Auto merge of #5411 - dtolnay:hasher, r=flip1995

Downgrade implicit_hasher to pedantic

From the [documentation](https://rust-lang.github.io/rust-clippy/master/index.html#implicit_hasher), this lint is intended to suggest:

```diff
- pub fn foo(map: &mut HashMap<i32, i32>) { }

+ pub fn foo<S: BuildHasher>(map: &mut HashMap<i32, i32, S>) { }
```

I think this is pedantic. I get that this lint can benefit core libraries like serde, but that's exactly the use case for pedantic lints; a library like serde will [enable clippy_pedantic](https://github.com/serde-rs/json/blob/fd6741f4b0b3fc90a58a6f578e33a9adc6403f3f/src/lib.rs#L304) and take the time to go through everything possible. Similar for libraries doing a libz blitz style checkup before committing to a 1.0 release; it would make sense to run through all the available pedantic lints then.

But otherwise, for most codebases and certainly for industrial codebases, the above suggested change just makes the codebase more obtuse for questionable benefit.

changelog: Remove implicit_hasher from default set of enabled lints

9 files changed, 31 insertions, 23 deletions

diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 3e8ce4006f6..b106113c2a9 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -1135,6 +1135,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&types::CAST_POSSIBLE_WRAP),
         LintId::of(&types::CAST_PRECISION_LOSS),
         LintId::of(&types::CAST_SIGN_LOSS),
+        LintId::of(&types::IMPLICIT_HASHER),
         LintId::of(&types::INVALID_UPCAST_COMPARISONS),
         LintId::of(&types::LET_UNIT_VALUE),
         LintId::of(&types::LINKEDLIST),
@@ -1383,7 +1384,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&types::CHAR_LIT_AS_U8),
         LintId::of(&types::FN_TO_NUMERIC_CAST),
         LintId::of(&types::FN_TO_NUMERIC_CAST_WITH_TRUNCATION),
-        LintId::of(&types::IMPLICIT_HASHER),
         LintId::of(&types::REDUNDANT_ALLOCATION),
         LintId::of(&types::TYPE_COMPLEXITY),
         LintId::of(&types::UNIT_ARG),
@@ -1494,7 +1494,6 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&try_err::TRY_ERR),
         LintId::of(&types::FN_TO_NUMERIC_CAST),
         LintId::of(&types::FN_TO_NUMERIC_CAST_WITH_TRUNCATION),
-        LintId::of(&types::IMPLICIT_HASHER),
         LintId::of(&unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME),
         LintId::of(&write::PRINTLN_EMPTY_STRING),
         LintId::of(&write::PRINT_LITERAL),
diff --git a/clippy_lints/src/types.rs b/clippy_lints/src/types.rs
index 13e275b3291..1e7e0adf390 100644
--- a/clippy_lints/src/types.rs
+++ b/clippy_lints/src/types.rs
@@ -2169,7 +2169,7 @@ declare_clippy_lint! {
     /// pub fn foo<S: BuildHasher>(map: &mut HashMap<i32, i32, S>) { }
     /// ```
     pub IMPLICIT_HASHER,
-    style,
+    pedantic,
     "missing generalization over different hashers"
 }
 
diff --git a/src/lintlist/mod.rs b/src/lintlist/mod.rs
index 3b275f43e17..b3c77f3f481 100644
--- a/src/lintlist/mod.rs
+++ b/src/lintlist/mod.rs
@@ -747,7 +747,7 @@ pub static ref ALL_LINTS: Vec<Lint> = vec![
     },
     Lint {
         name: "implicit_hasher",
-        group: "style",
+        group: "pedantic",
         desc: "missing generalization over different hashers",
         deprecation: None,
         module: "types",
diff --git a/tests/ui/crashes/ice-3717.rs b/tests/ui/crashes/ice-3717.rs
index 21c48f4749c..f50714643fd 100644
--- a/tests/ui/crashes/ice-3717.rs
+++ b/tests/ui/crashes/ice-3717.rs
@@ -1,3 +1,5 @@
+#![deny(clippy::implicit_hasher)]
+
 use std::collections::HashSet;
 
