Implement if let

src/doc/rust.md

@@ -2457,6 +2457,8 @@ The currently implemented features of the reference compiler are:
             whether this will continue as a feature or not. For these reasons,
             the glob import statement has been hidden behind this feature flag.
 
+* `if_let` - Allows use of the `if let` desugaring syntax.
+
 * `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
                  are inherently unstable and no promise about them is made.
 

src/librustc/diagnostics.rs

@@ -17,6 +17,11 @@ register_diagnostic!(E0001, r##"
     one is too specific or the ordering is incorrect.
 "##)
 
+register_diagnostic!(E0159, r##"
+    This error is produced by an `if let` expression where the pattern is irrefutable.
+    An `if let` that can never fail is considered an error.
+"##)
+
 register_diagnostics!(
     E0002,
     E0003,

src/librustc/front/config.rs

@@ -206,15 +206,15 @@ fn fold_block(cx: &mut Context, b: ast::P<ast::Block>) -> ast::P<ast::Block> {
 
 fn fold_expr(cx: &mut Context, expr: Gc<ast::Expr>) -> Gc<ast::Expr> {
     let expr = match expr.node {
-        ast::ExprMatch(ref m, ref arms) => {
+        ast::ExprMatch(ref m, ref arms, source) => {
             let arms = arms.iter()
                 .filter(|a| (cx.in_cfg)(a.attrs.as_slice()))
                 .map(|a| a.clone())
                 .collect();
             box(GC) ast::Expr {
                 id: expr.id,
                 span: expr.span.clone(),
-                node: ast::ExprMatch(m.clone(), arms),
+                node: ast::ExprMatch(m.clone(), arms, source),
             }
         }
         _ => expr.clone()

src/librustc/front/feature_gate.rs

@@ -70,6 +70,8 @@ static KNOWN_FEATURES: &'static [(&'static str, Status)] = &[
     ("unboxed_closures", Active),
     ("import_shadowing", Active),
 
+    ("if_let", Active),
+
     // if you change this list without updating src/doc/rust.md, cmr will be sad
 
     // A temporary feature gate used to enable parser extensions needed
@@ -339,6 +341,14 @@ impl<'a> Visitor<()> for Context<'a> {
                                   "unboxed closures are a work-in-progress \
                                    feature with known bugs");
             }
+            ast::ExprIfLet(..) |
+            ast::ExprMatch(_, _, ast::MatchIfLetDesugar) => {
+                // note: at the point where we check feature-gates, the AST desugaring
+                // should not have happened, but the ExprMatch check is included just
+                // in case.
+                self.gate_feature("if_let", e.span,
+                                  "`if let` desugaring is experimental");
+            }
             _ => {}
         }
         visit::walk_expr(self, e, ());

src/librustc/lint/builtin.rs

@@ -1080,7 +1080,10 @@ impl LintPass for UnnecessaryParens {
         let (value, msg, struct_lit_needs_parens) = match e.node {
             ast::ExprIf(cond, _, _) => (cond, "`if` condition", true),
             ast::ExprWhile(cond, _) => (cond, "`while` condition", true),
-            ast::ExprMatch(head, _) => (head, "`match` head expression", true),
+            ast::ExprMatch(head, _, source) => match source {
+                ast::MatchNormal => (head, "`match` head expression", true),
+                ast::MatchIfLetDesugar => (head, "`if let` head expression", true)
+            },
             ast::ExprRet(Some(value)) => (value, "`return` value", false),
             ast::ExprAssign(_, value) => (value, "assigned value", false),
             ast::ExprAssignOp(_, _, value) => (value, "assigned value", false),
@@ -1192,7 +1195,7 @@ impl LintPass for UnusedMut {
 
     fn check_expr(&mut self, cx: &Context, e: &ast::Expr) {
         match e.node {
-            ast::ExprMatch(_, ref arms) => {
+            ast::ExprMatch(_, ref arms, _) => {
                 for a in arms.iter() {
                     self.check_unused_mut_pat(cx, a.pats.as_slice())
                 }

src/librustc/middle/cfg/construct.rs

@@ -225,6 +225,8 @@ impl<'a> CFGBuilder<'a> {
                 self.add_node(expr.id, [then_exit, else_exit])           // 4, 5
             }
 
