nix repl: Render docs for attributes

src/libexpr/eval.cc

@@ -1415,6 +1415,22 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     v = *vAttrs;
 }
 
+Symbol ExprSelect::evalExceptFinalSelect(EvalState & state, Env & env, Value & attrs)
+{
+    Value vTmp;
+    Symbol name = getName(attrPath[attrPath.size() - 1], state, env);
+
+    if (attrPath.size() == 1) {
+        e->eval(state, env, vTmp);
+    } else {
+        ExprSelect init(*this);
+        init.attrPath.pop_back();
+        init.eval(state, env, vTmp);
+    }
+    attrs = vTmp;
+    return name;
+}
+
 
 void ExprOpHasAttr::eval(EvalState & state, Env & env, Value & v)
 {
@@ -2876,13 +2892,37 @@ Expr * EvalState::parse(
     const SourcePath & basePath,
     std::shared_ptr<StaticEnv> & staticEnv)
 {
-    auto result = parseExprFromBuf(text, length, origin, basePath, symbols, settings, positions, rootFS, exprSymbols);
+    DocCommentMap tmpDocComments; // Only used when not origin is not a SourcePath
+    DocCommentMap *docComments = &tmpDocComments;
+
+    if (auto sourcePath = std::get_if<SourcePath>(&origin)) {
+        auto [it, _] = positionToDocComment.try_emplace(*sourcePath);
+        docComments = &it->second;
+    }
+
+    auto result = parseExprFromBuf(text, length, origin, basePath, symbols, settings, positions, *docComments, rootFS, exprSymbols);
 
     result->bindVars(*this, staticEnv);
 
     return result;
 }
 
+DocComment EvalState::getDocCommentForPos(PosIdx pos)
+{
+    auto pos2 = positions[pos];
+    auto path = pos2.getSourcePath();
+    if (!path)
+        return {};
+
+    auto table = positionToDocComment.find(*path);
+    if (table == positionToDocComment.end())
+        return {};
+
+    auto it = table->second.find(pos);
+    if (it == table->second.end())
+        return {};
+    return it->second;
+}
 
 std::string ExternalValueBase::coerceToString(EvalState & state, const PosIdx & pos, NixStringContext & context, bool copyMore, bool copyToStore) const
 {

src/libexpr/eval.hh

@@ -130,6 +130,8 @@ struct Constant
     typedef std::map<std::string, Value *> ValMap;
 #endif
 
+typedef std::map<PosIdx, DocComment> DocCommentMap;
+
 struct Env
 {
     Env * up;
@@ -329,6 +331,12 @@ private:
 #endif
     FileEvalCache fileEvalCache;
 
+    /**
+     * Associate source positions of certain AST nodes with their preceding doc comment, if they have one.
+     * Grouped by file.
+     */
+    std::map<SourcePath, DocCommentMap> positionToDocComment;
+
     LookupPath lookupPath;
 
     std::map<std::string, std::optional<std::string>> lookupPathResolved;
@@ -771,6 +779,8 @@ public:
         std::string_view pathArg,
         PosIdx pos);
 
+    DocComment getDocCommentForPos(PosIdx pos);
+
 private:
 
     /**

src/libexpr/nixexpr.hh

@@ -202,6 +202,17 @@ struct ExprSelect : Expr
     ExprSelect(const PosIdx & pos, Expr * e, AttrPath attrPath, Expr * def) : pos(pos), e(e), def(def), attrPath(std::move(attrPath)) { };
     ExprSelect(const PosIdx & pos, Expr * e, Symbol name) : pos(pos), e(e), def(0) { attrPath.push_back(AttrName(name)); };
     PosIdx getPos() const override { return pos; }
+
+    /**
+     * Evaluate the `a.b.c` part of `a.b.c.d`. This exists mostly for the purpose of :doc in the repl.
+     *
+     * @param[out] v The attribute set that should contain the last attribute name (if it exists).
+     * @return The last attribute name in `attrPath`
+     * 
+     * @note This does *not* evaluate the final attribute, and does not fail if that's the only attribute that does not exist.
+     */
+    Symbol evalExceptFinalSelect(EvalState & state, Env & env, Value & attrs);
+
     COMMON_METHODS
 };
 

src/libexpr/parser-state.hh

@@ -64,7 +64,7 @@ struct LexerState
     /**
      * @brief Maps some positions to a DocComment, where the comment is relevant to the location.
      */
-    std::map<PosIdx, DocComment> positionToDocComment;
+    std::map<PosIdx, DocComment> & positionToDocComment;
 
     PosTable & positions;
     PosTable::Origin origin;

src/libexpr/parser.y

@@ -33,6 +33,8 @@
 
 namespace nix {
 
+typedef std::map<PosIdx, DocComment> DocCommentMap;
+
 Expr * parseExprFromBuf(
     char * text,
     size_t length,
@@ -41,6 +43,7 @@ Expr * parseExprFromBuf(
     SymbolTable & symbols,
     const EvalSettings & settings,
     PosTable & positions,
+    DocCommentMap & docComments,
     const ref<SourceAccessor> rootFS,
     const Expr::AstSymbols & astSymbols);
 
@@ -335,10 +338,12 @@ binds
       $$ = $1;
 
       auto pos = state->at(@2);
+      auto exprPos = state->at(@4);
       {
         auto it = state->lexerState.positionToDocComment.find(pos);
         if (it != state->lexerState.positionToDocComment.end()) {
           $4->setDocComment(it->second);
+          state->lexerState.positionToDocComment.emplace(exprPos, it->second);
         }
       }
 
@@ -463,11 +468,13 @@ Expr * parseExprFromBuf(
     SymbolTable & symbols,
     const EvalSettings & settings,
     PosTable & positions,
+    DocCommentMap & docComments,
     const ref<SourceAccessor> rootFS,
     const Expr::AstSymbols & astSymbols)
 {
     yyscan_t scanner;
     LexerState lexerState {
+        .positionToDocComment = docComments,
         .positions = positions,
         .origin = positions.addOrigin(origin, length),
     };