The code responsible for parsing this line is located at line 181 of swift-package-manager/Sources/PackageLoading/ToolsVersionLoader.swift, copied below:

// The regex to match swift tools version specification:
// * It should start with `//` followed by any amount of whitespace.
// * Following that it should contain the case insensitive string `swift-tools-version:`.
// * The text between the above string and `;` or string end becomes the tools version specifier.
static let regex = try! NSRegularExpression(
    pattern: "^// swift-tools-version:(.*?)(?:;.*|$)",
    options: [.caseInsensitive])

The regex intends to match "any amount of whitespace followed by //" (according to the comments in the code snippet above), but in its implementation, it matches only 1 space character.

This can be easily fixed, but I want to use this bug report to ask if it was a conscious design choice to allow only 1 space character, and shouldn't be changed.

Opened pull request #2937

resolved by apple/swift-package-manager#2937