Only print "irrefutable if-let pattern" errors once

When an `if let` translates into a `match` with more than two arms, we
don't want to print multiple "irrefutable if-let pattern" errors.

Author: lilyball

src/librustc/middle/check_match.rs

@@ -253,19 +253,26 @@ fn check_for_static_nan(cx: &MatchCheckCtxt, arms: &[Arm]) {
 // Check for unreachable patterns
 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 => {
                     if source == MatchIfLetDesugar {
-                        // 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")
+                        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")
+                        span_err!(cx.tcx.sess, pat.span, E0001, "unreachable pattern");
                     }
                 }
                 Useful => (),

src/test/compile-fail/if-let.rs

@@ -0,0 +1,58 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(macro_rules)]
+
+fn macros() {
+    macro_rules! foo{
+        ($p:pat, $e:expr, $b:block) => {{
+            if let $p = $e $b
+        }}
+    }
+    macro_rules! bar{
+        ($p:pat, $e:expr, $b:block) => {{
+            foo!($p, $e, $b)
+        }}
+    }
+
+    foo!(a, 1i, { //~ ERROR irrefutable if-let
+        println!("irrefutable pattern");
+    });
+    bar!(a, 1i, { //~ ERROR irrefutable if-let
+        println!("irrefutable pattern");
+    });
+}
+
+pub fn main() {
+    if let a = 1i { //~ ERROR irrefutable if-let
+        println!("irrefutable pattern");
+    }
+
+    if let a = 1i { //~ ERROR irrefutable if-let
+        println!("irrefutable pattern");
+    } else if true {
+        println!("else-if in irrefutable if-let");
+    } else {
+        println!("else in irrefutable if-let");
+    }
+
+    if let 1i = 2i {
+        println!("refutable pattern");
+    } else if let a = 1i { //~ ERROR irrefutable if-let
+        println!("irrefutable pattern");
+    }
+
+    if true {
+        println!("if");
+    } else if let a = 1i { //~ ERROR irrefutable if-let
+        println!("irrefutable pattern");
+    }
+}
+