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, 4 insertions, 4 deletions

diff --git a/src/librustc/middle/astencode.rs b/src/librustc/middle/astencode.rs
index 93a19a01f66..69dfab1e5c1 100644
--- a/src/librustc/middle/astencode.rs
+++ b/src/librustc/middle/astencode.rs
@@ -82,7 +82,7 @@ pub fn encode_inlined_item(ecx: &e::EncodeContext,
         e::IIImplItemRef(_, &ast::MethodImplItem(ref m)) => m.id,
         e::IIImplItemRef(_, &ast::TypeImplItem(ref ti)) => ti.id,
     };
-    debug!("> Encoding inlined item: {} ({})",
+    debug!("> Encoding inlined item: {} ({:?})",
            ecx.tcx.map.path_to_string(id),
            rbml_w.writer.tell());
 
@@ -96,7 +96,7 @@ pub fn encode_inlined_item(ecx: &e::EncodeContext,
     encode_side_tables_for_ii(ecx, rbml_w, &ii);
     rbml_w.end_tag();
 
-    debug!("< Encoded inlined fn: {} ({})",
+    debug!("< Encoded inlined fn: {} ({:?})",
            ecx.tcx.map.path_to_string(id),
            rbml_w.writer.tell());
 }
@@ -127,7 +127,7 @@ pub fn decode_inlined_item<'tcx>(cdata: &cstore::crate_metadata,
       None => Err(path),
       Some(ast_doc) => {
         let mut path_as_str = None;
-        debug!("> Decoding inlined fn: {}::?",
+        debug!("> Decoding inlined fn: {:?}::?",
         {
             // Do an Option dance to use the path after it is moved below.
             let s = ast_map::path_to_string(ast_map::Values(path.iter()));
@@ -1880,7 +1880,7 @@ impl<'a, 'tcx> rbml_decoder_decoder_helpers<'tcx> for reader::Decoder<'a> {
             NominalType | TypeWithId | RegionParameter => dcx.tr_def_id(did),
             TypeParameter | UnboxedClosureSource => dcx.tr_intern_def_id(did)
         };
-        debug!("convert_def_id(source={}, did={})={}", source, did, r);
+        debug!("convert_def_id(source={:?}, did={:?})={:?}", source, did, r);
         return r;
     }
 }