core: split into fmt::Show and fmt::String

fmt::Show is for debugging, and can and should be implemented for
all public types. This trait is used with `{:?}` syntax. There still
exists #[derive(Show)].

fmt::String is for types that faithfully be represented as a String.
Because of this, there is no way to derive fmt::String, all
implementations must be purposeful. It is used by the default format
syntax, `{}`.

This will break most instances of `{}`, since that now requires the type
to impl fmt::String. In most cases, replacing `{}` with `{:?}` is the
correct fix. Types that were being printed specifically for users should
receive a fmt::String implementation to fix this.

Part of #20013

[breaking-change]

1 files changed, 3 insertions, 3 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 32f8f5ee3d6..ed6e5c9204b 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -488,7 +488,7 @@ impl<'a> Parser<'a> {
     /// followed by some token from the set edible + inedible.  Recover
     /// from anticipated input errors, discarding erroneous characters.
     pub fn commit_expr(&mut self, e: &Expr, edible: &[token::Token], inedible: &[token::Token]) {
-        debug!("commit_expr {}", e);
+        debug!("commit_expr {:?}", e);
         if let ExprPath(..) = e.node {
             // might be unit-struct construction; check for recoverableinput error.
             let mut expected = edible.iter().map(|x| x.clone()).collect::<Vec<_>>();
@@ -5113,7 +5113,7 @@ impl<'a> Parser<'a> {
                 attrs = tmp;
                 first = false;
             }
-            debug!("parse_mod_items: parse_item_or_view_item(attrs={})",
+            debug!("parse_mod_items: parse_item_or_view_item(attrs={:?})",
                    attrs);
             match self.parse_item_or_view_item(attrs,
                                                true /* macros allowed */) {
@@ -5250,7 +5250,7 @@ impl<'a> Parser<'a> {
                                              format!("file not found for module `{}`",
                                                      mod_name)[],
                                              format!("name the file either {} or {} inside \
-                                                     the directory {}",
+                                                     the directory {:?}",
                                                      default_path_str,
                                                      secondary_path_str,
                                                      dir_path.display())[]);