rustc: provide more precise information about refutable patterns.

The compiler now points exactly which part(s) of a pattern are
refutable, rather than just highlighting the whole pattern.

Author: huonw

src/librustc/middle/check_match.rs

@@ -863,13 +863,17 @@ fn default(cx: &MatchCheckCtxt, r: &[@Pat]) -> Option<Vec<@Pat> > {
 
 fn check_local(cx: &mut MatchCheckCtxt, loc: &Local) {
     visit::walk_local(cx, loc, ());
-    if is_refutable(cx, loc.pat) {
-        let name = match loc.source {
-            LocalLet => "local",
-            LocalFor => "`for` loop"
-        };
 
-        cx.tcx.sess.span_err(loc.pat.span,
+    let name = match loc.source {
+        LocalLet => "local",
+        LocalFor => "`for` loop"
+    };
+
+    let mut spans = vec![];
+    find_refutable(cx, loc.pat, &mut spans);
+
+    for span in spans.iter() {
+        cx.tcx.sess.span_err(*span,
                              format!("refutable pattern in {} binding", name).as_slice());
     }
 
@@ -884,53 +888,65 @@ fn check_fn(cx: &mut MatchCheckCtxt,
             sp: Span) {
     visit::walk_fn(cx, kind, decl, body, sp, ());
     for input in decl.inputs.iter() {
-        if is_refutable(cx, input.pat) {
-            cx.tcx.sess.span_err(input.pat.span,
+        let mut spans = vec![];
+        find_refutable(cx, input.pat, &mut spans);
+
+        for span in spans.iter() {
+            cx.tcx.sess.span_err(*span,
                                  "refutable pattern in function argument");
         }
     }
 }
 
-fn is_refutable(cx: &MatchCheckCtxt, pat: &Pat) -> bool {
+fn find_refutable(cx: &MatchCheckCtxt, pat: &Pat, spans: &mut Vec<Span>) {
+    macro_rules! this_pattern {
+        () => {
+            {
+                spans.push(pat.span);
+                return
+            }
+        }
+    }
     let opt_def = cx.tcx.def_map.borrow().find_copy(&pat.id);
     match opt_def {
       Some(DefVariant(enum_id, _, _)) => {
         if ty::enum_variants(cx.tcx, enum_id).len() != 1u {
-            return true;
+            this_pattern!()
         }
       }
-      Some(DefStatic(..)) => return true,
+      Some(DefStatic(..)) => this_pattern!(),
       _ => ()
     }
 
     match pat.node {
       PatUniq(sub) | PatRegion(sub) | PatIdent(_, _, Some(sub)) => {
-        is_refutable(cx, sub)
+        find_refutable(cx, sub, spans)
       }
-      PatWild | PatWildMulti | PatIdent(_, _, None) => { false }
+      PatWild | PatWildMulti | PatIdent(_, _, None) => {}
       PatLit(lit) => {
           match lit.node {
             ExprLit(lit) => {
                 match lit.node {
-                    LitNil => false,    // `()`
-                    _ => true,
+                    LitNil => {}    // `()`
+                    _ => this_pattern!(),
                 }
             }
-            _ => true,
+            _ => this_pattern!(),
           }
       }
-      PatRange(_, _) => { true }
+      PatRange(_, _) => { this_pattern!() }
       PatStruct(_, ref fields, _) => {
-        fields.iter().any(|f| is_refutable(cx, f.pat))
-      }
-      PatTup(ref elts) => {
-        elts.iter().any(|elt| is_refutable(cx, *elt))
+          for f in fields.iter() {
+              find_refutable(cx, f.pat, spans);
+          }
       }
-      PatEnum(_, Some(ref args)) => {
-        args.iter().any(|a| is_refutable(cx, *a))
+      PatTup(ref elts) | PatEnum(_, Some(ref elts))=> {
+          for elt in elts.iter() {
+              find_refutable(cx, *elt, spans)
+          }
       }
-      PatEnum(_,_) => { false }
-      PatVec(..) => { true }
+      PatEnum(_,_) => {}
+      PatVec(..) => { this_pattern!() }
     }
 }
 

src/test/compile-fail/precise-refutable-pattern-errors.rs

@@ -0,0 +1,32 @@
+// 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.
+
+
+fn func(
+    (
+        1, //~ ERROR refutable pattern in function argument
+        (
+            Some( //~ ERROR refutable pattern in function argument
+                1), // nested, so no warning.
+            2..3 //~ ERROR refutable pattern in function argument
+            )
+        ): (int, (Option<int>, int))
+        ) {}
+
+fn main() {
+    let (
+        1, //~ ERROR refutable pattern in local binding
+        (
+            Some( //~ ERROR refutable pattern in local binding
+                1), // nested, so no warning.
+            2..3 //~ ERROR refutable pattern in local binding
+            )
+        ) = (1, (None, 2));
+}