PatternSource::Let);