Add a missing fmt::Debug impl lint

Closes #20855

src/libcore/fmt/mod.rs

@@ -245,6 +245,7 @@ pub trait Show {
 #[unstable = "I/O and core have yet to be reconciled"]
 #[rustc_on_unimplemented = "`{Self}` cannot be formatted using `:?`; if it is defined in your \
                             crate, add `#[derive(Debug)]` or manually implement it"]
+#[lang = "debug_trait"]
 pub trait Debug {
     /// Formats the value using the given formatter.
     fn fmt(&self, &mut Formatter) -> Result;

src/librustc/lint/builtin.rs

@@ -1627,6 +1627,69 @@ fn check_item(&mut self, cx: &Context, item: &ast::Item) {
     }
 }
 
+declare_lint! {
+    MISSING_DEBUG_IMPLEMENTATIONS,
+    Allow,
+    "detects missing implementations of fmt::Debug"
+}
+
+pub struct MissingDebugImplementations {
+    impling_types: Option<NodeSet>,
+}
+
+impl MissingDebugImplementations {
+    pub fn new() -> MissingDebugImplementations {
+        MissingDebugImplementations {
+            impling_types: None,
+        }
+    }
+}
+
+impl LintPass for MissingDebugImplementations {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(MISSING_DEBUG_IMPLEMENTATIONS)
+    }
+
+    fn check_item(&mut self, cx: &Context, item: &ast::Item) {
+        if !cx.exported_items.contains(&item.id) {
+            return;
+        }
+
+        match item.node {
+            ast::ItemStruct(..) | ast::ItemEnum(..) => {},
+            _ => return,
+        }
+
+        let debug = match cx.tcx.lang_items.debug_trait() {
+            Some(debug) => debug,
+            None => return,
+        };
+
+        if self.impling_types.is_none() {
+            let impls = cx.tcx.trait_impls.borrow();
+            let impls = match impls.get(&debug) {
+                Some(impls) => {
+                    impls.borrow().iter()
+                        .filter(|d| d.krate == ast::LOCAL_CRATE)
+                        .filter_map(|d| ty::ty_to_def_id(ty::node_id_to_type(cx.tcx, d.node)))
+                        .map(|d| d.node)
+                        .collect()
+                }
+                None => NodeSet(),
+            };
+            self.impling_types = Some(impls);
+            debug!("{:?}", self.impling_types);
+        }
+
+        if !self.impling_types.as_ref().unwrap().contains(&item.id) {
+            cx.span_lint(MISSING_DEBUG_IMPLEMENTATIONS,
+                         item.span,
+                         "type does not implement `fmt::Debug`; consider adding #[derive(Debug)] \
+                          or a manual implementation")
+        }
+    }
+}
+
 declare_lint! {
     DEPRECATED,
     Warn,

src/librustc/lint/context.rs

@@ -219,6 +219,7 @@ pub fn register_builtin(&mut self, sess: Option<&Session>) {
                               RawPointerDerive,
                               MissingDoc,
                               Stability,
+                              MissingDebugImplementations,
         );
 
         add_lint_group!(sess, "bad_style",

src/librustc/middle/lang_items.rs

@@ -328,4 +328,6 @@ pub fn collect_language_items(krate: &ast::Crate,
     IteratorItem,                    "iterator",                iterator;
 
     StackExhaustedLangItem,          "stack_exhausted",         stack_exhausted;
+
+    DebugTraitLangItem,              "debug_trait",             debug_trait;
 }

src/test/compile-fail/missing_debug_impls.rs

+// Copyright 2015 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.
+
+// compile-flags: --crate-type lib
+#![deny(missing_debug_implementations)]
+#![allow(unused, unstable, missing_copy_implementations)]
+
+use std::fmt;
+
+pub enum A {} //~ ERROR type does not implement `fmt::Debug`
+
+#[derive(Debug)]
+pub enum B {}
+
+pub enum C {}
+
+impl fmt::Debug for C {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        Ok(())
+    }
+}
+
+pub struct Foo; //~ ERROR type does not implement `fmt::Debug`
+
+#[derive(Debug)]
+pub struct Bar;
+
+pub struct Baz;
+
+impl fmt::Debug for Baz {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        Ok(())
+    }
+}
+
+struct PrivateStruct;
+
+enum PrivateEnum {}
+
+#[derive(Debug)]
+struct GenericType<T>(T);