iter conservation efforts: save the endangered .iter() and .into_iter()

Make explicit_iter_loop and explicit_into_iter_loop allow-by-default, so
that people can turn them on if they want to enforce that style; avoid
presenting them as *the* idiomatic Rust style, rather than just *a* style.

clippy_lints/src/lib.rs

@@ -449,6 +449,8 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         if_not_else::IF_NOT_ELSE,
         infinite_iter::MAYBE_INFINITE_ITER,
         items_after_statements::ITEMS_AFTER_STATEMENTS,
+        loops::EXPLICIT_INTO_ITER_LOOP,
+        loops::EXPLICIT_ITER_LOOP,
         matches::SINGLE_MATCH_ELSE,
         methods::FILTER_MAP,
         methods::OPTION_MAP_UNWRAP_OR,
@@ -546,8 +548,6 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         literal_representation::UNREADABLE_LITERAL,
         loops::EMPTY_LOOP,
         loops::EXPLICIT_COUNTER_LOOP,
-        loops::EXPLICIT_INTO_ITER_LOOP,
-        loops::EXPLICIT_ITER_LOOP,
         loops::FOR_KV_MAP,
         loops::FOR_LOOP_OVER_OPTION,
         loops::FOR_LOOP_OVER_RESULT,
@@ -718,8 +718,6 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry<'_>, conf: &Conf) {
         literal_representation::LARGE_DIGIT_GROUPS,
         literal_representation::UNREADABLE_LITERAL,
         loops::EMPTY_LOOP,
-        loops::EXPLICIT_INTO_ITER_LOOP,
-        loops::EXPLICIT_ITER_LOOP,
         loops::FOR_KV_MAP,
         loops::NEEDLESS_RANGE_LOOP,
         loops::WHILE_LET_ON_ITERATOR,

clippy_lints/src/loops.rs

@@ -85,7 +85,7 @@ declare_clippy_lint! {
 /// ```
 declare_clippy_lint! {
     pub EXPLICIT_ITER_LOOP,
-    style,
+    pedantic,
     "for-looping over `_.iter()` or `_.iter_mut()` when `&_` or `&mut _` would do"
 }
 
@@ -107,7 +107,7 @@ declare_clippy_lint! {
 /// ```
 declare_clippy_lint! {
     pub EXPLICIT_INTO_ITER_LOOP,
-    style,
+    pedantic,
     "for-looping over `_.into_iter()` when `_` would do"
 }
 
@@ -1209,7 +1209,7 @@ fn lint_iter_method(cx: &LateContext<'_, '_>, args: &[Expr], arg: &Expr, method_
         cx,
         EXPLICIT_ITER_LOOP,
         arg.span,
-        "it is more idiomatic to loop over references to containers instead of using explicit \
+        "it is more concise to loop over references to containers instead of using explicit \
          iteration methods",
         "to write this more concisely, try",
         format!("&{}{}", muta, object),
@@ -1247,7 +1247,7 @@ fn check_for_loop_arg(cx: &LateContext<'_, '_>, pat: &Pat, arg: &Expr, expr: &Ex
                         cx,
                         EXPLICIT_INTO_ITER_LOOP,
                         arg.span,
-                        "it is more idiomatic to loop over containers instead of using explicit \
+                        "it is more concise to loop over containers instead of using explicit \
                          iteration methods`",
                         "to write this more concisely, try",
                         object.to_string(),