A tool to find out what the earliest version that your project supports is!

rustup-bisect

#!/usr/bin/env bash
# With revisions from @parasyte for macOS compatibility
# Requires bash >= 5 (mapfile)

if [[ "$1" == "--keep-versions" ]]; then
    keep_versions="1"
fi

purple="\e[0;35m"
reset="\e[0m"

releases_url="https://raw.githubusercontent.com/rust-lang/rust/master/RELEASES.md"

mapfile -t versions < <(curl -s "$releases_url" \
    | grep -v alpha \
    | egrep -o '^Version [1-9][0-9.]+' \
    | cut -d\  -f2)

# Before that, we have to use `cargo build`
checkindex=$(echo ${versions[@]/1.15.0//} | cut -d/ -f1 | wc -w | tr -d ' ')

n_versions=${#versions[@]}
echo -e "${purple}We know of ${reset}${n_versions}${purple} stable Rust versions."

mapfile -t priors < <(rustup toolchain list \
    | egrep -o '^[0-9.]+')

echo -e "There are ${reset}${#priors[@]}${purple} past versions explicitly installed here."

current=$(rustc --version | cut -d\  -f2)
curindex=$(echo ${versions[@]/$current//} | cut -d/ -f1 | wc -w | tr -d ' ')

if [[ $curindex == 0 ]]; then
    echo -e "You are on the latest stable: ${reset}${current}${purple}."
elif [[ $curindex == $n_versions ]]; then
    echo -e "You are not currently on a stable: ${reset}${current}${purple}" \
        "(latest is ${reset}${versions[0]}${purple})."
else
    echo -e "You are not on the latest stable: ${reset}${current}${purple}" \
        "(latest is ${reset}${versions[0]}${purple})."
fi

# Exaggerated (+2) from theoretical given observations
est_steps=$(echo "l($n_versions - 1)/l(2) + 3" | bc -l | cut -d. -f1)

echo
echo "Now going to binary search over Rust versions to find the earliest one" \
    "which compiles your project. We do this by repeatedly installing and checking" \
    $(if [[ -z "$keep_versions" ]]; then
        echo -e "then uninstalling Rust versions through rustup. Versions that are" \
            "already installed will not be removed (at the end, your system should" \
            "look the same). If you want to keep the versions this script downloads" \
            "instead, quit and run again with the ${reset}--keep-versions${purple} flag."
    else
        echo -e "Rust versions through rustup. As you've passed the" \
            "${reset}--keep-versions${purple} flag, versions this script downloads will" \
            "be kept instead of being cleaned up immediately."
    fi) \
    | fold -s
echo
echo "You'll want a fast, unmetered internet connection. If you're running Windows," \
    "it will take forever due to https://github.com/rust-lang/rustup.rs/issues/1540." \
    | fold -s
if [[ -z "$keep_versions" ]]; then
echo
echo "To pause on Linux/macOS, prefer Ctrl-Z / fg." \
    | fold -s
fi
echo
echo -ne "If you still want to proceed (estimated ${reset}${est_steps}${purple} iterations)," \
    "press enter now. Otherwise, abort with Ctrl-C.\n" \
    "\n${reset}=> " \
    | fold -s
read
echo -e "${purple}"

# absolute smallest
absmin="$((n_versions - 1))"

# smallest unsupported
trymin="$absmin"

# smallest supported
trymax="0"

# what we're checking now
target="$trymin"

# default variable values, needed for cleanup
a_priori=
target_v="${versions[$target]}"

function cleanup {
    if [[ -z "$a_priori" ]] && [[ -z "$keep_versions" ]]; then
        echo -e "${reset}"
        rustup uninstall "$target_v"
        echo -e "${purple}"
    fi
}

# remove the last installed version when the script is interrupted
trap 'cleanup ; echo -e "${reset}" ; exit 1' SIGINT

while true; do
    #echo "target: $target trymin: $trymin trymax: $trymax"

    # the above are indexes, this is the version string
    target_v="${versions[$target]}"
    echo -ne "Going to check against version ${reset}${target_v}${purple}"

    a_priori=
    if [[ " ${priors[@]} " =~ " ${target_v} " ]]; then
        a_priori=1
        echo -e ", which already exists locally.${reset}"
    else
        echo -e ".${reset}"
        rustup install "$target_v"
    fi

    echo
    if if [[ $target > $checkindex ]]; then
        rustup run "$target_v" cargo build
    else
        rustup run "$target_v" cargo check
    fi; then
        echo -e "\n${purple}Version ${reset}${target_v}${purple} is supported!"

        trymax="$target"
        target=$(((trymin - trymax) / 2 + trymax))

        if [[ $trymax == $target ]] || [[ $target_v == "1.0.0" ]]; then
            echo "That's the earliest! We're done!"
            cleanup
            break
        fi
    else
        echo -e "\n${purple}Version ${reset}${target_v}${purple} is not supported! See above for why."

        trymin="$target"
        target=$(((trymin - trymax) / 2 + trymax))
    fi

    cleanup
done

echo -e "${reset}"

Here's a patch to get this script working on macOS. (NOTE: I updated bash to version 5 to get mapfile, but it could also be replaced with a read loop.)

--- rustup-bisect.txt	2019-02-12 14:27:47.000000000 -0800
+++ rustup-bisect	2019-02-12 14:31:22.000000000 -0800
@@ -11,7 +11,7 @@ releases_url="https://raw.githubusercont

 mapfile -t versions < <(curl -s "$releases_url" \
     | grep -v alpha \
-    | grep -oP '^Version [1-9][\d.]+' \
+    | egrep -o '^Version [1-9][0-9.]+' \
     | cut -d\  -f2)

 # Before that, we have to use `cargo build`
@@ -21,7 +21,7 @@ n_versions=${#versions[@]}
 echo -e "${purple}We know of ${reset}${n_versions}${purple} stable Rust versions."

 mapfile -t priors < <(rustup toolchain list \
-    | grep -oP '^[\d.]+')
+    | egrep -o '^[\d.]+')

 echo -e "There are ${reset}${#priors[@]}${purple} past versions explicitly installed here."

@@ -129,3 +129,5 @@ while true; do
         echo -e "${purple}"
     fi
 done
+
+echo -e "${reset}"

@parasyte I merged those in, thank you!

@passcod The latest change missed one of the required fixes to the regular expression for matching versions. egrep doesn't support \d inside character ranges. But you can still use 0-9.

I also decided to fix the toolchain uninstall when the script is interrupted. These changes are in my fork of the gist, making it a bit easier to contribute. :)

Thank you!