+            ast::ExprIfLet(..) => fail!("non-desugared ExprIfLet"),
+
             ast::ExprWhile(ref cond, ref body) => {
                 //
                 //         [pred]
@@ -325,7 +327,7 @@ impl<'a> CFGBuilder<'a> {
                 expr_exit
             }
 
-            ast::ExprMatch(ref discr, ref arms) => {
+            ast::ExprMatch(ref discr, ref arms, _) => {
                 //
                 //     [pred]
                 //       |

src/librustc/middle/check_match.rs

@@ -139,7 +139,7 @@ pub fn check_crate(tcx: &ty::ctxt, krate: &Crate) {
 fn check_expr(cx: &mut MatchCheckCtxt, ex: &Expr) {
     visit::walk_expr(cx, ex, ());
     match ex.node {
-        ExprMatch(scrut, ref arms) => {
+        ExprMatch(scrut, ref arms, source) => {
             // First, check legality of move bindings.
             for arm in arms.iter() {
                 check_legality_of_move_bindings(cx,
@@ -178,7 +178,7 @@ fn check_expr(cx: &mut MatchCheckCtxt, ex: &Expr) {
             check_for_static_nan(cx, inlined_arms.as_slice());
 
             // Fourth, check for unreachable arms.
-            check_arms(cx, inlined_arms.as_slice());
+            check_arms(cx, inlined_arms.as_slice(), source);
 
             // Finally, check if the whole match expression is exhaustive.
             // Check for empty enum, because is_useful only works on inhabited types.
@@ -251,13 +251,30 @@ fn check_for_static_nan(cx: &MatchCheckCtxt, arms: &[Arm]) {
 }
 
 // Check for unreachable patterns
-fn check_arms(cx: &MatchCheckCtxt, arms: &[Arm]) {
+fn check_arms(cx: &MatchCheckCtxt, arms: &[Arm], source: MatchSource) {
     let mut seen = Matrix(vec!());
+    let mut printed_if_let_err = false;
     for arm in arms.iter() {
         for &pat in arm.pats.iter() {
             let v = vec![pat];
             match is_useful(cx, &seen, v.as_slice(), LeaveOutWitness) {
-                NotUseful => span_err!(cx.tcx.sess, pat.span, E0001, "unreachable pattern"),
+                NotUseful => {
+                    if source == MatchIfLetDesugar {
+                        if printed_if_let_err {
+                            // we already printed an irrefutable if-let pattern error.
+                            // We don't want two, that's just confusing.
+                        } else {
+                            // find the first arm pattern so we can use its span
+                            let first_arm = &arms[0]; // we know there's at least 1 arm
+                            let first_pat = first_arm.pats.get(0); // and it's safe to assume 1 pat
+                            let span = first_pat.span;
+                            span_err!(cx.tcx.sess, span, E0159, "irrefutable if-let pattern");
+                            printed_if_let_err = true;
+                        }
+                    } else {
+                        span_err!(cx.tcx.sess, pat.span, E0001, "unreachable pattern");
+                    }
+                }
                 Useful => (),
                 UsefulWithWitness(_) => unreachable!()
             }

src/librustc/middle/expr_use_visitor.rs

@@ -356,7 +356,9 @@ impl<'d,'t,TYPER:mc::Typer> ExprUseVisitor<'d,'t,TYPER> {
                 }
             }
 
-            ast::ExprMatch(ref discr, ref arms) => {
+            ast::ExprIfLet(..) => fail!("non-desugared ExprIfLet"),
+
+            ast::ExprMatch(ref discr, ref arms, _) => {
                 // treatment of the discriminant is handled while
                 // walking the arms:
                 self.walk_expr(&**discr);

src/librustc/middle/liveness.rs

@@ -491,6 +491,7 @@ fn visit_expr(ir: &mut IrMaps, expr: &Expr) {
         ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
         visit::walk_expr(ir, expr, ());
       }
+      ExprIfLet(..) => fail!("non-desugared ExprIfLet"),
       ExprForLoop(ref pat, _, _, _) => {
         pat_util::pat_bindings(&ir.tcx.def_map, &**pat, |bm, p_id, sp, path1| {
             debug!("adding local variable {} from for loop with bm {:?}",
@@ -1017,6 +1018,8 @@ impl<'a> Liveness<'a> {
             self.propagate_through_expr(&**cond, ln)
           }
 
