diff --git a/commands.md b/commands.md
index 6020150c..ea73d56f 100644
--- a/commands.md
+++ b/commands.md
@@ -237,6 +237,14 @@ declare function dedentToRoot(doc: Doc): Doc;
 
 This will dedent the current indentation to the root marked by `markAsRoot`.
 
+### trim
+
+```ts
+declare var trim: Doc;
+```
+
+This will trim any whitespace or tab character on the current line. This is used for preprocessor directives.
+
 ### cursor
 
 ```ts
diff --git a/src/doc/doc-builders.js b/src/doc/doc-builders.js
index 08acf143..6965fc58 100644
--- a/src/doc/doc-builders.js
+++ b/src/doc/doc-builders.js
@@ -113,6 +113,7 @@ function lineSuffix(contents) {
 
 const lineSuffixBoundary = { type: "line-suffix-boundary" };
 const breakParent = { type: "break-parent" };
+const trim = { type: "trim" };
 const line = { type: "line" };
 const softline = { type: "line", soft: true };
 const hardline = concat([{ type: "line", hard: true }, breakParent]);
@@ -167,6 +168,7 @@ module.exports = {
   cursor,
   breakParent,
   ifBreak,
+  trim,
   indent,
   align,
   addAlignmentToDoc,
diff --git a/src/doc/doc-debug.js b/src/doc/doc-debug.js
index 0c9875b6..51d57e61 100644
--- a/src/doc/doc-debug.js
+++ b/src/doc/doc-debug.js
@@ -59,6 +59,10 @@ function printDoc(doc) {
     return "breakParent";
   }
 
+  if (doc.type === "trim") {
+    return "trim";
+  }
+
   if (doc.type === "concat") {
     return "[" + doc.parts.map(printDoc).join(", ") + "]";
   }
diff --git a/src/doc/doc-printer.js b/src/doc/doc-printer.js
index 0ed9b0f8..5d2e511b 100644
--- a/src/doc/doc-printer.js
+++ b/src/doc/doc-printer.js
@@ -109,9 +109,37 @@ function generateInd(ind, newPart, options) {
   }
 }
 
+function trim(out) {
+  if (out.length === 0) {
+    return 0;
+  }
+
+  let trimCount = 0;
+
+  // Trim whitespace at the end of line
+  while (
+    out.length > 0 &&
+    typeof out[out.length - 1] === "string" &&
+    out[out.length - 1].match(/^[ \t]*$/)
+  ) {
+    trimCount += out.pop().length;
+  }
+
+  if (out.length && typeof out[out.length - 1] === "string") {
+    const trimmed = out[out.length - 1].replace(/[ \t]*$/, "");
+    trimCount += out[out.length - 1].length - trimmed.length;
+    out[out.length - 1] = trimmed;
+  }
+
+  return trimCount;
+}
+
 function fits(next, restCommands, width, options, mustBeFlat) {
   let restIdx = restCommands.length;
   const cmds = [next];
+  // `out` is only used for width counting because `trim` requires to look
+  // backwards for space characters.
