From 9f641a1009e92431b4a49a835c4e80c51ce44c88 Mon Sep 17 00:00:00 2001
From: mcarton <cartonmartin+git@gmail.com>
Date: Wed, 13 Jan 2016 01:19:27 +0100
Subject: [PATCH] Add known enums to SINGLE_MATCH

---
 src/matches.rs                | 53 ++++++++++++++++++++++++++++++++---
 src/utils.rs                  |  1 +
 tests/compile-fail/matches.rs | 49 ++++++++++++++++++++++++++++----
 3 files changed, 94 insertions(+), 9 deletions(-)

diff --git a/src/matches.rs b/src/matches.rs
index 5e207093c15..217c4abf45b 100644
--- a/src/matches.rs
+++ b/src/matches.rs
@@ -8,7 +8,8 @@ use std::cmp::Ordering;
 use syntax::ast::Lit_::LitBool;
 use syntax::codemap::Span;
 
-use utils::{snippet, span_lint, span_note_and_lint, span_help_and_lint, in_external_macro, expr_block};
+use utils::{COW_PATH, OPTION_PATH, RESULT_PATH};
+use utils::{match_type, snippet, span_lint, span_note_and_lint, span_help_and_lint, in_external_macro, expr_block};
 
 /// **What it does:** This lint checks for matches with a single arm where an `if let` will usually suffice. It is `Warn` by default.
 ///
@@ -109,9 +110,20 @@ impl LateLintPass for MatchPass {
 }
 
 fn check_single_match(cx: &LateContext, ex: &Expr, arms: &[Arm], expr: &Expr) {
-    if arms.len() == 2 && arms[0].pats.len() == 1 && arms[0].guard.is_none() && arms[1].pats.len() == 1 &&
-       arms[1].guard.is_none() && arms[1].pats[0].node == PatWild && is_unit_expr(&arms[1].body) &&
-       (cx.tcx.expr_ty(ex).sty != ty::TyBool || cx.current_level(MATCH_BOOL) == Allow) {
+    if arms.len() == 2 &&
+       arms[0].pats.len() == 1 && arms[0].guard.is_none() &&
+       arms[1].pats.len() == 1 && arms[1].guard.is_none() &&
+       is_unit_expr(&arms[1].body) {
+           let ty = cx.tcx.expr_ty(ex);
+           if ty.sty != ty::TyBool || cx.current_level(MATCH_BOOL) == Allow {
+                check_single_match_single_pattern(cx, ex, arms, expr);
+                check_single_match_opt_like(cx, ex, arms, expr, ty);
+           }
+    }
+}
+
+fn check_single_match_single_pattern(cx: &LateContext, ex: &Expr, arms: &[Arm], expr: &Expr) {
+    if arms[1].pats[0].node == PatWild {
         span_help_and_lint(cx,
                            SINGLE_MATCH,
                            expr.span,
@@ -124,6 +136,39 @@ fn check_single_match(cx: &LateContext, ex: &Expr, arms: &[Arm], expr: &Expr) {
     }
 }
 
+fn check_single_match_opt_like(cx: &LateContext, ex: &Expr, arms: &[Arm], expr: &Expr, ty: ty::Ty) {
+    // list of candidate Enums we know will never get any more membre
+    let candidates = &[
+        (&COW_PATH, "Borrowed"),
+        (&COW_PATH, "Cow::Borrowed"),
+        (&COW_PATH, "Cow::Owned"),
+        (&COW_PATH, "Owned"),
+        (&OPTION_PATH, "None"),
+        (&RESULT_PATH, "Err"),
+        (&RESULT_PATH, "Ok"),
+    ];
+
+    let path = match arms[1].pats[0].node {
+        PatEnum(ref path, _) => path.to_string(),
+        PatIdent(BindByValue(MutImmutable), ident, None) => ident.node.to_string(),
+        _ => return
+    };
+
+    for &(ty_path, pat_path) in candidates {
+        if &path == pat_path && match_type(cx, ty, ty_path) {
+            span_help_and_lint(cx,
+                               SINGLE_MATCH,
+                               expr.span,
+                               "you seem to be trying to use match for destructuring a single pattern. Consider using \
+                                `if let`",
+                               &format!("try\nif let {} = {} {}",
+                                        snippet(cx, arms[0].pats[0].span, ".."),
+                                        snippet(cx, ex.span, ".."),
+                                        expr_block(cx, &arms[0].body, None, "..")));
+        }
+    }
+}
+
 fn check_match_bool(cx: &LateContext, ex: &Expr, arms: &[Arm], expr: &Expr) {
     // type of expression == bool
     if cx.tcx.expr_ty(ex).sty == ty::TyBool {
diff --git a/src/utils.rs b/src/utils.rs
index 74a15927ad1..77e63fe4458 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -29,6 +29,7 @@ pub const OPEN_OPTIONS_PATH: [&'static str; 3] = ["std", "fs", "OpenOptions"];
 pub const MUTEX_PATH: [&'static str; 4] = ["std", "sync", "mutex", "Mutex"];
 pub const CLONE_PATH: [&'static str; 2] = ["Clone", "clone"];
 pub const BEGIN_UNWIND: [&'static str; 3] = ["std", "rt", "begin_unwind"];
+pub const COW_PATH: [&'static str; 3] = ["collections", "borrow", "Cow"];
 
 /// Produce a nested chain of if-lets and ifs from the patterns:
 ///
diff --git a/tests/compile-fail/matches.rs b/tests/compile-fail/matches.rs
index b569f9566ef..c58e62419c6 100644
--- a/tests/compile-fail/matches.rs
+++ b/tests/compile-fail/matches.rs
@@ -4,8 +4,14 @@
 #![deny(clippy)]
 #![allow(unused)]
 
+use std::borrow::Cow;
+
+enum Foo { Bar, Baz(u8) }
+use Foo::*;
+
 fn single_match(){
     let x = Some(1u8);
+
     match x {  //~ ERROR you seem to be trying to use match
                //~^ HELP try
         Some(y) => {
@@ -13,11 +19,7 @@ fn single_match(){
         }
         _ => ()
     }
-    // Not linted
-    match x {
-        Some(y) => println!("{:?}", y),
-        None => ()
-    }
+
     let z = (1u8,1u8);
     match z { //~ ERROR you seem to be trying to use match
               //~^ HELP try
@@ -38,6 +40,43 @@ fn single_match(){
     }
 }
 
+fn single_match_know_enum() {
+    let x = Some(1u8);
+    let y : Result<_, i8> = Ok(1i8);
+
+    match x { //~ ERROR you seem to be trying to use match
+              //~^ HELP try
+        Some(y) => println!("{:?}", y),
+        None => ()
+    }
+
+    match y { //~ ERROR you seem to be trying to use match
+              //~^ HELP try
+        Ok(y) => println!("{:?}", y),
+        Err(..) => ()
+    }
+
+    let c = Cow::Borrowed("");
+
+    match c { //~ ERROR you seem to be trying to use match
+              //~^ HELP try
+        Cow::Borrowed(..) => println!("42"),
+        Cow::Owned(..) => (),
+    }
+
+    let z = Foo::Bar;
+    // no warning
+    match z {
+        Bar => println!("42"),
+        Baz(_) => (),
+    }
+
+    match z {
+        Baz(_) => println!("42"),
+        Bar => (),
+    }
+}
+
 fn match_bool() {
     let test: bool = true;