Metric spaces admitting only trivial weak contractions

If $(X,d)$ is a metric space then the map $f\text{\hspace{0.17em}⁣}:X\to X$ is defined to be a weak contraction if $d(f(x),f(y))<d(x,y)$ for all $x,y\in X$, $x\mathrm{\ne }y$. We determine the simplest non-closed sets $X\subseteq {\mathbb{R}}^n$ in the sense of descriptive set theoretic complexity such that every weak contraction $f\text{\hspace{0.17em}⁣}:X\to X$ is constant. In order to do so, we prove that there exists a non-closed $F_{\sigma }$ set $F\subseteq \mathbb{R}$ such that every weak contraction $f\text{\hspace{0.17em}⁣}:F\to F$ is constant. Similarly, there exists a non-closed $G_{\delta }$ set $G\subseteq \mathbb{R}$ such that every weak contraction $f\text{\hspace{0.17em}⁣}:G\to G$ is constant. These answer questions of M. Elekes. We use measure theoretic methods, first of all the concept of generalized Hausdorff measure.