+  const out = [];
   while (width >= 0) {
     if (cmds.length === 0) {
       if (restIdx === 0) {
@@ -130,6 +158,8 @@ function fits(next, restCommands, width, options, mustBeFlat) {
     const doc = x[2];
 
     if (typeof doc === "string") {
+      out.push(doc);
+
       width -= getStringWidth(doc);
     } else {
       switch (doc.type) {
@@ -146,6 +176,10 @@ function fits(next, restCommands, width, options, mustBeFlat) {
         case "align":
           cmds.push([makeAlign(ind, doc.n, options), mode, doc.contents]);
 
+          break;
+        case "trim":
+          width += trim(out);
+
           break;
         case "group":
           if (mustBeFlat && doc.break) {
@@ -184,6 +218,8 @@ function fits(next, restCommands, width, options, mustBeFlat) {
             case MODE_FLAT:
               if (!doc.hard) {
                 if (!doc.soft) {
+                  out.push(" ");
+
                   width -= 1;
                 }
 
@@ -244,6 +280,10 @@ function printDocToString(doc, options) {
         case "align":
           cmds.push([makeAlign(ind, doc.n, options), mode, doc.contents]);
 
+          break;
+        case "trim":
+          pos -= trim(out);
+
           break;
         case "group":
           switch (mode) {
@@ -471,24 +511,7 @@ function printDocToString(doc, options) {
                   pos = 0;
                 }
               } else {
-                if (out.length > 0) {
-                  // Trim whitespace at the end of line
-                  while (
-                    out.length > 0 &&
-                    typeof out[out.length - 1] === "string" &&
-                    out[out.length - 1].match(/^[ \t]*$/)
-                  ) {
-                    out.pop();
-                  }
-
-                  if (out.length && typeof out[out.length - 1] === "string") {
-                    out[out.length - 1] = out[out.length - 1].replace(
-                      /[ \t]*$/,
-                      ""
-                    );
-                  }
-                }
-
+                pos -= trim(out);
                 out.push(newLine + ind.value);
                 pos = ind.length;
               }
diff --git a/tests_integration/__tests__/doc-mark-as-root.js b/tests_integration/__tests__/doc-mark-as-root.js
new file mode 100644
index 00000000..7b1dbc78
--- /dev/null
+++ b/tests_integration/__tests__/doc-mark-as-root.js
@@ -0,0 +1,38 @@
+"use strict";
+
+const prettier = require("prettier/local");
+const docPrinter = prettier.doc.printer;
+const docBuilders = prettier.doc.builders;
+
+const printDocToString = docPrinter.printDocToString;
+const concat = docBuilders.concat;
+const hardline = docBuilders.hardline;
+const literalline = docBuilders.literalline;
+const trim = docBuilders.trim;
+const indent = docBuilders.indent;
+const markAsRoot = docBuilders.markAsRoot;
+
+describe("markAsRoot", () => {
+  test.each([
+    [
+      "with hardline will insert a newline with current indentation",
+      concat([indent(markAsRoot(indent(hardline))), "123"]),
+      "\n    123"
+    ],
+    [
+      "with literalline will insert a newline with root indentation",
+      concat([indent(markAsRoot(indent(literalline))), "123"]),
+      "\n  123"
+    ],
+    [
+      "followed by trim will trims up to the the first column, ignoring indented root",
+      concat([indent(markAsRoot(indent(literalline))), trim, "123"]),
+      "\n123"
+    ]
+  ])("%s", (_, doc, expected) => {
+    const result = printDocToString(doc, { printWidth: 80, tabWidth: 2 });
+
+    expect(result).toBeDefined();
+    expect(result.formatted).toEqual(expected);
+  });
+});
diff --git a/tests_integration/__tests__/doc-trim.js b/tests_integration/__tests__/doc-trim.js
new file mode 100644
index 00000000..aa5fdeb8
--- /dev/null
+++ b/tests_integration/__tests__/doc-trim.js
@@ -0,0 +1,59 @@
+"use strict";
+
+const prettier = require("prettier/local");
+const docPrinter = prettier.doc.printer;
+const docBuilders = prettier.doc.builders;
+
+const printDocToString = docPrinter.printDocToString;
+const concat = docBuilders.concat;
+const line = docBuilders.line;
+const trim = docBuilders.trim;
+const group = docBuilders.group;
+const indent = docBuilders.indent;
+
+// These tests don't use `runPrettier` because `trim` is not used by any
+// bundled parser (only third-party plugins).
+
+describe("trim", () => {
+  test.each([
+    ["trims the current line", group(concat(["hello    ", trim])), "hello"],
+    [
+      "trims existing indentation",
+      group(
+        concat([
+          "function()",
+          line,
+          "{",
+          indent(
+            concat([
+              line,
+              group(concat([trim, "#if DEBUG"])),
+              line,
+              "alert(42);",
+              line,
+              group(concat([trim, "#endif"]))
+            ])
+          ),
+          line,
+          "}"
+        ])
+      ),
+      `function()
+{
+#if DEBUG
+  alert(42);
+#endif
+}`
+    ],
+    [
+      "ignores trimmed characters when fitting the line",
+      group(concat(["hello  ", "  ", trim, line, "world!"])),
+      "hello world!"
+    ]
+  ])("%s", (_, doc, expected) => {
+    const result = printDocToString(doc, { printWidth: 12, tabWidth: 2 });
+
+    expect(result).toBeDefined();
+    expect(result.formatted).toEqual(expected);
+  });
+});