Implement validation for autoconfig files

.github/workflows/rust.yaml

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+name: Rust
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+    paths:
+      - tools/validation/**
+  schedule:
+    - cron: '30 2 * * *'
+
+env:
+  CARGO_TERM_COLOR: always
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    name: "Build & Test: ./tools/validation"
+    defaults:
+      run:
+        working-directory: ./tools/validation
+
+    steps:
+    - name: Checkout source
+      uses: actions/checkout@v4
+
+    - name: Install Rust
+      uses: dtolnay/rust-toolchain@stable
+      with:
+        components: rustfmt, clippy
+
+    - name: Check Formatting
+      run: cargo fmt --all -- --check
+
+    - name: Cargo Cache
+      uses: actions/cache@v3
+      with:
+        path: |
+          ~/.cargo/registry/index/
+          ~/.cargo/registry/cache/
+          target/
+        key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+
+    - name: Build project
+      run: cargo build
+
+    - name: Test project
+      run: cargo test --all
+
+    - name: Run clippy
+      uses: giraffate/clippy-action@v1
+      with:
+        reporter: 'github-pr-check'
+        clippy_flags: --no-deps
+        filter_mode: nofilter
+        github_token: ${{ secrets.GITHUB_TOKEN }}

.gitignore

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+target
+.idea

tools/validation/Cargo.toml

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+[workspace]
+resolver = "2"
+members = ["autoconfig_validate"]

tools/validation/README.md

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+# Autoconfig validation
+
+Tools to use for reading and validating autoconfig configuration files.
+
+For more information on autoconfig files and the autoconfig format,
+see https://github.com/thunderbird/autoconfig

tools/validation/autoconfig_validate/Cargo.toml

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+[package]
+name = "autoconfig_validate"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+anyhow = "1.0.86"
+quick-xml = { version = "0.34.0", features = ["serde", "serialize"] }
+regex = "1.10.5"
+serde = { version = "1.0.203", features = ["derive"] }
+serde_path_to_error = "0.1.16"
+thiserror = "1.0.61"

tools/validation/autoconfig_validate/resources/test_data/valid_config.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<clientConfig version="1.1">
+    <emailProvider id="office365.com">
+        <!-- Office365 customer domains -->
+        <domain>office365.com</domain>
+        <domain>onmicrosoft.com</domain>
+        <!-- MX e.g. example.mail.protection.outlook.com -->
+        <domain>mail.protection.outlook.com</domain>
+        <displayName>Microsoft 365</displayName>
+        <displayShortName>Microsoft 365</displayShortName>
+        <incomingServer type="imap">
+            <hostname>outlook.office365.com</hostname>
+            <port>993</port>
+            <socketType>SSL</socketType>
+            <authentication>OAuth2</authentication>
+            <username>%EMAILADDRESS%</username>
+        </incomingServer>
+        <incomingServer type="pop3">
+            <hostname>outlook.office365.com</hostname>
+            <port>995</port>
+            <socketType>SSL</socketType>
+            <authentication>OAuth2</authentication>
+            <username>%EMAILADDRESS%</username>
+            <pop3>
+                <leaveMessagesOnServer>true</leaveMessagesOnServer>
+            </pop3>
+        </incomingServer>
+        <incomingServer type="exchange">
+            <hostname>outlook.office365.com</hostname>
+            <port>443</port>
+            <username>%EMAILADDRESS%</username>
+            <socketType>SSL</socketType>
+            <authentication>OAuth2</authentication>
+            <owaURL>https://outlook.office365.com/owa/</owaURL>
+            <ewsURL>https://outlook.office365.com/ews/exchange.asmx</ewsURL>
+            <useGlobalPreferredServer>true</useGlobalPreferredServer>
+        </incomingServer>
+        <outgoingServer type="smtp">
+            <hostname>smtp.office365.com</hostname>
+            <port>587</port>
+            <socketType>STARTTLS</socketType>
+            <authentication>OAuth2</authentication>
+            <username>%EMAILADDRESS%</username>
+        </outgoingServer>
+    </emailProvider>
+
+    <oAuth2>
+        <issuer>login.microsoftonline.com</issuer>
+        <scope>https://outlook.office365.com/IMAP.AccessAsUser.All https://outlook.office365.com/POP.AccessAsUser.All
+            https://outlook.office365.com/SMTP.Send offline_access
+        </scope>
+        <!-- https://docs.microsoft.com/en-us/azure/active-directory/develop/active-directory-v2-protocols#endpoints -->
+        <authURL>https://login.microsoftonline.com/common/oauth2/v2.0/authorize</authURL>
+        <tokenURL>https://login.microsoftonline.com/common/oauth2/v2.0/token</tokenURL>
+    </oAuth2>
+
+</clientConfig>

tools/validation/autoconfig_validate/src/error.rs

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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::{env, io};
+use thiserror::Error;
+
+use crate::types::IncomingServerType;
+
+/// An error that arose while deserializing or validating the autoconfig file.
+#[derive(Debug, Error)]
+pub enum ValidationError {
+    #[error("failed to deserialize structure from XML: {0}")]
+    Deserialize(#[from] serde_path_to_error::Error<quick_xml::DeError>),
+
+    #[error("unsupported clientConfig version: {0}")]
+    UnsupportedVersion(String),
+
+    #[error(
+        "found protocol-specific config for {found:?} on incoming server of type {server_type:?}"
+    )]
+    InvalidIncomingConfig {
+        server_type: IncomingServerType,
+        found: IncomingServerType,
+    },
+
+    #[error("invalid username template: {0}")]
+    InvalidUsernameTemplate(String),
+
+    #[error("encountered an unexpected error: {0}")]
+    Unexpected(#[source] anyhow::Error),
+}
+
+/// An error that arose in a test.
+#[derive(Debug, Error)]
+pub(crate) enum TestError {
+    #[error("failed to read variable from environment: {0}")]
+    EnvRead(#[from] env::VarError),
+
+    #[error("failed to read file: {0}")]
+    Io(#[from] io::Error),
+
+    #[error("validation error: {0}")]
+    Validation(#[from] ValidationError),
+}