use literal span for concrete type suggestion

Fixes rust-lang#51874.

src/librustc_typeck/check/method/suggest.rs

@@ -245,12 +245,11 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                             "f32"
                         };
                         match expr.node {
-                            hir::ExprLit(_) => {  // numeric literal
-                                let snippet = tcx.sess.codemap().span_to_snippet(expr.span)
+                            hir::ExprLit(ref lit) => {  // numeric literal
+                                let snippet = tcx.sess.codemap().span_to_snippet(lit.span)
                                     .unwrap_or("<numeric literal>".to_string());
-                                // FIXME: use the literal for missing snippet
 
-                                err.span_suggestion(expr.span,
+                                err.span_suggestion(lit.span,
                                                     &format!("you must specify a concrete type for \
                                                               this numeric value, like `{}`",
                                                              concrete_type),

src/test/ui/issue-51874.rs

@@ -0,0 +1,13 @@
+// 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 main() {
+    let a = (1.0).pow(1.0); //~ ERROR can't call method `pow` on ambiguous numeric type
+}

src/test/ui/issue-51874.stderr

@@ -0,0 +1,13 @@
+error[E0689]: can't call method `pow` on ambiguous numeric type `{float}`
+  --> $DIR/issue-51874.rs:12:19
+   |
+LL |     let a = (1.0).pow(1.0); //~ ERROR can't call method `pow` on ambiguous numeric type
+   |                   ^^^
+help: you must specify a concrete type for this numeric value, like `f32`
+   |
+LL |     let a = (1.0_f32).pow(1.0); //~ ERROR can't call method `pow` on ambiguous numeric type
+   |              ^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0689`.