Do not suggest changes to str literal if it isn't one

src/librustc_typeck/check/demand.rs

@@ -264,9 +264,11 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 (&ty::TyStr, &ty::TySlice(arr)) if arr == self.tcx.types.u8 => {
                     if let hir::ExprLit(_) = expr.node {
                         if let Ok(src) = cm.span_to_snippet(sp) {
-                            return Some((sp,
-                                         "consider removing the leading `b`",
-                                         src[1..].to_string()));
+                            if src.starts_with("b\"") {
+                                return Some((sp,
+                                             "consider removing the leading `b`",
+                                             src[1..].to_string()));
+                            }
                         }
                     }
                 },
@@ -274,9 +276,11 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 (&ty::TySlice(arr), &ty::TyStr) if arr == self.tcx.types.u8 => {
                     if let hir::ExprLit(_) = expr.node {
                         if let Ok(src) = cm.span_to_snippet(sp) {
-                            return Some((sp,
-                                         "consider adding a leading `b`",
-                                         format!("b{}", src)));
+                            if src.starts_with("\"") {
+                                return Some((sp,
+                                             "consider adding a leading `b`",
+                                             format!("b{}", src)));
+                            }
                         }
                     }
                 }

src/test/ui/suggestions/issue-48364.rs

@@ -0,0 +1,16 @@
+// Copyright 2018 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 foo() -> bool {
+    b"".starts_with(stringify!(foo))
+    //~^ ERROR mismatched types
+}
+
+fn main() {}

src/test/ui/suggestions/issue-48364.stderr

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-48364.rs:12:21
+   |
+LL |     b"".starts_with(stringify!(foo))
+   |                     ^^^^^^^^^^^^^^^ expected slice, found str
+   |
+   = note: expected type `&[u8]`
+              found type `&'static str`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.