add script and documentation for rebasing over a version bump

Author: tjhance

source/docs/internal/version-bumping.md

@@ -0,0 +1,30 @@
+### Updating the code forked form rustc
+
+First, download the rust repository somewhere on your machine:
+
+```
+git clone https://github.com/rust-lang/rust/
+```
+
+Now `cd` back go the Verus repository that you want to apply the changes to. For the next step, you'll need to know the rustc version you're updating from and to. Run:
+
+```
+./source/tools/update-rustc-forks.sh /path/to/rust <old> <new>
+```
+
+Rust uses the version numbers as tags, so for example, if you're trying to update from 1.88.0 to 1.89.0, you can just do:
+
+```
+./source/tools/update-rustc-forks.sh /path/to/rust 1.88.0 1.89.0
+```
+
+This script will create a diff of all the relevant code and fetch it into your repository. If the script is successful, you'll see output like:
+
+```
+Diff prepared. When you're ready to apply it, run:
+git cherry-pick <some commit hash>
+```
+
+### Tips for resolving any conflicts
+
+In our fork, all the dependencies from the `rustc_mir_build/Cargo.toml` file were moved to be `extern crate` lines in `rustc_mir_build/src/lib.rs` file.  So if you see any conflicts in `Cargo.toml`, you may need to modify `lib.rs`.

source/tools/update-rustc-forks.sh

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+#!/bin/bash
+
+set -e
+
+RUST_DIR=$1
+BASE=$2
+UPDATED=$3
+
+SCRIPT_DIR=$(dirname "$0")
+VERUS_DIR=$(realpath $SCRIPT_DIR/../..)
+
+VERUS_TMP_DIR=$(mktemp -d)
+echo $VERUS_TMP_DIR
+
+cd $VERUS_DIR
+GIT_CONFIG_USER_NAME=$(git config user.name)
+GIT_CONFIG_USER_EMAIL=$(git config user.email)
+
+cd $VERUS_TMP_DIR
+git init
+
+cd $RUST_DIR
+git checkout $BASE
+GIT_BASE_HASH=$(git rev-parse HEAD)
+
+cd $VERUS_TMP_DIR
+mkdir source
+mkdir source/rustc_hir_typeck
+mkdir source/rustc_hir_typeck/src
+cp -r $RUST_DIR/compiler/rustc_mir_build $VERUS_TMP_DIR/source/
+cp -r $RUST_DIR/compiler/rustc_hir_typeck/src/upvar.rs $VERUS_TMP_DIR/source/rustc_hir_typeck/src/
+cp -r $RUST_DIR/compiler/rustc_hir_typeck/src/expr_use_visitor.rs $VERUS_TMP_DIR/source/rustc_hir_typeck/src/
+git add source
+git commit --author="nobody <nobody>" -m "base (from $BASE)" 
+
+cd $RUST_DIR
+git checkout $UPDATED
+GIT_UPDATED_HASH=$(git rev-parse HEAD)
+
+cd $VERUS_TMP_DIR
+git rm -r source
+mkdir source
+mkdir source/rustc_hir_typeck
+mkdir source/rustc_hir_typeck/src
+cp -r $RUST_DIR/compiler/rustc_mir_build $VERUS_TMP_DIR/source/
+cp -r $RUST_DIR/compiler/rustc_hir_typeck/src/upvar.rs $VERUS_TMP_DIR/source/rustc_hir_typeck/src/
+cp -r $RUST_DIR/compiler/rustc_hir_typeck/src/expr_use_visitor.rs $VERUS_TMP_DIR/source/rustc_hir_typeck/src/
+git add source
+git commit --author="nobody <nobody>" -m "Updating forked Rust code: from $BASE ($GIT_BASE_HASH) to $UPDATED ($GIT_UPDATED_HASH)" -m "Diff generated by: update-rustc-forks.sh" -m "Applied by: $GIT_CONFIG_USER_NAME <$GIT_CONFIG_USER_EMAIL>"
+
+GIT_COMMIT=$(git rev-parse HEAD)
+
+cd $VERUS_DIR
+git fetch $VERUS_TMP_DIR/.git
+
+echo ""
+echo "===="
+echo ""
+
+echo "Diff prepared. When you're ready to apply it, run:"
+echo "git cherry-pick $GIT_COMMIT"
+