 fn main() {}
diff --git a/tests/ui/crashes/ice-3717.stderr b/tests/ui/crashes/ice-3717.stderr
index 08c53c399c2..296c95abb96 100644
--- a/tests/ui/crashes/ice-3717.stderr
+++ b/tests/ui/crashes/ice-3717.stderr
@@ -1,10 +1,14 @@
 error: parameter of type `HashSet` should be generalized over different hashers
-  --> $DIR/ice-3717.rs:5:21
+  --> $DIR/ice-3717.rs:7:21
    |
 LL | pub fn ice_3717(_: &HashSet<usize>) {
    |                     ^^^^^^^^^^^^^^
    |
-   = note: `-D clippy::implicit-hasher` implied by `-D warnings`
+note: the lint level is defined here
+  --> $DIR/ice-3717.rs:1:9
+   |
+LL | #![deny(clippy::implicit_hasher)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
 help: consider adding a type parameter
    |
 LL | pub fn ice_3717<S: ::std::hash::BuildHasher + Default>(_: &HashSet<usize, S>) {
diff --git a/tests/ui/implicit_hasher.rs b/tests/ui/implicit_hasher.rs
index c0ffa6879ce..fdcc9a33f55 100644
--- a/tests/ui/implicit_hasher.rs
+++ b/tests/ui/implicit_hasher.rs
@@ -1,4 +1,5 @@
 // aux-build:implicit_hasher_macros.rs
+#![deny(clippy::implicit_hasher)]
 #![allow(unused)]
 
 #[macro_use]
diff --git a/tests/ui/implicit_hasher.stderr b/tests/ui/implicit_hasher.stderr
index 252e9eb5dd8..2b06d661772 100644
--- a/tests/ui/implicit_hasher.stderr
+++ b/tests/ui/implicit_hasher.stderr
@@ -1,10 +1,14 @@
 error: impl for `HashMap` should be generalized over different hashers
-  --> $DIR/implicit_hasher.rs:15:35
+  --> $DIR/implicit_hasher.rs:16:35
    |
 LL | impl<K: Hash + Eq, V> Foo<i8> for HashMap<K, V> {
    |                                   ^^^^^^^^^^^^^
    |
-   = note: `-D clippy::implicit-hasher` implied by `-D warnings`
+note: the lint level is defined here
+  --> $DIR/implicit_hasher.rs:2:9
+   |
+LL | #![deny(clippy::implicit_hasher)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
 help: consider adding a type parameter
    |
 LL | impl<K: Hash + Eq, V, S: ::std::hash::BuildHasher + Default> Foo<i8> for HashMap<K, V, S> {
@@ -15,7 +19,7 @@ LL |         (HashMap::default(), HashMap::with_capacity_and_hasher(10, Default:
    |          ^^^^^^^^^^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: impl for `HashMap` should be generalized over different hashers
-  --> $DIR/implicit_hasher.rs:24:36
+  --> $DIR/implicit_hasher.rs:25:36
    |
 LL | impl<K: Hash + Eq, V> Foo<i8> for (HashMap<K, V>,) {
    |                                    ^^^^^^^^^^^^^
@@ -30,7 +34,7 @@ LL |         ((HashMap::default(),), (HashMap::with_capacity_and_hasher(10, Defa
    |           ^^^^^^^^^^^^^^^^^^     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: impl for `HashMap` should be generalized over different hashers
-  --> $DIR/implicit_hasher.rs:29:19
+  --> $DIR/implicit_hasher.rs:30:19
    |
 LL | impl Foo<i16> for HashMap<String, String> {
    |                   ^^^^^^^^^^^^^^^^^^^^^^^
@@ -45,7 +49,7 @@ LL |         (HashMap::default(), HashMap::with_capacity_and_hasher(10, Default:
    |          ^^^^^^^^^^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: impl for `HashSet` should be generalized over different hashers
-  --> $DIR/implicit_hasher.rs:46:32
+  --> $DIR/implicit_hasher.rs:47:32
    |
 LL | impl<T: Hash + Eq> Foo<i8> for HashSet<T> {
    |                                ^^^^^^^^^^
@@ -60,7 +64,7 @@ LL |         (HashSet::default(), HashSet::with_capacity_and_hasher(10, Default:
    |          ^^^^^^^^^^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: impl for `HashSet` should be generalized over different hashers
-  --> $DIR/implicit_hasher.rs:51:19
+  --> $DIR/implicit_hasher.rs:52:19
    |
 LL | impl Foo<i16> for HashSet<String> {
    |                   ^^^^^^^^^^^^^^^
@@ -75,7 +79,7 @@ LL |         (HashSet::default(), HashSet::with_capacity_and_hasher(10, Default:
    |          ^^^^^^^^^^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: parameter of type `HashMap` should be generalized over different hashers
-  --> $DIR/implicit_hasher.rs:68:23
+  --> $DIR/implicit_hasher.rs:69:23
    |
 LL | pub fn foo(_map: &mut HashMap<i32, i32>, _set: &mut HashSet<i32>) {}
    |                       ^^^^^^^^^^^^^^^^^
@@ -86,7 +90,7 @@ LL | pub fn foo<S: ::std::hash::BuildHasher>(_map: &mut HashMap<i32, i32, S>, _s
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^
 