+          ExprIfLet(..) => fail!("non-desugared ExprIfLet"),
+
           ExprWhile(ref cond, ref blk) => {
             self.propagate_through_loop(expr,
                                         WhileLoop(cond.clone()),
@@ -1035,7 +1038,7 @@ impl<'a> Liveness<'a> {
             self.propagate_through_loop(expr, LoopLoop, &**blk, succ)
           }
 
-          ExprMatch(ref e, ref arms) => {
+          ExprMatch(ref e, ref arms, _) => {
             //
             //      (e)
             //       |
@@ -1458,6 +1461,7 @@ fn check_expr(this: &mut Liveness, expr: &Expr) {
       ExprPath(..) | ExprBox(..) | ExprForLoop(..) => {
         visit::walk_expr(this, expr, ());
       }
+      ExprIfLet(..) => fail!("non-desugared ExprIfLet")
     }
 }
 

src/librustc/middle/mem_categorization.rs

@@ -514,6 +514,8 @@ impl<'t,TYPER:Typer> MemCategorizationContext<'t,TYPER> {
           ast::ExprForLoop(..) => {
             Ok(self.cat_rvalue_node(expr.id(), expr.span(), expr_ty))
           }
+
+          ast::ExprIfLet(..) => fail!("non-desugared ExprIfLet")
         }
     }
 

src/librustc/middle/trans/debuginfo.rs

@@ -3603,6 +3603,11 @@ fn populate_scope_map(cx: &CrateContext,
                 }
             }
 
+            ast::ExprIfLet(..) => {
+                cx.sess().span_bug(exp.span, "debuginfo::populate_scope_map() - \
+                                              Found unexpanded if-let.");
+            }
+
             ast::ExprWhile(ref cond_exp, ref loop_body) => {
                 walk_expr(cx, &**cond_exp, scope_stack, scope_map);
 
@@ -3681,7 +3686,7 @@ fn populate_scope_map(cx: &CrateContext,
                 }
             }
 
-            ast::ExprMatch(ref discriminant_exp, ref arms) => {
+            ast::ExprMatch(ref discriminant_exp, ref arms, _) => {
                 walk_expr(cx, &**discriminant_exp, scope_stack, scope_map);
 
                 // For each arm we have to first walk the pattern as these might

src/librustc/middle/trans/expr.rs

@@ -739,7 +739,7 @@ fn trans_rvalue_dps_unadjusted<'a>(bcx: &'a Block<'a>,
         ast::ExprIf(ref cond, ref thn, els) => {
             controlflow::trans_if(bcx, expr.id, &**cond, thn.clone(), els, dest)
         }
-        ast::ExprMatch(ref discr, ref arms) => {
+        ast::ExprMatch(ref discr, ref arms, _) => {
             _match::trans_match(bcx, expr, &**discr, arms.as_slice(), dest)
         }
         ast::ExprBlock(ref blk) => {

src/librustc/middle/ty.rs

@@ -3429,6 +3429,8 @@ pub fn expr_kind(tcx: &ctxt, expr: &ast::Expr) -> ExprKind {
             RvalueDpsExpr
         }
 
