update compiler directives page of FSharp reference

docs/fsharp/language-reference/compiler-directives.md

@@ -1,57 +1,58 @@
 ---
 title: Compiler Directives
-description: Learn about F# language preprocessor directives, conditional compilation directives, line directives, and compiler directives.
+description: Learn about F# language conditional compilation directives, line directives, and warn directives.
 ms.date: 12/10/2018
 f1_keywords:
  - "#endif_FS"
 ---
 # Compiler Directives
 
-This topic describes processor directives and compiler directives.
+This topic describes compiler directives, for F# Interactive (`dotnet fsi`) directives, see [Interactive Programming with F#](../tools/fsharp-interactive/index.md).
 
-For F# Interactive (`dotnet fsi`) directives, see [Interactive Programming with F#](../tools/fsharp-interactive/index.md).
+A compiler directive is prefixed with the # symbol and appears on a line by itself.
 
-## Preprocessor Directives
-
-A preprocessor directive is prefixed with the # symbol and appears on a line by itself. It is interpreted by the preprocessor, which runs before the compiler itself.
-
-The following table lists the preprocessor directives that are available in F#.
+The following table lists the compiler directives that are available in F#.
 
 |Directive|Description|
 |---------|-----------|
-|`#if` *symbol*|Supports conditional compilation. Code in the section after the `#if` is included if the *symbol* is defined. The symbol can also be negated with `!`.|
-|`#else`|Supports conditional compilation. Marks a section of code to include if the symbol used with the previous `#if` is not defined.|
+|`#if` *if-expression*|Supports conditional compilation. Code in the section after the `#if` is included if the *if-expression* evaluates to `defined` (see below).|
+|`#else`|Supports conditional compilation. Marks a section of code to include if the symbol used with the previous `#if` does not evaluate to `defined`.|
 |`#endif`|Supports conditional compilation. Marks the end of a conditional section of code.|
 |`#`[line] *int*,<br/>`#`[line] *int* *string*,<br/>`#`[line] *int* *verbatim-string*|Indicates the original source code line and file name, for debugging. This feature is provided for tools that generate F# source code.|
-|`#nowarn` *warningcode*|Disables a compiler warning or warnings. To disable multiple warning numbers on the same line, separate each string by a space. <br/> For example: `#nowarn 9 42`|
-
-The effect of disabling a warning applies to the entire file, including portions of the file that precede the directive.|
+|`#nowarn` *warningcodes*|Disables one or more compiler warnings as specified by *warningcodes* (see below).|
+|`#warnon` *warningcodes*|Enables one or more compiler warnings as specified by *warningcodes* (see below).|
 
 ## Conditional Compilation Directives
 
 Code that is deactivated by one of these directives appears dimmed in the Visual Studio Code Editor.
 
-> [!NOTE]
-> The behavior of the conditional compilation directives is not the same as it is in other languages. For example, you cannot use Boolean expressions involving symbols, and `true` and `false` have no special meaning. Symbols that you use in the `if` directive must be defined by the command line or in the project settings; there is no `define` preprocessor directive.
-
 The following code illustrates the use of the `#if`, `#else`, and `#endif` directives. In this example, the code contains two versions of the definition of `function1`. When `VERSION1` is defined by using the [-define compiler option](./compiler-options.md), the code between the `#if` directive and the `#else` directive is activated. Otherwise, the code between `#else` and `#endif` is activated.
 
 [!code-fsharp[Main](~/samples/snippets/fsharp/lang-ref-2/snippet7301.fs)]
 
-There is no `#define` preprocessor directive in F#. You must use the compiler option or project settings to define the symbols used by the `#if` directive.
-
-Conditional compilation directives can be nested. Indentation is not significant for preprocessor directives.
-
-You can also negate a symbol with `!`. In this example, a string's value is something only when _not_ debugging:
+The `#if` directive also accepts logical expressions:
 
 ```fsharp
-#if !DEBUG
-let str = "Not debugging!"
-#else
-let str = "Debugging!"
+#if SILVERLIGHT || COMPILED && (NETCOREFX || !DEBUG)
 #endif
 ```
 
+The following expressions can be used.
+
+| if-expr | evaluation |
+| --- | --- |
+| `if-expr1 \|\| if-expr2` | `defined` if `if-expr1` or `if-expr2` is `defined`. |
+| `if-expr1 && if-expr2` | `defined` if `if-expr1` and `if-expr2` are `defined`. |
+| `!if-expr1` | `defined` if `if-expr1` is not `defined`. |
+| `( if-expr1 )` | defined if `if-expr1` is defined. |
+| `symbol` | `defined` if it is flagged as defined by the `-define` compiler option. |
+
+The logical operators have the usual logical precedence.
+
+There is no `#define` compiler directive in F#. You must use the compiler option or project settings to define the symbols used by the `#if` directive.
+
+Conditional compilation directives can be nested. Indentation is not significant for compiler directives.
+
 ## NULLABLE directive
 
 Starting with F# 9, you can enable nullable reference types in the project:
@@ -84,6 +85,42 @@ When you use the `#line` directive, file names must be enclosed in quotation mar
 
 These tokens indicate that the F# code generated at this location is derived from some constructs at or near line `25` in `Script1`.
 
+Note that `#line` directives do not influence the behavior of `#nowarn` / `#warnon`. These two directives always relate the the file that is being compiled.
+
+## Warn Directives
+
+Warn directives disable or enable specified compiler warnings for parts of a source file.
+
+A warn directive is a single line of source code that consists of
+
+- Optional leading whitespace
+- The string `#nowarn` or `#warnon`
+- Whitespace
+- One or more *warningcodes* (see below), separated by whitespace
+- Optional whitespace
+- Optional line comment
+
+A *warningcode* is a sequence of digits (representing the warning number), optionally preceded by `FS`, optionally surrounded by double quotes.
+
+A `#nowarn` directive disables a warning until a `#warnon` directive for the same warning number is found, or else until end of file. Similarly, a `#nowarn` directive disables a warning until a `#warnon` directive for the same warning numberis found, or else until end of file. Before and after such pairs, the compilation default applies, which is
+
+- no warning if disabled by a --nowarn compiler option (or the respective msbuild property)
+- no warning for [opt-in warnings](./compiler-options.md#opt-in-warnings), unless enabled by the --warnon compiler option (or the respective msbuild property)
+
+Here is a (contrived) example.
+
+```
+module A
+match None with None -> ()     // warning
+let x =
+    #nowarn 25
+    match None with None -> 1  // no warning
+    #warnon FS25
+match None with None -> ()     // warning
+#nowarn "FS25" FS007 "42"
+match None with None -> ()     // no warning
+```
+
 ## See also
 
 - [F# Language Reference](index.md)