diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 89814785e..850c5bae6 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -179,8 +179,8 @@ public:
     Expr * parseExprFromFile(const Path & path, StaticEnv & staticEnv);
 
     /* Parse a Nix expression from the specified string. */
-    Expr * parseExprFromString(std::string_view s, const Path & basePath, StaticEnv & staticEnv);
-    Expr * parseExprFromString(std::string_view s, const Path & basePath);
+    Expr * parseExprFromString(std::string s, const Path & basePath, StaticEnv & staticEnv);
+    Expr * parseExprFromString(std::string s, const Path & basePath);
 
     Expr * parseStdin();
 
@@ -308,7 +308,7 @@ private:
     friend struct ExprAttrs;
     friend struct ExprLet;
 
-    Expr * parse(const char * text, FileOrigin origin, const Path & path,
+    Expr * parse(char * text, size_t length, FileOrigin origin, const Path & path,
         const Path & basePath, StaticEnv & staticEnv);
 
 public:
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index c18877e29..a0e7a1877 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -64,29 +64,32 @@ static void adjustLoc(YYLTYPE * loc, const char * s, size_t len)
 }
 
 
-// FIXME: optimize
-static Expr * unescapeStr(SymbolTable & symbols, const char * s, size_t length)
+// we make use of the fact that the parser receives a private copy of the input
+// string and can munge around in it.
+static Expr * unescapeStr(SymbolTable & symbols, char * s, size_t length)
 {
-    string t;
-    t.reserve(length);
+    char * result = s;
+    char * t = s;
     char c;
+    // the input string is terminated with *two* NULs, so we can safely take
+    // *one* character after the one being checked against.
     while ((c = *s++)) {
         if (c == '\\') {
-            assert(*s);
             c = *s++;
-            if (c == 'n') t += '\n';
-            else if (c == 'r') t += '\r';
-            else if (c == 't') t += '\t';
-            else t += c;
+            if (c == 'n') *t = '\n';
+            else if (c == 'r') *t = '\r';
+            else if (c == 't') *t = '\t';
+            else *t = c;
         }
         else if (c == '\r') {
             /* Normalise CR and CR/LF into LF. */
-            t += '\n';
+            *t = '\n';
             if (*s == '\n') s++; /* cr/lf */
         }
-        else t += c;
+        else *t = c;
+        t++;
     }
-    return new ExprString(symbols.create(t));
+    return new ExprString(symbols.create({result, size_t(t - result)}));
 }
 
 
@@ -139,7 +142,7 @@ or          { return OR_KW; }
 \/\/        { return UPDATE; }
 \+\+        { return CONCAT; }
 
-{ID}        { yylval->id = strdup(yytext); return ID; }
+{ID}        { yylval->id = {yytext, (size_t) yyleng}; return ID; }
 {INT}       { errno = 0;
               try {
                   yylval->n = boost::lexical_cast<int64_t>(yytext);
@@ -221,14 +224,14 @@ or          { return OR_KW; }
 <PATH_START>{PATH_SEG} {
   POP_STATE();
   PUSH_STATE(INPATH_SLASH);
-  yylval->path = strdup(yytext);
+  yylval->path = {yytext, (size_t) yyleng};
   return PATH;
 }
 
 <PATH_START>{HPATH_START} {
   POP_STATE();
   PUSH_STATE(INPATH_SLASH);
-  yylval->path = strdup(yytext);
+  yylval->path = {yytext, (size_t) yyleng};
   return HPATH;
 }
 
@@ -237,7 +240,7 @@ or          { return OR_KW; }
     PUSH_STATE(INPATH_SLASH);
   else
     PUSH_STATE(INPATH);
-  yylval->path = strdup(yytext);
+  yylval->path = {yytext, (size_t) yyleng};
   return PATH;
 }
 {HPATH} {
@@ -245,7 +248,7 @@ or          { return OR_KW; }
     PUSH_STATE(INPATH_SLASH);
   else
     PUSH_STATE(INPATH);
-  yylval->path = strdup(yytext);
+  yylval->path = {yytext, (size_t) yyleng};
   return HPATH;
 }
 
@@ -280,8 +283,8 @@ or          { return OR_KW; }
   throw ParseError("path has a trailing slash");
 }
 
-{SPATH}     { yylval->path = strdup(yytext); return SPATH; }
-{URI}       { yylval->uri = strdup(yytext); return URI; }
+{SPATH}     { yylval->path = {yytext, (size_t) yyleng}; return SPATH; }
+{URI}       { yylval->uri = {yytext, (size_t) yyleng}; return URI; }
 
