added parent method, also changed match-if-let note to help

src/misc.rs

@@ -7,8 +7,7 @@ use rustc::lint::{Context, LintPass, LintArray, Lint, Level};
 use rustc::middle::ty;
 use syntax::codemap::{Span, Spanned};
 
-use types::span_note_and_lint;
+use utils::{match_path, snippet, span_lint, span_help_and_lint};
-use utils::{match_path, snippet, span_lint};
 
 pub fn walk_ty<'t>(ty: ty::Ty<'t>) -> ty::Ty<'t> {
     match ty.sty {
@@ -42,9 +41,12 @@ impl LintPass for MiscPass {
                     }
                     // In some cases, an exhaustive match is preferred to catch situations when
                     // an enum is extended. So we only consider cases where a `_` wildcard is used
-                    if arms[1].pats[0].node == PatWild(PatWildSingle) && arms[0].pats.len() == 1 {
+                    if arms[1].pats[0].node == PatWild(PatWildSingle) && 
-                        span_note_and_lint(cx, SINGLE_MATCH, expr.span,
+                            arms[0].pats.len() == 1 {
-                              "You seem to be trying to use match for destructuring a single type. Did you mean to use `if let`?",
+                        span_help_and_lint(cx, SINGLE_MATCH, expr.span,
+                              "You seem to be trying to use match for \
+                              destructuring a single type. Did you mean to \
+                              use `if let`?",
                               &*format!("Try if let {} = {} {{ ... }}",
                                       snippet(cx, arms[0].pats[0].span, ".."),
                                       snippet(cx, ex.span, ".."))
@@ -136,7 +138,8 @@ impl LintPass for CmpNan {
 
 fn check_nan(cx: &Context, path: &Path, span: Span) {
 	path.segments.last().map(|seg| if seg.identifier.name == "NAN" {
-		span_lint(cx, CMP_NAN, span, "Doomed comparison with NAN, use std::{f32,f64}::is_nan instead");
+		span_lint(cx, CMP_NAN, span, 
+			"Doomed comparison with NAN, use std::{f32,f64}::is_nan instead");
 	});
 }
 

src/utils.rs

@@ -1,7 +1,8 @@
-use rustc::lint::{Context, Lint};
+use rustc::lint::{Context, Lint, Level};
-use syntax::ast::{DefId, Name, Path};
+use syntax::ast::{DefId, Expr, Name, NodeId, Path};
 use syntax::codemap::{ExpnInfo, Span};
 use syntax::ptr::P;
+use rustc::ast_map::Node::NodeExpr;
 use rustc::middle::ty;
 use std::borrow::{Cow, IntoCow};
 use std::convert::From;
@@ -50,6 +51,16 @@ pub fn snippet<'a>(cx: &Context, span: Span, default: &'a str) -> Cow<'a, str> {
     cx.sess().codemap().span_to_snippet(span).map(From::from).unwrap_or(Cow::Borrowed(default))
 }
 
+/// get a parent expr if any – this is useful to constrain a lint
+pub fn get_parent_expr<'c>(cx: &'c Context, e: &Expr) -> Option<&'c Expr> {
+    let map = &cx.tcx.map;
+    let node_id : NodeId = e.id;
+    let parent_id : NodeId = map.get_parent_node(node_id);
+    if node_id == parent_id { return None; }
+    map.find(parent_id).and_then(|node| 
+        if let NodeExpr(parent) = node { Some(parent) } else { None } )
+}
+
 /// dereference a P<T> and return a ref on the result
 pub fn de_p<T>(p: &P<T>) -> &T { &*p }
 
@@ -65,3 +76,11 @@ pub fn span_lint(cx: &Context, lint: &'static Lint, sp: Span, msg: &str) {
     // http://doc.rust-lang.org/syntax/codemap/struct.CodeMap.html#method.span_to_string
     cx.span_lint(lint, sp, msg);
 }
+
+pub fn span_help_and_lint(cx: &Context, lint: &'static Lint, span: Span, 
+        msg: &str, help: &str) {
+    span_lint(cx, lint, span, msg);
+    if cx.current_level(lint) != Level::Allow {
+        cx.sess().span_help(span, help);
+    }
+}

tests/compile-fail/match_if_let.rs

@@ -6,7 +6,7 @@
 fn main(){
     let x = Some(1u8);
     match x {  //~ ERROR You seem to be trying to use match
-               //~^ NOTE Try if let Some(y) = x { ... }
+               //~^ HELP Try if let Some(y) = x { ... }
         Some(y) => println!("{:?}", y),
         _ => ()
     }
@@ -17,7 +17,7 @@ fn main(){
     }
     let z = (1u8,1u8);
     match z { //~ ERROR You seem to be trying to use match
-              //~^ NOTE Try if let (2...3, 7...9) = z { ... }
+              //~^ HELP Try if let (2...3, 7...9) = z { ... }
         (2...3, 7...9) => println!("{:?}", z),
         _ => {}
     }