Declare & implement space_around_attr_eq config.

Configurations.md

@@ -2001,6 +2001,28 @@ fn main() {
 }
 ```
 
+## `space_around_attr_eq`
+
+Determines if '=' are wrapped in spaces in attributes.
+
+- **Default value**: `true`
+- **Possible values**: `true`, `false`
+- **Stable**: No
+
+#### `true` (default):
+
+```rust
+#[cfg(not(target_os = "pi"))]
+println!("os is not pi!");
+```
+
+#### `false`
+
+```rust
+#[cfg(not(target_os="pi"))]
+println!("os is not pi!");
+```
+
 ## `struct_field_align_threshold`
 
 The maximum diff of width between struct fields to be aligned with each other.

rustfmt-core/rustfmt-config/src/lib.rs

@@ -73,6 +73,8 @@ create_config! {
         "Determines if '+' or '=' are wrapped in spaces in the punctuation of types";
     space_before_colon: bool, false, false, "Leave a space before the colon";
     space_after_colon: bool, true, false, "Leave a space after the colon";
+    space_around_attr_eq: bool, true, false,
+        "Determines if '=' are wrapped in spaces in attributes.";
     spaces_around_ranges: bool, false, false, "Put spaces around the  .. and ..= range operators";
     binop_separator: SeparatorPlace, SeparatorPlace::Front, false,
         "Where to put a binary operator when a binary expression goes multiline";
@@ -517,6 +519,7 @@ reorder_impl_items = false
 type_punctuation_density = "Wide"
 space_before_colon = false
 space_after_colon = true
+space_around_attr_eq = true
 spaces_around_ranges = false
 binop_separator = "Front"
 remove_nested_parens = true

rustfmt-core/rustfmt-lib/src/attr.rs

@@ -252,7 +252,11 @@ impl Rewrite for ast::MetaItem {
                 // See #2479 for example.
                 let value = rewrite_literal(context, literal, lit_shape)
                     .unwrap_or_else(|| context.snippet(literal.span).to_owned());
-                format!("{} = {}", path, value)
+                if context.config.space_around_attr_eq() {
+                    format!("{} = {}", path, value)
+                } else {
+                    format!("{}={}", path, value)
+                }
             }
         })
     }

rustfmt-core/rustfmt-lib/tests/source/configs/space_around_attr_eq/false.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_around_attr_eq: false
+
+fn attr_eq_test() {
+    #[cfg(not(target_os = "pi"))]
+    println!("os is not pi!");
+}

rustfmt-core/rustfmt-lib/tests/source/configs/space_around_attr_eq/true.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_around_attr_eq: true
+
+fn attr_eq_test() {
+    #[cfg(not(target_os="pi"))]
+    println!("os is not pi!");
+}

rustfmt-core/rustfmt-lib/tests/target/configs/space_around_attr_eq/false.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_around_attr_eq: false
+
+fn attr_eq_test() {
+    #[cfg(not(target_os="pi"))]
+    println!("os is not pi!");
+}

rustfmt-core/rustfmt-lib/tests/target/configs/space_around_attr_eq/true.rs

@@ -0,0 +1,6 @@
+// rustfmt-space_around_attr_eq: true
+
+fn attr_eq_test() {
+    #[cfg(not(target_os = "pi"))]
+    println!("os is not pi!");
+}