auto merge of #17634 : jakub-/rust/if_let, r=kballard

Continuation of #16741.

Author: bors

src/doc/reference.md

@@ -2441,6 +2441,8 @@ The currently implemented features of the reference compiler are:
 * `default_type_params` - Allows use of default type parameters. The future of
                           this feature is uncertain.
 
+* `if_let` - Allows use of the `if let` syntax.
+
 * `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics
                  are inherently unstable and no promise about them is made.
 
@@ -3229,7 +3231,7 @@ for i in range(0u, 256) {
 if_expr : "if" no_struct_literal_expr '{' block '}'
           else_tail ? ;
 
-else_tail : "else" [ if_expr
+else_tail : "else" [ if_expr | if_let_expr
                    | '{' block '}' ] ;
 ```
 
@@ -3434,6 +3436,19 @@ let message = match maybe_digit {
 };
 ```
 
+### If let expressions
+
+```{.ebnf .gram}
+if_let_expr : "if" "let" pat '=' expr '{' block '}'
+               else_tail ? ;
+else_tail : "else" [ if_expr | if_let_expr | '{' block '}' ] ;
+```
+
+An `if let` expression is semantically identical to an `if` expression but in place
+of a condition expression it expects a refutable let statement. If the value of the
+expression on the right hand side of the let statement matches the pattern, the corresponding
+block will execute, otherwise flow proceeds to the first `else` block that follows.
+
 ### Return expressions
 
 ```{.ebnf .gram}

src/librustc/diagnostics.rs

@@ -151,5 +151,6 @@ register_diagnostics!(
     E0157,
     E0158,
     E0159,
-    E0161
+    E0161,
+    E0162
 )

src/librustc/lint/builtin.rs

@@ -1092,7 +1092,10 @@ impl LintPass for UnnecessaryParens {
         let (value, msg, struct_lit_needs_parens) = match e.node {
             ast::ExprIf(ref cond, _, _) => (cond, "`if` condition", true),
             ast::ExprWhile(ref cond, _, _) => (cond, "`while` condition", true),
-            ast::ExprMatch(ref head, _) => (head, "`match` head expression", true),
+            ast::ExprMatch(ref head, _, source) => match source {
+                ast::MatchNormal => (head, "`match` head expression", true),
+                ast::MatchIfLetDesugar => (head, "`if let` head expression", true)
+            },
             ast::ExprRet(Some(ref value)) => (value, "`return` value", false),
             ast::ExprAssign(_, ref value) => (value, "assigned value", false),
             ast::ExprAssignOp(_, _, ref value) => (value, "assigned value", false),
@@ -1242,7 +1245,7 @@ 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

@@ -222,6 +222,10 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
                 self.add_node(expr.id, [then_exit, else_exit])           // 4, 5
             }
 
+            ast::ExprIfLet(..) => {
+                self.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
+            }
+
             ast::ExprWhile(ref cond, ref body, _) => {
                 //
                 //         [pred]
@@ -322,7 +326,7 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
                 expr_exit
             }
 
-            ast::ExprMatch(ref discr, ref arms) => {
+            ast::ExprMatch(ref discr, ref arms, _) => {
                 //
                 //     [pred]
                 //       |

src/librustc/middle/check_match.rs

@@ -147,7 +147,7 @@ pub fn check_crate(tcx: &ty::ctxt) {
 fn check_expr(cx: &mut MatchCheckCtxt, ex: &Expr) {
     visit::walk_expr(cx, ex);
     match ex.node {
-        ExprMatch(ref scrut, ref arms) => {
+        ExprMatch(ref scrut, ref arms, source) => {
             // First, check legality of move bindings.
             for arm in arms.iter() {
                 check_legality_of_move_bindings(cx,
@@ -184,7 +184,7 @@ fn check_expr(cx: &mut MatchCheckCtxt, ex: &Expr) {
             }
 
             // 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.
@@ -252,13 +252,31 @@ fn check_for_static_nan(cx: &MatchCheckCtxt, pats: &[P<Pat>]) {
 }
 
 // Check for unreachable patterns
-fn check_arms(cx: &MatchCheckCtxt, arms: &[(Vec<P<Pat>>, Option<&Expr>)]) {
+fn check_arms(cx: &MatchCheckCtxt, arms: &[(Vec<P<Pat>>, Option<&Expr>)], source: MatchSource) {
     let mut seen = Matrix(vec![]);
+    let mut printed_if_let_err = false;
     for &(ref pats, guard) in arms.iter() {
         for pat in 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 &(ref first_arm_pats, _) = &arms[0];
+                            let first_pat = first_arm_pats.get(0);
+                            let span = first_pat.span;
+                            span_err!(cx.tcx.sess, span, E0162, "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

@@ -374,7 +374,11 @@ impl<'d,'t,'tcx,TYPER:mc::Typer<'tcx>> ExprUseVisitor<'d,'t,TYPER> {
                 }
             }
 
-            ast::ExprMatch(ref discr, ref arms) => {
+            ast::ExprIfLet(..) => {
+                self.tcx().sess.span_bug(expr.span, "non-desugared ExprIfLet");
+            }
+
+            ast::ExprMatch(ref discr, ref arms, _) => {
                 let discr_cmt = return_if_err!(self.mc.cat_expr(&**discr));
                 self.borrow_expr(&**discr, ty::ReEmpty, ty::ImmBorrow, MatchDiscriminant);
 

src/librustc/middle/liveness.rs

@@ -481,6 +481,9 @@ 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(..) => {
+          ir.tcx.sess.span_bug(expr.span, "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 {:?}",
@@ -1011,6 +1014,10 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             self.propagate_through_expr(&**cond, ln)
           }
 
+          ExprIfLet(..) => {
+              self.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
+          }
+
           ExprWhile(ref cond, ref blk, _) => {
             self.propagate_through_loop(expr, WhileLoop(&**cond), &**blk, succ)
           }
@@ -1026,7 +1033,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             self.propagate_through_loop(expr, LoopLoop, &**blk, succ)
           }
 
-          ExprMatch(ref e, ref arms) => {
+          ExprMatch(ref e, ref arms, _) => {
             //
             //      (e)
             //       |
@@ -1470,6 +1477,9 @@ fn check_expr(this: &mut Liveness, expr: &Expr) {
       ExprPath(..) | ExprBox(..) => {
         visit::walk_expr(this, expr);
       }
+      ExprIfLet(..) => {
+        this.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
+      }
     }
 }
 

src/librustc/middle/mem_categorization.rs

@@ -505,6 +505,10 @@ impl<'t,'tcx,TYPER:Typer<'tcx>> MemCategorizationContext<'t,TYPER> {
           ast::ExprForLoop(..) => {
             Ok(self.cat_rvalue_node(expr.id(), expr.span(), expr_ty))
           }
+
+          ast::ExprIfLet(..) => {
+            self.tcx().sess.span_bug(expr.span, "non-desugared ExprIfLet");
+          }
         }
     }
 

src/librustc/middle/trans/debuginfo.rs

@@ -3576,6 +3576,11 @@ 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);
 
@@ -3654,7 +3659,7 @@ 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

@@ -1013,7 +1013,7 @@ fn trans_rvalue_dps_unadjusted<'blk, 'tcx>(bcx: Block<'blk, 'tcx>,
         ast::ExprIf(ref cond, ref thn, ref els) => {
             controlflow::trans_if(bcx, expr.id, &**cond, &**thn, els.as_ref().map(|e| &**e), 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) => {