Improve docs for single_line_if_else_max_width & add tests

Configurations.md

@@ -2381,12 +2381,74 @@ Don't reformat out of line modules
 
 Maximum line length for single line if-else expressions. A value of `0` (zero) results in if-else expressions always being broken into multiple lines. Note this occurs when `use_small_heuristics` is set to `Off`.
 
+Common if-else expressions that may be formatted on a single line are:
+* Expressions on the right hand side of an equals (`=`) sign. e.g. `let x = if y == 1 { 0 } else { 1 };`
+* Conditional expression in function calls. e.g. `f(if x== 0 { 10 } else { 0 })`
+* Single expression closures. e.g. `let x = |y| if y == 0 { 0 } else { 10 };` 
+
+In the following scenarios the if-else expression will be formatted over multiple lines.
+* Empty if-else blocks.
+* An outer if-else expression when if-else expression are nested.
+* If-else blocks that contain statements (e.g. `let x = a;`), comments, or attributes (e.g. `let _ = if true { #[must_use] 1 } else { 2 };`).
+
+When `version=One` is set (the default) an if-else expression at the end of a block is treated as a statement and is not formatted on a single line. When using `version=Two` the if-else expression may be formatted on a single line if it meets the criteria described above.
+
 - **Default value**: `50`
 - **Possible values**: any positive integer that is less than or equal to the value specified for [`max_width`](#max_width)
 - **Stable**: Yes
 
 By default this option is set as a percentage of [`max_width`](#max_width) provided by [`use_small_heuristics`](#use_small_heuristics), but a value set directly for `single_line_if_else_max_width` will take precedence.
 
+#### `50` (default):
+
+```rust
+fn foo() -> usize {
+    fun(if some_long_name && some_other_long_name {
+        0
+    } else {
+        10
+    });
+
+    closure(|super_long_closure_variable| {
+        if super_long_closure_variable == 0 {
+            0
+        } else {
+            10
+        }
+    });
+
+    let bar = if some_long_name && some_other_long_name {
+        baz()
+    } else {
+        buzz()
+    };
+
+    if some_long_name && some_other_long_name {
+        1
+    } else {
+        2
+    }
+}
+```
+
+#### `70`:
+
+```rust
+fn foo() -> usize {
+    fun(if some_long_name && some_other_long_name { 0 } else { 10 });
+
+    closure(|super_long_closure_variable| if super_long_closure_variable == 0 { 0 } else { 10 });
+
+    let bar = if some_long_name && some_other_long_name { baz() } else { buzz() };
+
+    if some_long_name && some_other_long_name {
+        1
+    } else {
+        2
+    }
+}
+```
+
 See also [`max_width`](#max_width) and [`use_small_heuristics`](#use_small_heuristics)
 
 ## `space_after_colon`

tests/source/issue-5496/70.rs

@@ -0,0 +1,8 @@
+// rustfmt-version: One
+// rustfmt-single_line_if_else_max_width: 70
+fn foo() -> usize {
+    let some_long_name = true;
+    let some_other_long_name = false;
+    let bar = if some_long_name && some_other_long_name { baz() } else { buzz() };
+    if some_long_name && some_other_long_name { 1 } else { 2 }
+}

tests/source/issue-5496/70_version_two.rs

@@ -0,0 +1,12 @@
+// rustfmt-version: Two
+// rustfmt-single_line_if_else_max_width: 70
+fn foo() -> usize {
+    let some_long_name = true;
+    let some_other_long_name = false;
+    let bar = if some_long_name && some_other_long_name { baz() } else { buzz() };
+    if some_long_name && some_other_long_name {
+        1
+    } else {
+        2
+    }
+}

tests/source/issue-5496/default.rs

@@ -0,0 +1,6 @@
+fn foo() -> usize {
+    let some_long_name = true;
+    let some_other_long_name = false;
+    let bar = if some_long_name && some_other_long_name { baz() } else { buzz() };
+    if some_long_name && some_other_long_name { 1 } else { 2 }
+}

tests/source/issue-5496/nested_if_one.rs

@@ -0,0 +1,10 @@
+// rustfmt-version: One
+// rustfmt-single_line_if_else_max_width: 75
+fn foo() -> usize {
+    // nested
+    let _ = if true { if false { 1 } else { 2 } } else { 3 };
+    let _ = if true { 3 } else { if false { 1 } else { 2 } };
+    let _ = if true { if false { 1 } else { 2 } } else { if false { 3 } else { 4 } };
+
+    1
+}

tests/source/issue-5496/nested_if_two.rs

@@ -0,0 +1,22 @@
+// rustfmt-version: Two
+// rustfmt-single_line_if_else_max_width: 75
+fn foo() -> usize {
+    // nested
+    let _ = if true {
+        if false { 1 } else { 2 }
+    } else {
+        3
+    };
+    let _ = if true {
+        3
+    } else {
+        if false { 1 } else { 2 }
+    };
+    let _ = if true {
+        if false { 1 } else { 2 }
+    } else {
+        if false { 3 } else { 4 }
+    };
+
+    1
+}

