C++ (Qt)
label->setText( tr( "Text <b>Bold text</b> <font color=red>Red text</font>" ) );

C++ (Qt)
#include <QtGui/QTextLayout>
#include <QtGui/private/qlineedit_p.h>
#include <QtGui/QLineEdit>
...
class myEdit : public QLineEdit
{
        Q_OBJECT
public:
        myEdit(QWidget *parent) : QLineEdit(parent)
        {
        }
        ~myEdit(){}
        using QObject::d_ptr;
};
...
        myEdit *edit = new myEdit(this);
        edit->setText("test");
        edit->show();
        QLineEditPrivate *pedit = (QLineEditPrivate *)edit->d_ptr;
        QTextLayout &lay = pedit->textLayout;
        qDebug() << lay.text();
 

C++ (Qt)
// hardcoded in qlineedit.cpp
#define verticalMargin 1
#define horizontalMargin 2
 
/*!\reimp
*/
void MyLineEdit::paintEvent(QPaintEvent *)
{
    Q_D(QLineEdit);
    QPainter p(this);
 
    QRect r = rect();
    QPalette pal = palette();
 
    QStyleOptionFrameV2 panel;
    initStyleOption(&panel);
    style()->drawPrimitive(QStyle::PE_PanelLineEdit, &panel, &p, this);
    r = style()->subElementRect(QStyle::SE_LineEditContents, &panel, this);
#if QT_VERSION >= 0x040500
    int left, top, right, bottom;
    getTextMargins(&left, &top, &right, &bottom);
    r.adjust(left, top, -right, -bottom);
#endif
    p.setClipRect(r);
 
    QFontMetrics fm = fontMetrics();
    Qt::Alignment va = QStyle::visualAlignment(layoutDirection(), QFlag(d->alignment));
    switch (va & Qt::AlignVertical_Mask) {
     case Qt::AlignBottom:
         d->vscroll = r.y() + r.height() - fm.height() - verticalMargin;
         break;
     case Qt::AlignTop:
         d->vscroll = r.y() + verticalMargin;
         break;
     default:
         //center
         d->vscroll = r.y() + (r.height() - fm.height() + 1) / 2;
         break;
    }
    QRect lineRect(r.x() + horizontalMargin, d->vscroll, r.width() - 2*horizontalMargin, fm.height());
    QTextLine line = d->textLayout.lineAt(0);
 
    int cursor = d->cursor;
    if (d->preeditCursor != -1)
        cursor += d->preeditCursor;
    // locate cursor position
    int cix = qRound(line.cursorToX(cursor));
 
    // horizontal scrolling. d->hscroll is the left indent from the beginning
    // of the text line to the left edge of lineRect. we update this value
    // depending on the delta from the last paint event; in effect this means
    // the below code handles all scrolling based on the textline (widthUsed,
    // minLB, minRB), the line edit rect (lineRect) and the cursor position
    // (cix).
    int minLB = qMax(0, -fm.minLeftBearing());
    int minRB = qMax(0, -fm.minRightBearing());
    int widthUsed = qRound(line.naturalTextWidth()) + 1 + minRB;
    if ((minLB + widthUsed) <=  lineRect.width()) {
        // text fits in lineRect; use hscroll for alignment
        switch (va & ~(Qt::AlignAbsolute|Qt::AlignVertical_Mask)) {
        case Qt::AlignRight:
            d->hscroll = widthUsed - lineRect.width() + 1;
            break;
        case Qt::AlignHCenter:
            d->hscroll = (widthUsed - lineRect.width()) / 2;
            break;
        default:
            // Left
            d->hscroll = 0;
            break;
        }
        d->hscroll -= minLB;
    } else if (cix - d->hscroll >= lineRect.width()) {
        // text doesn't fit, cursor is to the right of lineRect (scroll right)
        d->hscroll = cix - lineRect.width() + 1;
    } else if (cix - d->hscroll < 0 && d->hscroll < widthUsed) {
        // text doesn't fit, cursor is to the left of lineRect (scroll left)
        d->hscroll = cix;
    } else if (widthUsed - d->hscroll < lineRect.width()) {
        // text doesn't fit, text document is to the left of lineRect; align
        // right
        d->hscroll = widthUsed - lineRect.width() + 1;
    }
    // the y offset is there to keep the baseline constant in case we have script changes in the text.
    QPoint topLeft = lineRect.topLeft() - QPoint(d->hscroll, d->ascent - fm.ascent());
 
    // draw text, selections and cursors
#ifndef QT_NO_STYLE_STYLESHEET
    if (QStyleSheetStyle* cssStyle = qobject_cast<QStyleSheetStyle*>(style())) {
        cssStyle->focusPalette(this, &panel, &pal);
    }
#endif
    p.setPen(pal.text().color());
 
    QVector<QTextLayout::FormatRange> selections;
#ifdef QT_KEYPAD_NAVIGATION
    if (!QApplication::keypadNavigationEnabled() || hasEditFocus())
#endif
    if (d->selstart < d->selend || (d->cursorVisible && d->maskData && !d->readOnly)) {
        QTextLayout::FormatRange o;
        if (d->selstart < d->selend) {
            o.start = d->selstart;
            o.length = d->selend - d->selstart;
            o.format.setBackground(pal.brush(QPalette::Highlight));
            o.format.setForeground(pal.brush(QPalette::HighlightedText));
        } else {
            // mask selection
            o.start = d->cursor;
            o.length = 1;
            o.format.setBackground(pal.brush(QPalette::Text));
            o.format.setForeground(pal.brush(QPalette::Window));
        }
        selections.append(o);
    }
 
/*+*/    if (/* условия для выделения блока текста жирным */) {
/*+*/        QTextLayout::FormatRange o;
/*+*/        o.start = 3; // начало блока (первый символ)
/*+*/        o.length = 4; // длина блока в символах
/*+*/        o.format.setFontWeight(QFont::Bold);
/*+*/        selections.append(o);
/*+*/    }
 
    // Asian users see an IM selection text as cursor on candidate
    // selection phase of input method, so the ordinary cursor should be
    // invisible if we have a preedit string.
    d->textLayout.draw(&p, topLeft, selections, r);
    if (d->cursorVisible && !d->readOnly && !d->hideCursor)
        d->textLayout.drawCursor(&p, topLeft, cursor, style()->pixelMetric(QStyle::PM_TextCursorWidth));
}