 [ \t\r\n]+    /* eat up whitespace */
 \#[^\r\n]*    /* single-line comments */
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index a75357871..640c44c01 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -473,7 +473,7 @@ string ExprLambda::showNamePos() const
 size_t SymbolTable::totalSize() const
 {
     size_t n = 0;
-    for (auto & i : symbols)
+    for (auto & i : store)
         n += i.size();
     return n;
 }
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index f8aaea582..a3e713937 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -273,9 +273,16 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, ParseData * data, const char * err
   nix::Formal * formal;
   nix::NixInt n;
   nix::NixFloat nf;
-  const char * id; // !!! -> Symbol
-  char * path;
-  char * uri;
+  // using C a struct allows us to avoid having to define the special
+  // members that using string_view here would implicitly delete.
+  struct StringToken {
+    const char * p;
+    size_t l;
+    operator std::string_view() const { return {p, l}; }
+  };
+  StringToken id; // !!! -> Symbol
+  StringToken path;
+  StringToken uri;
   std::vector<nix::AttrName> * attrNames;
   std::vector<std::pair<nix::Pos, nix::Expr *> > * string_parts;
 }
@@ -397,7 +404,7 @@ expr_select
 
 expr_simple
   : ID {
-      if (strcmp($1, "__curPos") == 0)
+      if (strncmp($1.p, "__curPos", $1.l) == 0)
           $$ = new ExprPos(CUR_POS);
       else
           $$ = new ExprVar(CUR_POS, data->symbols.create($1));
@@ -414,7 +421,7 @@ expr_simple
       $$ = new ExprConcatStrings(CUR_POS, false, $2);
   }
   | SPATH {
-      string path($1 + 1, strlen($1) - 2);
+      string path($1.p + 1, $1.l - 2);
       $$ = new ExprCall(CUR_POS,
           new ExprVar(data->symbols.create("__findFile")),
           {new ExprVar(data->symbols.create("__nixPath")),
@@ -460,14 +467,14 @@ string_parts_interpolated
 
 path_start
   : PATH {
-    Path path(absPath($1, data->basePath));
+    Path path(absPath({$1.p, $1.l}, data->basePath));
     /* add back in the trailing '/' to the first segment */
-    if ($1[strlen($1)-1] == '/' && strlen($1) > 1)
+    if ($1.p[$1.l-1] == '/' && $1.l > 1)
       path += "/";
     $$ = new ExprPath(path);
   }
   | HPATH {
-    Path path(getHome() + string($1 + 1));
+    Path path(getHome() + string($1.p + 1, $1.l - 1));
     $$ = new ExprPath(path);
   }
   ;
@@ -543,7 +550,7 @@ attrpath
 
 attr
   : ID { $$ = $1; }
-  | OR_KW { $$ = "or"; }
+  | OR_KW { $$ = {"or", 2}; }
   ;
 
 string_attr
@@ -589,7 +596,7 @@ formal
 namespace nix {
 
 
-Expr * EvalState::parse(const char * text, FileOrigin origin,
+Expr * EvalState::parse(char * text, size_t length, FileOrigin origin,
     const Path & path, const Path & basePath, StaticEnv & staticEnv)
 {
     yyscan_t scanner;
@@ -609,7 +616,7 @@ Expr * EvalState::parse(const char * text, FileOrigin origin,
     data.basePath = basePath;
 
     yylex_init(&scanner);
-    yy_scan_string(text, scanner);
+    yy_scan_buffer(text, length, scanner);
     int res = yyparse(scanner, &data);
     yylex_destroy(scanner);
 
@@ -655,26 +662,33 @@ Expr * EvalState::parseExprFromFile(const Path & path)
 
 Expr * EvalState::parseExprFromFile(const Path & path, StaticEnv & staticEnv)
 {
-    return parse(readFile(path).c_str(), foFile, path, dirOf(path), staticEnv);
+    auto buffer = readFile(path);
+    // readFile should have left some extra space for terminators
+    buffer.append("\0\0", 2);
+    return parse(buffer.data(), buffer.size(), foFile, path, dirOf(path), staticEnv);
 }
 
 
-Expr * EvalState::parseExprFromString(std::string_view s, const Path & basePath, StaticEnv & staticEnv)
+Expr * EvalState::parseExprFromString(std::string s, const Path & basePath, StaticEnv & staticEnv)
 {
-    return parse(s.data(), foString, "", basePath, staticEnv);
+    s.append("\0\0", 2);
+    return parse(s.data(), s.size(), foString, "", basePath, staticEnv);
 }
 
 
-Expr * EvalState::parseExprFromString(std::string_view s, const Path & basePath)
+Expr * EvalState::parseExprFromString(std::string s, const Path & basePath)
 {
-    return parseExprFromString(s, basePath, staticBaseEnv);
+    return parseExprFromString(std::move(s), basePath, staticBaseEnv);
 }
 