 error: parameter of type `HashSet` should be generalized over different hashers
-  --> $DIR/implicit_hasher.rs:68:53
+  --> $DIR/implicit_hasher.rs:69:53
    |
 LL | pub fn foo(_map: &mut HashMap<i32, i32>, _set: &mut HashSet<i32>) {}
    |                                                     ^^^^^^^^^^^^
@@ -97,7 +101,7 @@ LL | pub fn foo<S: ::std::hash::BuildHasher>(_map: &mut HashMap<i32, i32>, _set:
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                          ^^^^^^^^^^^^^^^
 
 error: impl for `HashMap` should be generalized over different hashers
-  --> $DIR/implicit_hasher.rs:72:43
+  --> $DIR/implicit_hasher.rs:73:43
    |
 LL |         impl<K: Hash + Eq, V> Foo<u8> for HashMap<K, V> {
    |                                           ^^^^^^^^^^^^^
@@ -116,7 +120,7 @@ LL |                 (HashMap::default(), HashMap::with_capacity_and_hasher(10,
    |                  ^^^^^^^^^^^^^^^^^^  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: parameter of type `HashMap` should be generalized over different hashers
-  --> $DIR/implicit_hasher.rs:80:33
+  --> $DIR/implicit_hasher.rs:81:33
    |
 LL |         pub fn $name(_map: &mut HashMap<i32, i32>, _set: &mut HashSet<i32>) {}
    |                                 ^^^^^^^^^^^^^^^^^
@@ -131,7 +135,7 @@ LL |         pub fn $name<S: ::std::hash::BuildHasher>(_map: &mut HashMap<i32, i
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^            ^^^^^^^^^^^^^^^^^^^^
 
 error: parameter of type `HashSet` should be generalized over different hashers
-  --> $DIR/implicit_hasher.rs:80:63
+  --> $DIR/implicit_hasher.rs:81:63
    |
 LL |         pub fn $name(_map: &mut HashMap<i32, i32>, _set: &mut HashSet<i32>) {}
    |                                                               ^^^^^^^^^^^^
diff --git a/tests/ui/mut_key.rs b/tests/ui/mut_key.rs
index d45cf8278a8..2d227e6654c 100644
--- a/tests/ui/mut_key.rs
+++ b/tests/ui/mut_key.rs
@@ -1,5 +1,3 @@
-#![allow(clippy::implicit_hasher)]
-
 use std::collections::{HashMap, HashSet};
 use std::hash::{Hash, Hasher};
 use std::sync::atomic::{AtomicUsize, Ordering::Relaxed};
diff --git a/tests/ui/mut_key.stderr b/tests/ui/mut_key.stderr
index 5af28f18d3d..8d6a259c7e3 100644
--- a/tests/ui/mut_key.stderr
+++ b/tests/ui/mut_key.stderr
@@ -1,5 +1,5 @@
 error: mutable key type
-  --> $DIR/mut_key.rs:29:32
+  --> $DIR/mut_key.rs:27:32
    |
 LL | fn should_not_take_this_arg(m: &mut HashMap<Key, usize>, _n: usize) -> HashSet<Key> {
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^
@@ -7,19 +7,19 @@ LL | fn should_not_take_this_arg(m: &mut HashMap<Key, usize>, _n: usize) -> Hash
    = note: `#[deny(clippy::mutable_key_type)]` on by default
 
 error: mutable key type
-  --> $DIR/mut_key.rs:29:72
+  --> $DIR/mut_key.rs:27:72
    |
 LL | fn should_not_take_this_arg(m: &mut HashMap<Key, usize>, _n: usize) -> HashSet<Key> {
    |                                                                        ^^^^^^^^^^^^
 
 error: mutable key type
-  --> $DIR/mut_key.rs:30:5
+  --> $DIR/mut_key.rs:28:5
    |
 LL |     let _other: HashMap<Key, bool> = HashMap::new();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: mutable key type
-  --> $DIR/mut_key.rs:49:22
+  --> $DIR/mut_key.rs:47:22
    |
 LL | fn tuples_bad<U>(_m: &mut HashMap<(Key, U), bool>) {}
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^