Explicitly set MSRV for rust extension (Qiskit#8196)

* Explicitly set MSRV for rust extension

This commit explicitly sets the MSRV (minimum supported rust version)
for Qiskit Terra to 1.56 which was released in October 2021 (with 1.56.1
which fixed two CVEs being released Nov. 1st 2021). Previously we had
avoided setting a hard MSRV and opted to try and just loosely support the
past 6 months of rust releases. However, managing it loosely has proven
tricky to manage in practice. This commit sets a hard version for MSRV
and modifies a test job to validate we're able to compile with the MSRV.
This should ensure we're able to avoid breaking compatibility for that
version.

* Explicitly set MSRV to 1.56.1 instead of loosely 1.56

* Explicitly set version as variable in CI config

* Assign MSRV to azure variable

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.azure/test-linux.yml

@@ -15,6 +15,10 @@ parameters:
     type: boolean
     default: false
 
+  - name: "rustVersion"
+    type: string
+    default: "stable"
+
 
 jobs:
   - job: "Linux_Tests_Python${{ replace(parameters.pythonVersion, '.', '') }}"
@@ -52,6 +56,10 @@ jobs:
           path: .stestr
         displayName: "Cache stestr"
 
+      - bash: |
+          rustup default "${{ parameters.rustVersion }}"
+        displayName: "Use rust version ${{ parameters.rustVersion }}"
+
       - bash: |
           set -e
           python -m pip install --upgrade pip setuptools wheel virtualenv

CONTRIBUTING.md

@@ -242,6 +242,10 @@ the Visual C++ build tools so that rust can link against the system c/c++
 libraries. You can see more details on this in the
 [rustup documentation](https://rust-lang.github.io/rustup/installation/windows.html).
 
+Qiskit Terra has a minimum supported Rust version (MSRV) of 1.56.1. This means
+to build Qiskit Terra from source you must have at least rustc version 1.56.1
+installed, older versions will not be able to compile Qiskit.
+
 Once you have a rust compiler installed you can rely on the normal Python
 build/install steps to install Qiskit Terra. This means you just run
 `pip install .` in your local git clone to build and install Qiskit Terra.

README.md

@@ -1,5 +1,5 @@
 # Qiskit Terra
-[![License](https://img.shields.io/github/license/Qiskit/qiskit-terra.svg?style=popout-square)](https://opensource.org/licenses/Apache-2.0)<!--- long-description-skip-begin -->[![Build Status](https://img.shields.io/travis/com/Qiskit/qiskit-terra/master.svg?style=popout-square)](https://travis-ci.com/Qiskit/qiskit-terra)[![Release](https://img.shields.io/github/release/Qiskit/qiskit-terra.svg?style=popout-square)](https://github.com/Qiskit/qiskit-terra/releases)[![Downloads](https://img.shields.io/pypi/dm/qiskit-terra.svg?style=popout-square)](https://pypi.org/project/qiskit-terra/)[![Coverage Status](https://coveralls.io/repos/github/Qiskit/qiskit-terra/badge.svg?branch=main)](https://coveralls.io/github/Qiskit/qiskit-terra?branch=main)<!--- long-description-skip-end -->
+[![License](https://img.shields.io/github/license/Qiskit/qiskit-terra.svg?style=popout-square)](https://opensource.org/licenses/Apache-2.0)<!--- long-description-skip-begin -->[![Build Status](https://img.shields.io/travis/com/Qiskit/qiskit-terra/master.svg?style=popout-square)](https://travis-ci.com/Qiskit/qiskit-terra)[![Release](https://img.shields.io/github/release/Qiskit/qiskit-terra.svg?style=popout-square)](https://github.com/Qiskit/qiskit-terra/releases)[![Downloads](https://img.shields.io/pypi/dm/qiskit-terra.svg?style=popout-square)](https://pypi.org/project/qiskit-terra/)[![Coverage Status](https://coveralls.io/repos/github/Qiskit/qiskit-terra/badge.svg?branch=main)](https://coveralls.io/github/Qiskit/qiskit-terra?branch=main)[![Minimum rustc 1.56.1](https://img.shields.io/badge/rustc-1.56.1+-blue.svg)](https://rust-lang.github.io/rfcs/2495-min-rust-version.html)<!--- long-description-skip-end -->
 
 **Qiskit** is an open-source framework for working with noisy quantum computers at the level of pulses, circuits, and algorithms.
 

azure-pipelines.yml

@@ -1,6 +1,6 @@
 # Trigger types.  This needs to include all the triggers for the different
 # pipelines we might want to run.
-# 
+#
 # Each of these individually triggers the pipeline compilation step, but it's
 # the templating expressions in the `stages` list that actually set what runs.
 trigger:
@@ -45,6 +45,12 @@ parameters:
     type: string
     default: "3.10"
 
+  - name: "minimumRustVersion"
+    displayName: "Minimum supported version of Rust"
+    type: string
+    default: "1.56.1"
+
+
   # These two versions of Python can be chosen somewhat arbitrarily, but we get
   # slightly better coverage per PR if they're neither the maximum nor minimum
   # supported versions.
@@ -144,6 +150,7 @@ stages:
             pythonVersion: ${{ parameters.minimumPythonVersion }}
             testQPY: false
             testImages: false
+            rustVersion: ${{ parameters.minimumRustVersion }}
 
     # The rest of the PR pipeline is to test the oldest and newest supported
     # versions of Python, along with the integration tests (via the tutorials).

releasenotes/notes/msrv-0b626e1cfb415abf.yaml

@@ -0,0 +1,10 @@
+---
+upgrade:
+  - |
+    Qiskit Terra's compiled Rust extensions now have a minimum supported Rust
+    version (MSRV) of 1.56.1. This means when building Qiskit Terra from source
+    the oldest version of the Rust compiler supported is 1.56.1. If you are using
+    an older version of the Rust compiler you will need to update to a newer
+    version to continue to build Qiskit from source. This change was necessary
+    as a number of upstream dependencies have updated their minimum supported
+    versions too.