 
 Expr * EvalState::parseStdin()
 {
     //Activity act(*logger, lvlTalkative, format("parsing standard input"));
-    return parse(drainFD(0).data(), foStdin, "", absPath("."), staticBaseEnv);
+    auto buffer = drainFD(0);
+    // drainFD should have left some extra space for terminators
+    buffer.append("\0\0", 2);
+    return parse(buffer.data(), buffer.size(), foStdin, "", absPath("."), staticBaseEnv);
 }
 
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 1d2a7d5d2..b819918ad 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -378,7 +378,7 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     auto output = runProgram(program, true, commandArgs);
     Expr * parsed;
     try {
-        parsed = state.parseExprFromString(output, pos.file);
+        parsed = state.parseExprFromString(std::move(output), pos.file);
     } catch (Error & e) {
         e.addTrace(pos, "While parsing the output from '%1%'", program);
         throw;
@@ -3915,9 +3915,12 @@ void EvalState::createBaseEnv()
 
     /* Note: we have to initialize the 'derivation' constant *after*
        building baseEnv/staticBaseEnv because it uses 'builtins'. */
-    eval(parse(
+    char code[] =
         #include "primops/derivation.nix.gen.hh"
-        , foFile, sDerivationNix, "/", staticBaseEnv), *vDerivation);
+        // the parser needs two NUL bytes as terminators; one of them
+        // is implied by being a C string.
+        "\0";
+    eval(parse(code, sizeof(code), foFile, sDerivationNix, "/", staticBaseEnv), *vDerivation);
 }
 
 
diff --git a/src/libexpr/symbol-table.hh b/src/libexpr/symbol-table.hh
index 4eb6dac81..a090ebae5 100644
--- a/src/libexpr/symbol-table.hh
+++ b/src/libexpr/symbol-table.hh
@@ -1,7 +1,8 @@
 #pragma once
 
+#include <list>
 #include <map>
-#include <unordered_set>
+#include <unordered_map>
 
 #include "types.hh"
 
@@ -70,15 +71,21 @@ public:
 class SymbolTable
 {
 private:
-    typedef std::unordered_set<string> Symbols;
-    Symbols symbols;
+    std::unordered_map<std::string_view, Symbol> symbols;
+    std::list<string> store;
 
 public:
     Symbol create(std::string_view s)
     {
-        // FIXME: avoid allocation if 's' already exists in the symbol table.
-        std::pair<Symbols::iterator, bool> res = symbols.emplace(std::string(s));
-        return Symbol(&*res.first);
+        // Most symbols are looked up more than once, so we trade off insertion performance
+        // for lookup performance.
+        // TODO: could probably be done more efficiently with transparent Hash and Equals
+        // on the original implementation using unordered_set
+        auto it = symbols.find(s);
+        if (it != symbols.end()) return it->second;
+
+        const string & rawSym = store.emplace_back(s);
+        return symbols.emplace(rawSym, Symbol(&rawSym)).first->second;
     }
 
     size_t size() const
@@ -91,7 +98,7 @@ public:
     template<typename T>
     void dump(T callback)
     {
-        for (auto & s : symbols)
+        for (auto & s : store)
             callback(s);
     }
 };
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 9edd69c64..938808276 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -668,7 +668,9 @@ void writeFull(int fd, std::string_view s, bool allowInterrupts)
 
 string drainFD(int fd, bool block, const size_t reserveSize)
 {
-    StringSink sink(reserveSize);
+    // the parser needs two extra bytes to append terminating characters, other users will
+    // not care very much about the extra memory.
+    StringSink sink(reserveSize + 2);
     drainFD(fd, sink, block);
     return std::move(*sink.s);
 }
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index b73eb0795..471a03ac7 100755
--- a/src/nix-build/nix-build.cc
+++ b/src/nix-build/nix-build.cc
@@ -292,7 +292,7 @@ static void main_nix_build(int argc, char * * argv)
     else
         for (auto i : left) {
             if (fromArgs)
-                exprs.push_back(state->parseExprFromString(i, absPath(".")));
+                exprs.push_back(state->parseExprFromString(std::move(i), absPath(".")));
             else {
                 auto absolute = i;
                 try {
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index a62ad66c2..be9f96836 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -703,7 +703,7 @@ void NixRepl::addVarToScope(const Symbol & name, Value & v)
 
 Expr * NixRepl::parseString(string s)
 {
-    Expr * e = state->parseExprFromString(s, curDir, staticEnv);
+    Expr * e = state->parseExprFromString(std::move(s), curDir, staticEnv);
     return e;
 }