Auto merge of rust-lang#12859 - cookie-s:dedup-single-char-name-diag, r=Alexendoo

[`many_single_char_names`]: Deduplicate diagnostics

Relates to rust-lang#12379

Fix `many_single_char_names` lint so that it doesn't emit diagnostics when the current level of the scope doesn't contain any single character name.

```rust
let (a, b, c, d): (i32, i32, i32, i32);
match 1 {
  1 => (),
  e => {},
}
```
produced the exact same MANY_SINGLE_CHAR_NAMES diagnostic at each of the Arm `e => {}` and the Block `{}`.

---

changelog: [`many_single_char_names`]: Fix duplicate diagnostics

Author: bors

clippy_lints/src/non_expressive_names.rs

@@ -98,6 +98,10 @@ struct SimilarNamesLocalVisitor<'a, 'tcx> {
 
 impl<'a, 'tcx> SimilarNamesLocalVisitor<'a, 'tcx> {
     fn check_single_char_names(&self) {
+        if self.single_char_names.last().map(Vec::len) == Some(0) {
+            return;
+        }
+
         let num_single_char_names = self.single_char_names.iter().flatten().count();
         let threshold = self.lint.single_char_binding_names_threshold;
         if num_single_char_names as u64 > threshold {

tests/ui/many_single_char_names.rs

@@ -1,5 +1,3 @@
-//@compile-flags: -Zdeduplicate-diagnostics=yes
-
 #![allow(clippy::too_many_arguments, clippy::diverging_sub_expression)]
 #![warn(clippy::many_single_char_names)]
 

tests/ui/many_single_char_names.stderr

@@ -1,5 +1,5 @@
 error: 5 bindings with single-character names in scope
-  --> tests/ui/many_single_char_names.rs:7:9
+  --> tests/ui/many_single_char_names.rs:5:9
    |
 LL |     let a: i32;
    |         ^
@@ -14,7 +14,7 @@ LL |             let e: i32;
    = help: to override `-D warnings` add `#[allow(clippy::many_single_char_names)]`
 
 error: 6 bindings with single-character names in scope
-  --> tests/ui/many_single_char_names.rs:7:9
+  --> tests/ui/many_single_char_names.rs:5:9
    |
 LL |     let a: i32;
    |         ^
@@ -28,7 +28,7 @@ LL |             let f: i32;
    |                 ^
 
 error: 5 bindings with single-character names in scope
-  --> tests/ui/many_single_char_names.rs:7:9
+  --> tests/ui/many_single_char_names.rs:5:9
    |
 LL |     let a: i32;
    |         ^
@@ -40,13 +40,13 @@ LL |             e => panic!(),
    |             ^
 
 error: 8 bindings with single-character names in scope
-  --> tests/ui/many_single_char_names.rs:36:13
+  --> tests/ui/many_single_char_names.rs:34:13
    |
 LL | fn bindings(a: i32, b: i32, c: i32, d: i32, e: i32, f: i32, g: i32, h: i32) {}
    |             ^       ^       ^       ^       ^       ^       ^       ^
 
 error: 8 bindings with single-character names in scope
-  --> tests/ui/many_single_char_names.rs:40:10
+  --> tests/ui/many_single_char_names.rs:38:10
    |
 LL |     let (a, b, c, d, e, f, g, h): (bool, bool, bool, bool, bool, bool, bool, bool) = unimplemented!();
    |          ^  ^  ^  ^  ^  ^  ^  ^