needless_collect: For BTreeMap and HashMap lint only is_empty

- `len` might produce different results than `count`
- they don't have `contain` but `contains_key` method

clippy_lints/src/loops/needless_collect.rs

@@ -10,7 +10,6 @@ use rustc_hir::intravisit::{walk_block, walk_expr, NestedVisitorMap, Visitor};
 use rustc_hir::{Block, Expr, ExprKind, GenericArg, GenericArgs, HirId, Local, Pat, PatKind, QPath, StmtKind, Ty};
 use rustc_lint::LateContext;
 use rustc_middle::hir::map::Map;
-
 use rustc_span::symbol::{sym, Ident};
 use rustc_span::{MultiSpan, Span};
 
@@ -28,13 +27,14 @@ fn check_needless_collect_direct_usage<'tcx>(expr: &'tcx Expr<'_>, cx: &LateCont
         if let Some(generic_args) = chain_method.args;
         if let Some(GenericArg::Type(ref ty)) = generic_args.args.get(0);
         if let Some(ty) = cx.typeck_results().node_type_opt(ty.hir_id);
-        if is_type_diagnostic_item(cx, ty, sym::vec_type)
+        then {
-            || is_type_diagnostic_item(cx, ty, sym::vecdeque_type)
+            let is_empty_sugg = Some("next().is_none()".to_string());
-            || match_type(cx, ty, &paths::BTREEMAP)
+            let method_name = &*method.ident.name.as_str();
-            || is_type_diagnostic_item(cx, ty, sym::hashmap_type);
+            let sugg = if is_type_diagnostic_item(cx, ty, sym::vec_type) || 
-        if let Some(sugg) = match &*method.ident.name.as_str() {
+                        is_type_diagnostic_item(cx, ty, sym::vecdeque_type) {
+                match method_name {
                     "len" => Some("count()".to_string()),
-            "is_empty" => Some("next().is_none()".to_string()),
+                    "is_empty" => is_empty_sugg,
                     "contains" => {
                         let contains_arg = snippet(cx, args[1].span, "??");
                         let (arg, pred) = contains_arg
@@ -43,8 +43,19 @@ fn check_needless_collect_direct_usage<'tcx>(expr: &'tcx Expr<'_>, cx: &LateCont
                         Some(format!("any(|{}| x == {})", arg, pred))
                     }
                     _ => None,
+                }
+            }
+            else if match_type(cx, ty, &paths::BTREEMAP) ||
+                is_type_diagnostic_item(cx, ty, sym::hashmap_type) {
+                match method_name {
+                    "is_empty" => is_empty_sugg,
+                    _ => None,
+                }
+            }
+            else {
+                None
             };
-        then {
+            if let Some(sugg) = sugg {
                 span_lint_and_sugg(
                     cx,
                     NEEDLESS_COLLECT,
@@ -56,6 +67,7 @@ fn check_needless_collect_direct_usage<'tcx>(expr: &'tcx Expr<'_>, cx: &LateCont
                 );
             }
         }
+    }
 }
 
 fn check_needless_collect_indirect_usage<'tcx>(expr: &'tcx Expr<'_>, cx: &LateContext<'tcx>) {

tests/ui/needless_collect.fixed

@@ -2,7 +2,7 @@
 
 #![allow(unused, clippy::suspicious_map, clippy::iter_count)]
 
-use std::collections::{BTreeSet, HashMap, HashSet};
+use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 
 #[warn(clippy::needless_collect)]
 #[allow(unused_variables, clippy::iter_cloned_collect, clippy::iter_next_slice)]
@@ -13,7 +13,13 @@ fn main() {
         // Empty
     }
     sample.iter().cloned().any(|x| x == 1);
-    sample.iter().map(|x| (x, x)).count();
+    // #7164 HashMap's and BTreeMap's `len` usage should not be linted
+    sample.iter().map(|x| (x, x)).collect::<HashMap<_, _>>().len();
+    sample.iter().map(|x| (x, x)).collect::<BTreeMap<_, _>>().len();
+
+    sample.iter().map(|x| (x, x)).next().is_none();
+    sample.iter().map(|x| (x, x)).next().is_none();
+
     // Notice the `HashSet`--this should not be linted
     sample.iter().collect::<HashSet<_>>().len();
     // Neither should this

tests/ui/needless_collect.rs

@@ -2,7 +2,7 @@
 
 #![allow(unused, clippy::suspicious_map, clippy::iter_count)]
 
-use std::collections::{BTreeSet, HashMap, HashSet};
+use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 
 #[warn(clippy::needless_collect)]
 #[allow(unused_variables, clippy::iter_cloned_collect, clippy::iter_next_slice)]
@@ -13,7 +13,13 @@ fn main() {
         // Empty
     }
     sample.iter().cloned().collect::<Vec<_>>().contains(&1);
+    // #7164 HashMap's and BTreeMap's `len` usage should not be linted
     sample.iter().map(|x| (x, x)).collect::<HashMap<_, _>>().len();
+    sample.iter().map(|x| (x, x)).collect::<BTreeMap<_, _>>().len();
+
+    sample.iter().map(|x| (x, x)).collect::<HashMap<_, _>>().is_empty();
+    sample.iter().map(|x| (x, x)).collect::<BTreeMap<_, _>>().is_empty();
+
     // Notice the `HashSet`--this should not be linted
     sample.iter().collect::<HashSet<_>>().len();
     // Neither should this

tests/ui/needless_collect.stderr

@@ -19,10 +19,16 @@ LL |     sample.iter().cloned().collect::<Vec<_>>().contains(&1);
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `any(|x| x == 1)`
 
 error: avoid using `collect()` when not needed
-  --> $DIR/needless_collect.rs:16:35
+  --> $DIR/needless_collect.rs:20:35
    |
-LL |     sample.iter().map(|x| (x, x)).collect::<HashMap<_, _>>().len();
+LL |     sample.iter().map(|x| (x, x)).collect::<HashMap<_, _>>().is_empty();
-   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `count()`
+   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `next().is_none()`
 
-error: aborting due to 4 previous errors
+error: avoid using `collect()` when not needed
+  --> $DIR/needless_collect.rs:21:35
+   |
+LL |     sample.iter().map(|x| (x, x)).collect::<BTreeMap<_, _>>().is_empty();
+   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `next().is_none()`
+
+error: aborting due to 5 previous errors