+        ast::ExprIfLet(..) => fail!("non-desugared ExprIfLet"),
+
         ast::ExprLit(lit) if lit_is_str(lit) => {
             RvalueDpsExpr
         }

src/librustc/middle/typeck/check/mod.rs

@@ -3430,6 +3430,7 @@ fn check_expr_with_unifier(fcx: &FnCtxt,
         check_then_else(fcx, &**cond, &**then_blk, opt_else_expr.clone(),
                         id, expr.span, expected);
       }
+      ast::ExprIfLet(..) => fail!("non-desugared ExprIfLet"),
       ast::ExprWhile(ref cond, ref body) => {
         check_expr_has_type(fcx, &**cond, ty::mk_bool());
         check_block_no_value(fcx, &**body);
@@ -3468,7 +3469,7 @@ fn check_expr_with_unifier(fcx: &FnCtxt,
             fcx.write_nil(id);
         }
       }
-      ast::ExprMatch(ref discrim, ref arms) => {
+      ast::ExprMatch(ref discrim, ref arms, _) => {
         _match::check_match(fcx, expr, &**discrim, arms.as_slice());
       }
       ast::ExprFnBlock(_, ref decl, ref body) => {

src/librustc/middle/typeck/check/regionck.rs

@@ -591,7 +591,7 @@ fn visit_expr(rcx: &mut Rcx, expr: &ast::Expr) {
             visit::walk_expr(rcx, expr, ());
         }
 
-        ast::ExprMatch(ref discr, ref arms) => {
+        ast::ExprMatch(ref discr, ref arms, _) => {
             link_match(rcx, &**discr, arms.as_slice());
 
             visit::walk_expr(rcx, expr, ());

src/librustc/util/ppaux.rs

@@ -80,6 +80,7 @@ pub fn explain_region_and_span(cx: &ctxt, region: ty::Region)
               ast::ExprMethodCall(..) => {
                 explain_span(cx, "method call", expr.span)
               },
+              ast::ExprMatch(_, _, ast::MatchIfLetDesugar) => explain_span(cx, "if let", expr.span),
               ast::ExprMatch(..) => explain_span(cx, "match", expr.span),
               _ => explain_span(cx, "expression", expr.span)
             }

src/librustc_back/svh.rs

@@ -291,6 +291,7 @@ mod svh_visitor {
             ExprForLoop(..)          => SawExprForLoop,
 
             // just syntactic artifacts, expanded away by time of SVH.
+            ExprIfLet(..)            => unreachable!(),
             ExprMac(..)              => unreachable!(),
         }
     }

src/libsyntax/ast.rs

@@ -534,13 +534,14 @@ pub enum Expr_ {
     ExprLit(Gc<Lit>),
     ExprCast(Gc<Expr>, P<Ty>),
     ExprIf(Gc<Expr>, P<Block>, Option<Gc<Expr>>),
+    ExprIfLet(P<Pat>, P<Expr>, P<Block>, Option<P<Expr>>),
     ExprWhile(Gc<Expr>, P<Block>),
     // FIXME #6993: change to Option<Name> ... or not, if these are hygienic.
     ExprForLoop(Gc<Pat>, Gc<Expr>, P<Block>, Option<Ident>),
     // Conditionless loop (can be exited with break, cont, or ret)
     // FIXME #6993: change to Option<Name> ... or not, if these are hygienic.
     ExprLoop(P<Block>, Option<Ident>),
-    ExprMatch(Gc<Expr>, Vec<Arm>),
+    ExprMatch(Gc<Expr>, Vec<Arm>, MatchSource),
     ExprFnBlock(CaptureClause, P<FnDecl>, P<Block>),
     ExprProc(P<FnDecl>, P<Block>),
     ExprUnboxedFn(CaptureClause, UnboxedClosureKind, P<FnDecl>, P<Block>),
@@ -574,6 +575,12 @@ pub enum Expr_ {
     ExprParen(Gc<Expr>)
 }
 
+#[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
+pub enum MatchSource {
+    MatchNormal,
+    MatchIfLetDesugar
+}
+
 #[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
 pub enum CaptureClause {
     CaptureByValue,

src/libsyntax/ext/build.rs

@@ -860,7 +860,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
 
     fn expr_match(&self, span: Span, arg: Gc<ast::Expr>,
                   arms: Vec<ast::Arm>) -> Gc<Expr> {
-        self.expr(span, ast::ExprMatch(arg, arms))
+        self.expr(span, ast::ExprMatch(arg, arms, ast::MatchNormal))
     }
 
     fn expr_if(&self, span: Span,