Implement OneSpaceInsideBraces (swiftlang#84)

Author: LaurenWhite

tools/swift-format/Sources/Rules/OneSpaceBeforeExpressions.swift

@@ -0,0 +1,47 @@
+import Core
+import Foundation
+import SwiftSyntax
+
+/// A single space is required between an expression and a brace on the same line.
+///
+/// Lint: If an expression appears on the same line as a brace with invalid spaces between
+///       them, a lint error is raised.
+///
+/// Format: A single space will be inserted between braces and same-line expressions.
+///
+/// - SeeAlso: https://google.github.io/swift#horizontal-whitespace
+public final class OneSpaceInsideBraces: SyntaxFormatRule {
+
+  public override func visit(_ token: TokenSyntax) -> Syntax {
+    
+    guard let next = token.nextToken else { return token }
+    guard !next.leadingTrivia.containsNewlines else { return token }
+    
+    let trailingSpaces = token.trailingTrivia.numberOfSpaces
+    guard trailingSpaces != 1 else { return token }
+
+    // Insert single space between open brace and next token
+    if token.tokenKind == .leftBrace {
+      diagnose(trailingSpaces > 1 ? .removeSpaceAfterOpenBrace : .insertSpaceAfterOpenBrace,
+               on: token)
+      return token.withTrailingTrivia(next.tokenKind == .rightBrace ? [] : .spaces(1))
+    // Insert single space between token and close brace
+    } else if next.tokenKind == .rightBrace {
+      diagnose(trailingSpaces > 1 ? .removeSpaceBeforeCloseBrace : .insertSpaceBeforeCloseBrace,
+               on: next)
+      return token.withTrailingTrivia(.spaces(1))
+    }
+    return token
+  }
+}
+
+extension Diagnostic.Message {
+  static let removeSpaceAfterOpenBrace =
+    Diagnostic.Message(.warning, "remove extra spaces after this '{'")
+  static let insertSpaceAfterOpenBrace =
+    Diagnostic.Message(.warning, "insert a space after this '{'")
+  static let removeSpaceBeforeCloseBrace =
+    Diagnostic.Message(.warning, "remove extra spaces before this '}'")
+  static let insertSpaceBeforeCloseBrace =
+    Diagnostic.Message(.warning, "insert a space before this '}'")
+}

tools/swift-format/Sources/Rules/OneSpaceInsideBraces.swift

@@ -0,0 +1,47 @@
+import Foundation
+import XCTest
+import SwiftSyntax
+
+@testable import Rules
+
+public class OneSpaceInsideBracesTests: DiagnosingTestCase {
+  public func testInvalidSpacesInsideBraces() {
+    XCTAssertFormatting(
+      OneSpaceInsideBraces.self,
+      input: """
+             let nonNegativeCubes = numbers.map {$0 * $0 * $0    }.filter {  $0 >= 0}
+             func foo() {}
+             func bar() {  }
+             guard x = y else {return nil  }
+             if y > x {
+               // ...
+             }
+             func baz() {if(true) {  print("yes!")}}
+             """,
+      expected: """
+                let nonNegativeCubes = numbers.map { $0 * $0 * $0 }.filter { $0 >= 0 }
+                func foo() {}
+                func bar() {}
+                guard x = y else { return nil }
+                if y > x {
+                  // ...
+                }
+                func baz() { if(true) { print("yes!") } }
+                """)
+    XCTAssertDiagnosed(.insertSpaceAfterOpenBrace)
+    XCTAssertDiagnosed(.removeSpaceBeforeCloseBrace)
+    XCTAssertDiagnosed(.removeSpaceAfterOpenBrace)
+    XCTAssertDiagnosed(.insertSpaceAfterOpenBrace)
+    XCTAssertDiagnosed(.removeSpaceBeforeCloseBrace)
+    XCTAssertDiagnosed(.insertSpaceAfterOpenBrace)
+    XCTAssertDiagnosed(.removeSpaceAfterOpenBrace)
+    XCTAssertDiagnosed(.insertSpaceBeforeCloseBrace)
+    XCTAssertDiagnosed(.insertSpaceBeforeCloseBrace)
+  }
+
+  #if !os(macOS)
+  static let allTests = [
+    OneSpaceInsideBracesTests.testInvalidSpacesInsideBraces,
+    ]
+  #endif
+}