[similar_names] don't raise if the first character is different

A lot of cases of the "noise" cases of `similar_names` come from two
idents with a different first letter, which is easy enough to
differentiate visually but causes this lint to be raised.

Do not raise the lint in these cases, as long as the first character
does not have a lookalike.

Link: rust-lang#10926

clippy_lints/src/non_expressive_names.rs

@@ -131,6 +131,14 @@ const ALLOWED_TO_BE_SIMILAR: &[&[&str]] = &[
     &["iter", "item"],
 ];
 
+/// Characters that look visually similar
+const SIMILAR_CHARS: &[(char, char)] = &[('l', 'i'), ('l', '1'), ('i', '1'), ('u', 'v')];
+
+/// Return true if two characters are visually similar
+fn chars_are_similar(a: char, b: char) -> bool {
+    a == b || SIMILAR_CHARS.contains(&(a, b)) || SIMILAR_CHARS.contains(&(b, a))
+}
+
 struct SimilarNamesNameVisitor<'a, 'tcx, 'b>(&'b mut SimilarNamesLocalVisitor<'a, 'tcx>);
 
 impl<'a, 'tcx, 'b> Visitor<'tcx> for SimilarNamesNameVisitor<'a, 'tcx, 'b> {
@@ -219,6 +227,16 @@ impl<'a, 'tcx, 'b> SimilarNamesNameVisitor<'a, 'tcx, 'b> {
             }
 
             let existing_str = existing_name.interned.as_str();
+
+            // The first char being different is usually enough to set identifiers apart, as long
+            // as the characters aren't too similar.
+            if !chars_are_similar(
+                interned_name.chars().next().expect("len >= 1"),
+                existing_str.chars().next().expect("len >= 1"),
+            ) {
+                continue;
+            }
+
             let dissimilar = match existing_name.len.cmp(&count) {
                 Ordering::Greater => existing_name.len - count != 1 || levenstein_not_1(interned_name, existing_str),
                 Ordering::Less => count - existing_name.len != 1 || levenstein_not_1(existing_str, interned_name),

tests/ui/similar_names.rs

@@ -17,13 +17,10 @@ fn main() {
     let specter: i32;
     let spectre: i32;
 
+    // ok; first letter is different enough
     let apple: i32;
-
     let bpple: i32;
-    //~^ ERROR: binding's name is too similar to existing binding
-
     let cpple: i32;
-    //~^ ERROR: binding's name is too similar to existing binding
 
     let a_bar: i32;
     let b_bar: i32;

tests/ui/similar_names.stderr

@@ -1,88 +1,64 @@
 error: binding's name is too similar to existing binding
-  --> $DIR/similar_names.rs:22:9
-   |
-LL |     let bpple: i32;
-   |         ^^^^^
-   |
-note: existing binding defined here
-  --> $DIR/similar_names.rs:20:9
-   |
-LL |     let apple: i32;
-   |         ^^^^^
-   = note: `-D clippy::similar-names` implied by `-D warnings`
-   = help: to override `-D warnings` add `#[allow(clippy::similar_names)]`
-
-error: binding's name is too similar to existing binding
-  --> $DIR/similar_names.rs:25:9
-   |
-LL |     let cpple: i32;
-   |         ^^^^^
-   |
-note: existing binding defined here
-  --> $DIR/similar_names.rs:20:9
-   |
-LL |     let apple: i32;
-   |         ^^^^^
-
-error: binding's name is too similar to existing binding
-  --> $DIR/similar_names.rs:50:9
+  --> $DIR/similar_names.rs:47:9
    |
 LL |     let bluby: i32;
    |         ^^^^^
    |
 note: existing binding defined here
-  --> $DIR/similar_names.rs:49:9
+  --> $DIR/similar_names.rs:46:9
    |
 LL |     let blubx: i32;
    |         ^^^^^
+   = note: `-D clippy::similar-names` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::similar_names)]`
 
 error: binding's name is too similar to existing binding
-  --> $DIR/similar_names.rs:55:9
+  --> $DIR/similar_names.rs:52:9
    |
 LL |     let coke: i32;
    |         ^^^^
    |
 note: existing binding defined here
-  --> $DIR/similar_names.rs:53:9
+  --> $DIR/similar_names.rs:50:9
    |
 LL |     let cake: i32;
    |         ^^^^
 
 error: binding's name is too similar to existing binding
-  --> $DIR/similar_names.rs:74:9
+  --> $DIR/similar_names.rs:71:9
    |
 LL |     let xyzeabc: i32;
    |         ^^^^^^^
    |
 note: existing binding defined here
-  --> $DIR/similar_names.rs:72:9
+  --> $DIR/similar_names.rs:69:9
    |
 LL |     let xyz1abc: i32;
    |         ^^^^^^^
 
 error: binding's name is too similar to existing binding
-  --> $DIR/similar_names.rs:79:9
+  --> $DIR/similar_names.rs:76:9
    |
 LL |     let parsee: i32;
    |         ^^^^^^
    |
 note: existing binding defined here
-  --> $DIR/similar_names.rs:77:9
+  --> $DIR/similar_names.rs:74:9
    |
 LL |     let parser: i32;
    |         ^^^^^^
 
 error: binding's name is too similar to existing binding
-  --> $DIR/similar_names.rs:101:16
+  --> $DIR/similar_names.rs:98:16
    |
 LL |         bpple: sprang,
    |                ^^^^^^
    |
 note: existing binding defined here
-  --> $DIR/similar_names.rs:100:16
+  --> $DIR/similar_names.rs:97:16
    |
 LL |         apple: spring,
    |                ^^^^^^
 
-error: aborting due to 7 previous errors
+error: aborting due to 5 previous errors