Remove as_str if the type is already &str

src/librustc/ty/sty.rs

@@ -1776,6 +1776,10 @@ impl<'tcx> TyS<'tcx> {
     #[inline]
     pub fn is_bool(&self) -> bool { self.kind == Bool }
 
+    /// Returns `true` if this type is a `str`.
+    #[inline]
+    pub fn is_str(&self) -> bool { self.kind == Str }
+
     #[inline]
     pub fn is_param(&self, index: u32) -> bool {
         match self.kind {

src/librustc_typeck/check/method/suggest.rs

@@ -518,7 +518,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     }
                 }
 
-                if let Some(lev_candidate) = lev_candidate {
+                if item_name.as_str() == "as_str" && actual.peel_refs().is_str() {
+                    // FIXME: the span is not quite correct, it should point to ".as_str()" instead
+                    // of just "as_str".
+                    err.span_label(
+                        span,
+                        "try removing `as_str`"
+                    );
+                } else if let Some(lev_candidate) = lev_candidate {
                     let def_kind = lev_candidate.def_kind();
                     err.span_suggestion(
                         span,

src/test/ui/suggestions/remove-as_str.rs

@@ -0,0 +1,21 @@
+fn foo1(s: &str) {
+    s.as_str();
+    //~^ ERROR no method named `as_str` found for type `&str` in the current scope
+}
+
+fn foo2<'a>(s: &'a str) {
+    s.as_str();
+    //~^ ERROR no method named `as_str` found for type `&'a str` in the current scope
+}
+
+fn foo3(s: &mut str) {
+    s.as_str();
+    //~^ ERROR no method named `as_str` found for type `&mut str` in the current scope
+}
+
+fn foo4(s: &&str) {
+    s.as_str();
+    //~^ ERROR no method named `as_str` found for type `&&str` in the current scope
+}
+
+fn main() {}

src/test/ui/suggestions/remove-as_str.stderr

@@ -0,0 +1,27 @@
+error[E0599]: no method named `as_str` found for type `&str` in the current scope
+  --> $DIR/remove-as_str.rs:2:7
+   |
+LL |     s.as_str();
+   |       ^^^^^^ try removing `as_str`
+
+error[E0599]: no method named `as_str` found for type `&'a str` in the current scope
+  --> $DIR/remove-as_str.rs:7:7
+   |
+LL |     s.as_str();
+   |       ^^^^^^ try removing `as_str`
+
+error[E0599]: no method named `as_str` found for type `&mut str` in the current scope
+  --> $DIR/remove-as_str.rs:12:7
+   |
+LL |     s.as_str();
+   |       ^^^^^^ try removing `as_str`
+
+error[E0599]: no method named `as_str` found for type `&&str` in the current scope
+  --> $DIR/remove-as_str.rs:17:7
+   |
+LL |     s.as_str();
+   |       ^^^^^^ try removing `as_str`
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0599`.