tests/source/issue-5496/simple_one.rs

@@ -0,0 +1,24 @@
+// rustfmt-version: One
+fn foo() -> usize {
+    // empty
+    let _ = if true { 1 } else { };
+    let _ = if true { } else { 2 };
+    let _ = if true { } else { };
+
+     // attribute
+    let _ = if true { #[must_use] 1 } else { 2 };
+    let _ = if true { 1 } else { #[must_use] 2 };
+    let _ = if true { #[must_use] 1 } else { #[must_use] 2 };
+
+    // comment
+    let _ = if true { 1 /*1*/ } else { 2 };
+    let _ = if true { 1 } else { 2 /*2*/};
+    let _ = if true { 1 /*1*/} else { 2 /*2*/};
+
+    // a statement
+    let _ = if true { let a = 1; a } else { 2 };
+    let _ = if true { 1 } else { let b = 2; b };
+    let _ = if true { let a = 1; a } else { let b = 2; b };
+
+    1
+}

tests/source/issue-5496/simple_two.rs

@@ -0,0 +1,24 @@
+// rustfmt-version: Two
+fn foo() -> usize {
+    // empty
+    let _ = if true { 1 } else { };
+    let _ = if true { } else { 2 };
+    let _ = if true { } else { };
+
+     // attribute
+    let _ = if true { #[must_use] 1 } else { 2 };
+    let _ = if true { 1 } else { #[must_use] 2 };
+    let _ = if true { #[must_use] 1 } else { #[must_use] 2 };
+
+    // comment
+    let _ = if true { 1 /*1*/ } else { 2 };
+    let _ = if true { 1 } else { 2 /*2*/};
+    let _ = if true { 1 /*1*/} else { 2 /*2*/};
+
+    // a statement
+    let _ = if true { let a = 1; a } else { 2 };
+    let _ = if true { 1 } else { let b = 2; b };
+    let _ = if true { let a = 1; a } else { let b = 2; b };
+
+    1
+}

tests/target/issue-5496/70.rs

@@ -0,0 +1,12 @@
+// rustfmt-version: One
+// rustfmt-single_line_if_else_max_width: 70
+fn foo() -> usize {
+    let some_long_name = true;
+    let some_other_long_name = false;
+    let bar = if some_long_name && some_other_long_name { baz() } else { buzz() };
+    if some_long_name && some_other_long_name {
+        1
+    } else {
+        2
+    }
+}

tests/target/issue-5496/70_version_two.rs

@@ -0,0 +1,8 @@
+// rustfmt-version: Two
+// rustfmt-single_line_if_else_max_width: 70
+fn foo() -> usize {
+    let some_long_name = true;
+    let some_other_long_name = false;
+    let bar = if some_long_name && some_other_long_name { baz() } else { buzz() };
+    if some_long_name && some_other_long_name { 1 } else { 2 }
+}

tests/target/issue-5496/default.rs

@@ -0,0 +1,14 @@
+fn foo() -> usize {
+    let some_long_name = true;
+    let some_other_long_name = false;
+    let bar = if some_long_name && some_other_long_name {
+        baz()
+    } else {
+        buzz()
+    };
+    if some_long_name && some_other_long_name {
+        1
+    } else {
+        2
+    }
+}

tests/target/issue-5496/nested_if_one.rs

@@ -0,0 +1,38 @@
+// rustfmt-version: One
+// rustfmt-single_line_if_else_max_width: 75
+fn foo() -> usize {
+    // nested
+    let _ = if true {
+        if false {
+            1
+        } else {
+            2
+        }
+    } else {
+        3
+    };
+    let _ = if true {
+        3
+    } else {
+        if false {
+            1
+        } else {
+            2
+        }
+    };
+    let _ = if true {
+        if false {
+            1
+        } else {
+            2
+        }
+    } else {
+        if false {
+            3
+        } else {
+            4
+        }
+    };
+
+    1
+}

tests/target/issue-5496/nested_if_two.rs

@@ -0,0 +1,22 @@
+// rustfmt-version: Two
+// rustfmt-single_line_if_else_max_width: 75
+fn foo() -> usize {
+    // nested
+    let _ = if true {
+        if false { 1 } else { 2 }
+    } else {
+        3
+    };
+    let _ = if true {
+        3
+    } else {
+        if false { 1 } else { 2 }
+    };
+    let _ = if true {
+        if false { 1 } else { 2 }
+    } else {
+        if false { 3 } else { 4 }
+    };
+
+    1
+}

tests/target/issue-5496/simple_one.rs

@@ -0,0 +1,76 @@
+// rustfmt-version: One
+fn foo() -> usize {
+    // empty
+    let _ = if true {
+        1
+    } else {
+    };
+    let _ = if true {
+    } else {
+        2
+    };
+    let _ = if true {
+    } else {
+    };
+
+    // attribute
+    let _ = if true {
+        #[must_use]
+        1
+    } else {
+        2
+    };
+    let _ = if true {
+        1
+    } else {
+        #[must_use]
+        2
+    };
+    let _ = if true {
+        #[must_use]
+        1
+    } else {
+        #[must_use]
+        2
+    };
+
+    // comment
+    let _ = if true {
+        1 /*1*/
+    } else {
+        2
+    };
+    let _ = if true {
+        1
+    } else {
+        2 /*2*/
+    };
+    let _ = if true {
+        1 /*1*/
+    } else {
+        2 /*2*/
+    };
+
+    // a statement
+    let _ = if true {
+        let a = 1;
+        a
+    } else {
+        2
+    };
+    let _ = if true {
+        1
+    } else {
+        let b = 2;
+        b
+    };
+    let _ = if true {
+        let a = 1;
+        a
+    } else {
+        let b = 2;
+        b
+    };
+
+    1
+}