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Correct 2 x grammar rules for compilation unit name in #line #1120

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Correct 2 x grammar rules for compilation unit name in #line #1120

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4 changes: 2 additions & 2 deletions standard/lexical-structure.md
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Original file line number Diff line number Diff line change
Expand Up @@ -1488,12 +1488,12 @@ fragment PP_Line_Indicator
;

fragment PP_Compilation_Unit_Name
: '"' PP_Compilation_Unit_Name_Character+ '"'
: '"' PP_Compilation_Unit_Name_Character* '"'
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@Nigel-Ecma Nigel-Ecma May 25, 2024 •
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We should check this is by design before making this change. Regardless of the answer there is probably some work to do:

  • §14.2 Compilation units defines a compilation unit, the definition does not include them having names… or being files… However the example (non-normative) uses two files A.cs and B.cs and refers to them as two compilation units.
  • §22.5.6.3 The CallerFilePath attribute which provides the file path (which is implementation-dependent) states “The file path may be affected by #line directives ([§6.5.8]”.
  • Here in §6.5.8 the #line allows the setting of the “compilation unit name”
  • So:
    • §6.5.8 states compilation units have names, which is omitted in §14.2; and
    • §22.5.6.3 tells us that the name is the file path, but leaves what that is implementation-dependent

What already exists isn’t overly clear and this change seeks to allow the compilation unit name to be the empty string, which is probably not a valid implementation-dependent path on any implementation… So if this
observed compiler behaviour is by design then it surely needs to have a defined meaning in the Standard.

If this change is to be made, and even if not, this all needs to tided up – either in this PR or spin it all off into a new one.

I might ask what the intended use of an empty file path/compilation unit name is but I might know – it was requested by the NSA so that the file names in NSA distributed software is not leaked and so endanger National Security! I’m only partially joking here, but that’s a story for another time ;-)

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@BillWagner told me that allowing an empty string was indeed a conscious decision, so I propose keeping that edit.

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@Nigel-Ecma

§6.1 Programs states:

A C# program consists of one or more source files, known formally as compilation units (§14.2). Although a compilation unit might have a one-to-one correspondence with a file in a file system, such correspondence is not required.

I propose appending to this, the following:

… As such, the accepted spelling of a compilation unit name, and its mapping, if any, to a filename is outside the scope of this specification.

I'm deliberately avoiding using any of the following terms:

  • behavior, implementation-defined – unspecified behavior where each implementation documents how the choice is made
  • behavior, undefined – behavior, upon use of a non-portable or erroneous construct or of erroneous data, for which this specification imposes no requirements
  • behavior, unspecified – behavior where this specification provides two or more possibilities and imposes no further requirements on which is chosen in any instance

;

fragment PP_Compilation_Unit_Name_Character
// Any Input_Character except "
: ~('\u000D' | '\u000A' | '\u0085' | '\u2028' | '\u2029' | '#')
: ~('\u000D' | '\u000A' | '\u0085' | '\u2028' | '\u2029' | '"')
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The # was a typo so should be fixed…

However §22.5.6.3 defines the format of the compilation unit name/file path as “implementation-dependent”. So this section might need a semantic rule saying this arbitrary string must conform to the same implementation-dependent rules §22.5.6.3, or that is does not need to (i.e. not be valid as a file path).

;
```

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