UIImage+Rotation.swift

extension UIImage {
    public func imageRotatedByDegrees(degrees: CGFloat) -> UIImage {
        let radiansToDegrees: (CGFloat) -> CGFloat = {
            return $0 * (180.0 / CGFloat(M_PI))
        }
        let degreesToRadians: (CGFloat) -> CGFloat = {
            return $0 / (180.0 * CGFloat(M_PI))
        }
        
        // calculate the size of the rotated view's containing box for our drawing space
        let rotatedViewBox = UIView(frame: CGRect(origin: CGPointZero, size: size))
        let t = CGAffineTransformMakeRotation(degreesToRadians(degrees));
        rotatedViewBox.transform = t
        let rotatedSize = rotatedViewBox.frame.size
        
        // Create the bitmap context
        UIGraphicsBeginImageContext(rotatedSize)
        let bitmap = UIGraphicsGetCurrentContext()
        
        // Move the origin to the middle of the image so we will rotate and scale around the center.
        CGContextTranslateCTM(bitmap, rotatedSize.width / 2.0, rotatedSize.height / 2.0);
        
        //   // Rotate the image context
        CGContextRotateCTM(bitmap, degreesToRadians(degrees));
        
        // Now, draw the rotated/scaled image into the context
        CGContextScaleCTM(bitmap, 1.0, -1.0)
        CGContextDrawImage(bitmap, CGRectMake(-size.width / 2, -size.height / 2, size.width, size.height), CGImage)
        
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        return newImage
    }
}

UIImage-imageRotatedByDegrees.m

// Requires macros:

#define RADIANS_TO_DEGREES(radians) ((radians) * (180.0 / M_PI))
#define DEGREES_TO_RADIANS(angle) (angle / 180.0 * M_PI)

- (UIImage *)imageRotatedByDegrees:(CGFloat)degrees
{
    // calculate the size of the rotated view's containing box for our drawing space
    UIView *rotatedViewBox = [[UIView alloc] initWithFrame:CGRectMake(0,0,self.size.width, self.size.height)];
    CGAffineTransform t = CGAffineTransformMakeRotation(DEGREES_TO_RADIANS(degrees));
    rotatedViewBox.transform = t;
    CGSize rotatedSize = rotatedViewBox.frame.size;
    
    // Create the bitmap context
    UIGraphicsBeginImageContext(rotatedSize);
    CGContextRef bitmap = UIGraphicsGetCurrentContext();
    
    // Move the origin to the middle of the image so we will rotate and scale around the center.
    CGContextTranslateCTM(bitmap, rotatedSize.width/2, rotatedSize.height/2);
    
    //   // Rotate the image context
    CGContextRotateCTM(bitmap, DEGREES_TO_RADIANS(degrees));
    
    // Now, draw the rotated/scaled image into the context
    CGContextScaleCTM(bitmap, 1.0, -1.0);
    CGContextDrawImage(bitmap, CGRectMake(-self.size.width / 2, -self.size.height / 2, self.size.width, self.size.height), [self CGImage]);
    
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
    
}

Swift 5 version:

public extension CGFloat {
    
    func toRadians() -> CGFloat {
        return self / (180 * .pi)
    }
    
    func toDegrees() -> CGFloat {
        return self * (180 * .pi)
    }
}

public extension UIImage {

func imageRotatedByDegrees(degrees: CGFloat) -> UIImage {
        
        // calculate the size of the rotated view's containing box for our drawing space
        let rotatedViewBox = UIView(frame: CGRect(origin: .zero, size: size))
        let t = CGAffineTransform(rotationAngle: degrees.toRadians());
        rotatedViewBox.transform = t
        let rotatedSize = rotatedViewBox.frame.size
        
        // Create the bitmap context
        UIGraphicsBeginImageContext(rotatedSize)
        if let bitmap = UIGraphicsGetCurrentContext() {
            
            bitmap.translateBy(x: rotatedSize.width / 2.0, y: rotatedSize.height / 2.0)
            
            //   // Rotate the image context
            bitmap.rotate(by: degrees.toRadians())
            
            // Now, draw the rotated/scaled image into the context
            bitmap.scaleBy(x: 1.0, y: -1.0)
            
            if let cgImage = self.cgImage {
                bitmap.draw(cgImage, in: CGRect(x: -size.width / 2, y: -size.height / 2, width: size.width, height: size.height))
            }
            
            guard let newImage = UIGraphicsGetImageFromCurrentImageContext() else { debugPrint("Failed to rotate image. Returning the same as input..."); return self }
            UIGraphicsEndImageContext()
            
            return newImage
        }else {
            debugPrint("Failed to create graphics context. Returning the same as input...")
            return self
        }

    }

}

The conversion from degrees to radians should be:

func toRadians() -> CGFloat {
        return self * (.pi / 180)
    }

But now it creates an image that is too small.