remove newlines immediately after an assignment operator

Fixes #76.

README.md

@@ -15,6 +15,25 @@ The added formatting rules are in the `format` package.
 
 ### Added rules
 
+No empty lines following an assignment operator
+
+<details><summary><i>example</i></summary>
+
+```
+func foo() {
+    foo :=
+        "bar"
+}
+```
+
+```
+func foo() {
+	foo := "bar"
+}
+```
+
+</details>
+
 No empty lines at the beginning or end of a function
 
 <details><summary><i>example</i></summary>

format/format.go

@@ -599,6 +599,10 @@ func (f *fumpter) applyPre(c *astutil.Cursor) {
 				c.Replace(node)
 			}
 		}
+
+	case *ast.AssignStmt:
+		// Only remove lines between the assignment token and the first right-hand side expression
+		f.removeLines(f.Line(node.TokPos), f.Line(node.Rhs[0].Pos()))
 	}
 }
 

testdata/scripts/assignment-newlines.txt

@@ -0,0 +1,60 @@
+gofumpt -w foo.go
+cmp foo.go foo.go.golden
+
+gofumpt -d foo.go.golden
+! stdout .
+
+-- foo.go --
+package p
+
+func f() {
+    foo :=
+
+
+        "bar"
+
+    foo :=
+        "bar"
+
+	_, _ =
+		0,
+		1
+
+	_, _ = 0,
+		1
+
+    _ =
+        `
+            foo
+        `
+
+	_ = /* inline */
+		"foo"
+
+	_ = // inline
+		"foo"
+}
+
+-- foo.go.golden --
+package p
+
+func f() {
+	foo := "bar"
+
+	foo := "bar"
+
+	_, _ = 0,
+		1
+
+	_, _ = 0,
+		1
+
+	_ = `
+            foo
+        `
+
+	_ = /* inline */ "foo"
+
+	_ = // inline
+	"foo"
+}