[Parser] String interpolation with trailing comma skips following expression during parsing

[kntkymt]

Description

When using string interpolation with a trailing comma, the parser appears to skip the subsequent expression entirely. As a result, the expression is not properly parsed and is missing from the AST.

Reproduction

compile the following code.

"\(1,)" // following expression are not parsed
print("a") // won't be executed
lethoge:;:;{}:;$$@@@/po
(・w・)っ@MainActor!!!!
今日は星が綺麗

output (not output, only the warning appears)

swift Source.swift
Source.swift:1:1: warning: string literal is unused
1 | "\(1,)" // following expression are not parsed
  | `- warning: string literal is unused
2 | print("a") // won't be executed
3 | lethoge:;:;{}:;$$@@@/po

Expected behavior

compiler should emit error/correctly parse the subsequent expressions into AST.

Environment

swift-driver version: 1.120.5 Apple Swift version 6.1 (swiftlang-6.1.0.110.21 clang-1700.0.13.3)
Target: arm64-apple-macosx15.0

Additional information

dump AST

swiftc -dump-ast Source.swift
(source_file "Source.swift"
  (top_level_code_decl range=[Source.swift:1:1 - line:1:1]
    (brace_stmt implicit range=[Source.swift:1:1 - line:1:1]
      (interpolated_string_literal_expr type="String" location=Source.swift:1:1 range=[Source.swift:1:1 - line:1:1] literal_capacity=0 interpolation_count=1 builder_init="Swift.(file).DefaultStringInterpolation.init(literalCapacity:interpolationCount:)" result_init="Swift.(file).String extension.init(stringInterpolation:)"
        (tap_expr implicit type="String.StringInterpolation" location=Source.swift:1:1 range=[Source.swift:1:1 - line:1:7] var="Source.(file).top-level code.$interpolation"
          (opaque_value_expr implicit type="String.StringInterpolation" location=Source.swift:1:1 range=[Source.swift:1:1 - line:1:1] "0x1229da660")
          (brace_stmt implicit range=[Source.swift:1:2 - line:1:7]
            (var_decl implicit "$interpolation" interface_type="String.StringInterpolation" access=fileprivate readImpl=stored writeImpl=stored readWriteImpl=stored)
            (call_expr implicit type="()" location=Source.swift:1:2 range=[Source.swift:1:2 - line:1:1] nothrow isolation_crossing="none"
              (dot_syntax_call_expr implicit type="(String) -> ()" location=Source.swift:1:2 range=[Source.swift:1:2 - line:1:2] nothrow isolation_crossing="none"
                (declref_expr implicit type="(inout DefaultStringInterpolation) -> (String) -> ()" decl="Swift.(file).DefaultStringInterpolation.appendLiteral" function_ref=single_apply)
                (argument_list implicit
                  (argument inout
                    (inout_expr implicit type="inout DefaultStringInterpolation" location=Source.swift:1:2 range=[Source.swift:1:2 - line:1:2]
                      (declref_expr implicit type="@lvalue String.StringInterpolation" location=Source.swift:1:2 range=[Source.swift:1:2 - line:1:2] decl="Source.(file).top-level code.$interpolation" function_ref=unapplied)))))
              (argument_list implicit
                (argument
                  (string_literal_expr type="String" location=Source.swift:1:1 range=[Source.swift:1:1 - line:1:1] encoding=utf8 value="" builtin_initializer="Swift.(file).String extension.init(_builtinStringLiteral:utf8CodeUnitCount:isASCII:)" initializer="**NULL**"))))
            (call_expr type="()" location=Source.swift:1:3 range=[Source.swift:1:3 - line:1:6] nothrow isolation_crossing="none"
              (dot_syntax_call_expr implicit type="(Int) -> ()" location=Source.swift:1:3 range=[Source.swift:1:3 - line:1:3] nothrow isolation_crossing="none"
                (declref_expr implicit type="(inout DefaultStringInterpolation) -> (Int) -> ()" location=Source.swift:1:3 range=[Source.swift:1:3 - line:1:3] decl="Swift.(file).DefaultStringInterpolation.appendInterpolation [with (substitution_map generic_signature=<T where T : CustomStringConvertible> T -> Int)]" function_ref=single_apply)
                (argument_list implicit
                  (argument inout
                    (inout_expr implicit type="inout DefaultStringInterpolation" location=Source.swift:1:3 range=[Source.swift:1:3 - line:1:3]
                      (declref_expr implicit type="@lvalue String.StringInterpolation" location=Source.swift:1:3 range=[Source.swift:1:3 - line:1:3] decl="Source.(file).top-level code.$interpolation" function_ref=unapplied)))))
              (argument_list
                (argument
                  (integer_literal_expr type="Int" location=Source.swift:1:4 range=[Source.swift:1:4 - line:1:4] value="1" builtin_initializer="Swift.(file).Int.init(_builtinIntegerLiteral:)" initializer="**NULL**"))))
            (call_expr implicit type="()" location=Source.swift:1:7 range=[Source.swift:1:7 - line:1:7] nothrow isolation_crossing="none"
              (dot_syntax_call_expr implicit type="(String) -> ()" location=Source.swift:1:7 range=[Source.swift:1:7 - line:1:7] nothrow isolation_crossing="none"
                (declref_expr implicit type="(inout DefaultStringInterpolation) -> (String) -> ()" decl="Swift.(file).DefaultStringInterpolation.appendLiteral" function_ref=single_apply)
                (argument_list implicit
                  (argument inout
                    (inout_expr implicit type="inout DefaultStringInterpolation" location=Source.swift:1:7 range=[Source.swift:1:7 - line:1:7]
                      (declref_expr implicit type="@lvalue String.StringInterpolation" location=Source.swift:1:7 range=[Source.swift:1:7 - line:1:7] decl="Source.(file).top-level code.$interpolation" function_ref=unapplied)))))
              (argument_list implicit
                (argument
                  (string_literal_expr type="String" location=Source.swift:1:7 range=[Source.swift:1:7 - line:1:7] encoding=utf8 value="" builtin_initializer="Swift.(file).String extension.init(_builtinStringLiteral:utf8CodeUnitCount:isASCII:)" initializer="**NULL**"))))))))))
Source.swift:1:1: warning: string literal is unused
1 | "\(1,)" // following expression are not parsed
  | `- warning: string literal is unused
2 | print("a") // won't be executed
3 | lethoge:;:;{}:;$$@@@/po

[kntkymt]

(I'm not familiar with the codebase but,) it seems that expressions after string interpolation with trailing comma will be skipped by skipUntilDeclStmtRBrace() in Parser::parseBraceItems.

    if (NeedParseErrorRecovery) {
      // If we had a parse error, skip to the start of the next stmt or decl.
      //
      // It would be ideal to stop at the start of the next expression (e.g.
      // "X = 4"), but distinguishing the start of an expression from the middle
      // of one is "hard".
      skipUntilDeclStmtRBrace();

      // If we have to recover, pretend that we had a semicolon; it's less
      // noisy that way.
      PreviousHadSemi = true;
    }

Because parser still returns nullptr when it found trailing comma in string interpolation (that represent initializing ParserResult with setIsParseError()).

    // Enable trailing comma in string literal interpolation
    // Note that 'Tok.is(tok::r_paren)' is not used because the text is ")\"\n" but the kind is actualy 'eof'
    if (Tok.is(tok::eof) && Tok.getText() == ")") {
      return nullptr;
    }

I would suggest to fix the logic returning nullptr like my PR

#80928

(this PR is originally intended to fix #80927)

[kntkymt]

I believe this issue has been fixed by #80927. if someone find the issue, feel free to re-open this!