Document and test use of char::is_whitespace in collapsible_if (#16840)

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comments](https://triagebot.infra.rust-lang.org/gh-comments/rust-lang/rust-clippy/pull/16840)*

changelog: none

This PR addresses the whitespace check in collapsible_if.

```rust
// ./clippy_lints/src/collapsible_if.rs:145

let requires_space = snippet(cx, up_to_else, "..").ends_with(|c: char| !c.is_whitespace());
```

My investigation revealed that the current char::is_whitespace check is
actually intentional. If we switched to rustc_lexer::is_whitespace,
zero-width characters (like \u{200E}) would be treated as valid spacing,
which would make Clippy to output suggestions that visibly look like
elseif. To prevent future regressions, this PR adds a UI test with a
zero-width space and an inline comment explaining why we must keep the
current check.

The issue is for outreachy applicants and is being tracked here:
rustfoundation/interop-initiative#53

Author: dswij

clippy_lints/src/collapsible_if.rs

@@ -140,6 +140,8 @@ impl CollapsibleIf {
 
                     // Prevent "elseif"
                     // Check that the "else" is followed by whitespace
+                    // Note: We intentionally use char::is_whitespace instead of rustc_lexer::is_whitespace here to
+                    // avoid visual issues with zero-width spaces. See ui tests.
                     let requires_space = snippet(cx, up_to_else, "..").ends_with(|c: char| !c.is_whitespace());
                     let mut applicability = Applicability::MachineApplicable;
                     diag.span_suggestion(

tests/ui/collapsible_else_if.fixed

@@ -161,3 +161,32 @@ fn in_brackets() {
         { if y == "world" { println!("world") } else { println!("!") } }
     }
 }
+
+#[rustfmt::skip]
+fn ends_with_zero_width_whitespace() {
+    // Test out snippets ending with the 2 zero-width characters recognized as whitespaces by the lexer,
+    // but not by char::is_whitespace
+    // Behaviour shows a whitespace is inserted between else and if here which is desirable in this case
+
+    let x = "hello";
+    let y = "world";
+
+
+    // LRM (U+200E)
+    if x == "hello" {
+        println!("hello LRM");
+    } else‎ if y == "world" {
+        println!("LRM world");
+    }
+    //~^^^^^ collapsible_else_if
+
+    // RLM (U+200F)
+    if x == "hello" {
+        println!("hello RLM");
+    } else‏ if y == "world" {
+        println!("RLM world");
+    }
+    //~^^^^^ collapsible_else_if
+
+
+}

tests/ui/collapsible_else_if.rs

@@ -181,3 +181,36 @@ fn in_brackets() {
         { if y == "world" { println!("world") } else { println!("!") } }
     }
 }
+
+#[rustfmt::skip]
+fn ends_with_zero_width_whitespace() {
+    // Test out snippets ending with the 2 zero-width characters recognized as whitespaces by the lexer,
+    // but not by char::is_whitespace
+    // Behaviour shows a whitespace is inserted between else and if here which is desirable in this case
+
+    let x = "hello";
+    let y = "world";
+
+
+    // LRM (U+200E)
+    if x == "hello" {
+        println!("hello LRM");
+    } else‎{
+        if y == "world" {
+            println!("LRM world");
+        }
+    }
+    //~^^^^^ collapsible_else_if
+
+    // RLM (U+200F)
+    if x == "hello" {
+        println!("hello RLM");
+    } else‏{
+        if y == "world" {
+            println!("RLM world");
+        }
+    }
+    //~^^^^^ collapsible_else_if
+
+
+}

tests/ui/collapsible_else_if.stderr

@@ -177,5 +177,41 @@ LL | |         (if y == "world" { println!("world") } else { println!("!") })
 LL | |     }
    | |_____^ help: collapse nested if block: `if y == "world" { println!("world") } else { println!("!") }`
 
-error: aborting due to 10 previous errors
+error: this `else { if .. }` block can be collapsed
+  --> tests/ui/collapsible_else_if.rs:198:12
+   |
+LL |       } else‎{
+   |  ___________^
+LL | |         if y == "world" {
+LL | |             println!("LRM world");
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+help: collapse nested if block
+   |
+LL ~     } else‎ if y == "world" {
+LL +         println!("LRM world");
+LL +     }
+   |
+
+error: this `else { if .. }` block can be collapsed
+  --> tests/ui/collapsible_else_if.rs:208:12
+   |
+LL |       } else‏{
+   |  ___________^
+LL | |         if y == "world" {
+LL | |             println!("RLM world");
+LL | |         }
+LL | |     }
+   | |_____^
+   |
+help: collapse nested if block
+   |
+LL ~     } else‏ if y == "world" {
+LL +         println!("RLM world");
+LL +     }
+   |
+
+error: aborting due to 12 previous errors