in which the trivial-casts word to the wise is tucked into a help note

The top level message shouldn't be too long; the
replaced-by-coercion/temporary-variable advice can live in a note. Also,
don't mention type ascription when it's not actually available as a real
thing. (The current state of discussion on the type ascription tracking
issue #23416 makes one rather suspect it will never be a stable thing in
its current form, but that's not for us to adjudicate in this commit.)

While we're here, yank out the differentiating parts of the
numeric/other conditional and only have one codepath emitting the
diagnostic.

src/librustc_typeck/check/cast.rs

@@ -365,28 +365,27 @@ impl<'a, 'gcx, 'tcx> CastCheck<'tcx> {
     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) {
         let t_cast = self.cast_ty;
         let t_expr = self.expr_ty;
-        if t_cast.is_numeric() && t_expr.is_numeric() {
-            fcx.tcx.lint_node(
-                lint::builtin::TRIVIAL_NUMERIC_CASTS,
-                self.expr.id,
-                self.span,
-                &format!("trivial numeric cast: `{}` as `{}`. Cast can be \
-                          replaced by coercion, this might require type \
-                          ascription or a temporary variable",
-                         fcx.ty_to_string(t_expr),
-                         fcx.ty_to_string(t_cast)));
+        let type_asc_or = if fcx.tcx.features().type_ascription {
+            "type ascription or "
         } else {
-            fcx.tcx.lint_node(
-                lint::builtin::TRIVIAL_CASTS,
-                self.expr.id,
-                self.span,
-                &format!("trivial cast: `{}` as `{}`. Cast can be \
-                          replaced by coercion, this might require type \
-                          ascription or a temporary variable",
-                         fcx.ty_to_string(t_expr),
-                         fcx.ty_to_string(t_cast)));
-        }
-
+            ""
+        };
+        let (adjective, lint) = if t_cast.is_numeric() && t_expr.is_numeric() {
+            ("numeric ", lint::builtin::TRIVIAL_NUMERIC_CASTS)
+        } else {
+            ("", lint::builtin::TRIVIAL_CASTS)
+        };
+        let mut err = fcx.tcx.struct_span_lint_node(
+            lint,
+            self.expr.id,
+            self.span,
+            &format!("trivial {}cast: `{}` as `{}`",
+                     adjective,
+                     fcx.ty_to_string(t_expr),
+                     fcx.ty_to_string(t_cast)));
+        err.help(&format!("cast can be replaced by coercion; this might \
+                           require {}a temporary variable", type_asc_or));
+        err.emit();
     }
 
     pub fn check(mut self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) {

src/test/ui/lint/trivial-casts-featuring-type-ascription.rs

@@ -0,0 +1,20 @@
+// 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.
+
+#![deny(trivial_casts, trivial_numeric_casts)]
+#![feature(type_ascription)]
+
+fn main() {
+    let lugubrious = 12i32 as i32;
+    //~^ ERROR trivial numeric cast
+    let haunted: &u32 = &99;
+    let _ = haunted as *const u32;
+    //~^ ERROR trivial cast
+}

src/test/ui/lint/trivial-casts-featuring-type-ascription.stderr

@@ -0,0 +1,28 @@
+error: trivial numeric cast: `i32` as `i32`
+  --> $DIR/trivial-casts-featuring-type-ascription.rs:15:22
+   |
+LL |     let lugubrious = 12i32 as i32;
+   |                      ^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/trivial-casts-featuring-type-ascription.rs:11:24
+   |
+LL | #![deny(trivial_casts, trivial_numeric_casts)]
+   |                        ^^^^^^^^^^^^^^^^^^^^^
+   = help: cast can be replaced by coercion; this might require type ascription or a temporary variable
+
+error: trivial cast: `&u32` as `*const u32`
+  --> $DIR/trivial-casts-featuring-type-ascription.rs:18:13
+   |
+LL |     let _ = haunted as *const u32;
+   |             ^^^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/trivial-casts-featuring-type-ascription.rs:11:9
+   |
+LL | #![deny(trivial_casts, trivial_numeric_casts)]
+   |         ^^^^^^^^^^^^^
+   = help: cast can be replaced by coercion; this might require type ascription or a temporary variable
+
+error: aborting due to 2 previous errors
+

src/test/ui/lint/trivial-casts.rs

@@ -0,0 +1,19 @@
+// 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.
+
+#![deny(trivial_casts, trivial_numeric_casts)]
+
+fn main() {
+    let lugubrious = 12i32 as i32;
+    //~^ ERROR trivial numeric cast
+    let haunted: &u32 = &99;
+    let _ = haunted as *const u32;
+    //~^ ERROR trivial cast
+}

src/test/ui/lint/trivial-casts.stderr

@@ -0,0 +1,28 @@
+error: trivial numeric cast: `i32` as `i32`
+  --> $DIR/trivial-casts.rs:14:22
+   |
+LL |     let lugubrious = 12i32 as i32;
+   |                      ^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/trivial-casts.rs:11:24
+   |
+LL | #![deny(trivial_casts, trivial_numeric_casts)]
+   |                        ^^^^^^^^^^^^^^^^^^^^^
+   = help: cast can be replaced by coercion; this might require a temporary variable
+
+error: trivial cast: `&u32` as `*const u32`
+  --> $DIR/trivial-casts.rs:17:13
+   |
+LL |     let _ = haunted as *const u32;
+   |             ^^^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/trivial-casts.rs:11:9
+   |
+LL | #![deny(trivial_casts, trivial_numeric_casts)]
+   |         ^^^^^^^^^^^^^
+   = help: cast can be replaced by coercion; this might require a temporary variable
+
+error: aborting due to 2 previous errors
+