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A generic out-of-sample error estimate is proposed for robust $M$-estimators regularized with a convex penalty in high-dimensional linear regression where $(X,y)$ is observed and $p,n$ are of the same order. If $ψ$ is the derivative of the robust data-fitting loss $ρ$, the estimate depends on the observed data only through the quantities $\hatψ= ψ(y-X\hatβ)$, $X^\top \hatψ$ and the derivatives $(\partial/\partial y) \hatψ$ and $(\partial/\partial y) X\hatβ$ for fixed $X$. The out-of-sample error estimate enjoys a relative error of order $n^{-1/2}$ in a linear model with Gaussian covariates and independent noise, either non-asymptotically when $p/n\le γ$ or asymptotically in the high-dimensional asymptotic regime $p/n\toγ'\in(0,\infty)$. General differentiable loss functions $ρ$ are allowed provided that $ψ=ρ'$ is 1-Lipschitz. The validity of the out-of-sample error estimate holds either under a strong convexity assumption, or for the $\ell_1$-penalized Huber M-estimator if the number of corrupted observations and sparsity of the true $β$ are bounded from above by $s_*n$ for some small enough constant $s_*\in(0,1)$ independent of $n,p$. For the square loss and in the absence of corruption in the response, the results additionally yield $n^{-1/2}$-consistent estimates of the noise variance and of the generalization error. This generalizes, to arbitrary convex penalty, estimates that were previously known for the Lasso.