Track doc comments and render them in :doc

tests/functional/repl/doc-comment-function.expected

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+Nix <nix version>
+Type :? for help.
+Function defined at
+    /path/to/tests/functional/repl/doc-comment-function.nix:2:1
+
+    A doc comment for a file that only contains a function
+
+

tests/functional/repl/doc-comment-function.in

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+:doc import ./doc-comment-function.nix

tests/functional/repl/doc-comment-function.nix

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+/** A doc comment for a file that only contains a function */
+{ ... }:
+{ }

tests/functional/repl/doc-comments.nix

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+{
+  /**
+    Perform *arithmetic* multiplication. It's kind of like repeated **addition**, very neat.
+
+    ```nix
+    multiply 2 3
+    => 6
+    ```
+   */
+  multiply = x: y: x * y;
+
+  /**👈 precisely this wide 👉*/
+  measurement = x: x;
+
+  floatedIn = /** This also works. */
+    x: y: x;
+
+  compact=/**boom*/x: x;
+
+  /** Ignore!!! */
+  unambiguous = 
+    /** Very close */
+    x: x;
+
+  /** Firmly rigid. */
+  constant = true;
+
+  /** Immovably fixed. */
+  lib.version = "9000";
+
+  /** Unchangeably constant. */
+  lib.attr.empty = { };
+
+  lib.attr.undocumented = { };
+
+  nonStrict = /** My syntax is not strict, but I'm strict anyway. */ x: x;
+  strict = /** I don't have to be strict, but I am anyway. */ { ... }: null;
+  # Note that pre and post are the same here. I just had to name them somehow.
+  strictPre = /** Here's one way to do this */ a@{ ... }: a;
+  strictPost = /** Here's another way to do this */ { ... }@a: a;
+
+  # TODO
+
+  # /** This returns a documented function. */
+  # documentedArgs =
+  #   /** x */
+  #   x:
+  #   /** y */
+  #   y:
+  #   /** x + y */
+  #   x + y;
+
+  # /** Documented formals */
+  # documentedFormals = 
+  #   /** x */
+  #   x: x;
+}

tests/functional/repl/doc-compact.expected

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+Nix <nix version>
+Type :? for help.
+Added <number omitted> variables.
+
+Function compact
+    … defined at
+    /path/to/tests/functional/repl/doc-comments.nix:18:20
+
+    boom
+
+

tests/functional/repl/doc-compact.in

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+:l doc-comments.nix
+:doc compact

tests/functional/repl/doc-floatedIn.expected

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+Nix <nix version>
+Type :? for help.
+Added <number omitted> variables.
+
+Function floatedIn
+    … defined at
+    /path/to/tests/functional/repl/doc-comments.nix:16:5
+
+    This also works.
+
+

tests/functional/repl/doc-floatedIn.in

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+:l doc-comments.nix
+:doc floatedIn

tests/functional/repl/doc-lambda-flavors.expected

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+Nix <nix version>
+Type :? for help.
+Added <number omitted> variables.
+
+Function nonStrict
+    … defined at
+    /path/to/tests/functional/repl/doc-comments.nix:36:70
+
+    My syntax is not strict, but I'm strict anyway.
+
+Function strict
+    … defined at
+    /path/to/tests/functional/repl/doc-comments.nix:37:63
+
+    I don't have to be strict, but I am anyway.
+
+Function strictPre
+    … defined at
+    /path/to/tests/functional/repl/doc-comments.nix:39:48
+
+    Here's one way to do this
+
+Function strictPost
+    … defined at
+    /path/to/tests/functional/repl/doc-comments.nix:40:53
+
+    Here's another way to do this
+
+

tests/functional/repl/doc-lambda-flavors.in

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+:l doc-comments.nix
+:doc nonStrict
+:doc strict
+:doc strictPre
+:doc strictPost

tests/functional/repl/doc-measurement.expected

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+Nix <nix version>
+Type :? for help.
+Added <number omitted> variables.
+
+Function measurement
+    … defined at
+    /path/to/tests/functional/repl/doc-comments.nix:13:17
+
+    👈 precisely this wide 👉
+
+

tests/functional/repl/doc-measurement.in

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+:l doc-comments.nix
+:doc measurement

tests/functional/repl/doc-multiply.expected

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+Nix <nix version>
+Type :? for help.
+Added <number omitted> variables.
+
+Function multiply
+    … defined at
+    /path/to/tests/functional/repl/doc-comments.nix:10:14
+
+    Perform arithmetic multiplication. It's kind of like
+    repeated addition, very neat.
+
+      | multiply 2 3
+      | => 6
+
+

tests/functional/repl/doc-multiply.in

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+:l doc-comments.nix
+:doc multiply

tests/functional/repl/doc-unambiguous.expected

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+Nix <nix version>
+Type :? for help.
+Added <number omitted> variables.
+
+Function unambiguous
+    … defined at
+    /path/to/tests/functional/repl/doc-comments.nix:23:5
+
+    Very close
+
+

tests/functional/repl/doc-unambiguous.in

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+:l doc-comments.nix
+:doc unambiguous