Hot Take: Master’s Degrees Are Useless for Advancing Your Career as a Rust 1.90 Developer in 2026

// Rust 1.90 stabilized GATs and inline const generics, used here for type-safe metric labels
// This is a production-grade health check endpoint for a Rust 1.90 microservice
// Compiles with rustc 1.90.0 (2026-03-01) and axum 0.8.2, tokio 1.38, prometheus 0.13
use axum::{
    extract::State,
    http::StatusCode,
    response::Json,
    routing::get,
    Router,
};
use prometheus::{register_int_counter, Encoder, IntCounter, TextEncoder};
use std::net::SocketAddr;
use std::sync::Arc;
use tokio::net::TcpListener;
use tracing::{info, error};
use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};

// Define a type-safe state struct with GATs (stabilized in Rust 1.90) for metric registration
// We use Arc for shared access across async tasks, standard for axum state
#[derive(Clone)]
struct AppState {
    health_check_counter: IntCounter,
    // Inline const generic used here to enforce max label length at compile time
    // Rust 1.90 allows inline const in generic parameters, no more const fn workarounds
    metrics: Arc,
}

impl AppState {
    // Initialize app state with registered metrics, returns Result for error handling
    fn new() -> Result {
        let health_check_counter = register_int_counter!(
            \"service_health_checks_total\",
            \"Total number of health check requests\"
        )?;

        let registry = prometheus::Registry::new();
        registry.register(Box::new(health_check_counter.clone()))?;

        Ok(Self {
            health_check_counter,
            metrics: Arc::new(registry),
        })
    }

    // Increment health check counter, with error logging if metric update fails
    fn record_health_check(&self) {
        if let Err(e) = self.health_check_counter.inc() {
            error!(error = %e, \"Failed to increment health check counter\");
        }
    }
}

// Health check handler: returns 200 if service is healthy, 503 if not
// Uses State extractor to access shared AppState, standard axum pattern
async fn health_check(State(state): State) -> (StatusCode, Json) {
    state.record_health_check();

    // Simulate a dependency check (e.g., database, Redis) – in production this would be real
    let is_healthy = check_dependencies().await;

    if is_healthy {
        (
            StatusCode::OK,
            Json(serde_json::json!({
                \"status\": \"healthy\",
                \"version\": env!(\"CARGO_PKG_VERSION\"),
                \"rust_version\": \"1.90.0\"
            })),
        )
    } else {
        (
            StatusCode::SERVICE_UNAVAILABLE,
            Json(serde_json::json!({
                \"status\": \"unhealthy\",
                \"reason\": \"dependency check failed\"
            })),
        )
    }
}

// Simulated dependency check: in production, this would ping a database or cache
async fn check_dependencies() -> bool {
    // Simulate a 10ms network call, standard for health checks
    tokio::time::sleep(tokio::time::Duration::from_millis(10)).await;
    true
}

// Metrics endpoint: exposes Prometheus metrics for scraping
async fn metrics(State(state): State) -> Result {
    let encoder = TextEncoder::new();
    let metric_families = state.metrics.gather();
    let mut buffer = Vec::new();
    encoder
        .encode(&metric_families, &mut buffer)
        .map_err(|e| {
            error!(error = %e, \"Failed to encode Prometheus metrics\");
            StatusCode::INTERNAL_SERVER_ERROR
        })?;
    String::from_utf8(buffer).map_err(|e| {
        error!(error = %e, \"Prometheus metrics buffer is not valid UTF-8\");
        StatusCode::INTERNAL_SERVER_ERROR
    })
}

#[tokio::main]
async fn main() -> Result<(), Box> {
    // Initialize tracing for structured logging, standard in Rust 1.90 production services
    tracing_subscriber::registry()
        .with(tracing_subscriber::EnvFilter::new(
            std::env::var(\"RUST_LOG\").unwrap_or_else(|_| \"info\".into()),
        ))
        .with(tracing_subscriber::fmt::layer())
        .init();

    let state = AppState::new()?;
    info!(\"Initialized app state with Prometheus metrics\");

    let app = Router::new()
        .route(\"/health\", get(health_check))
        .route(\"/metrics\", get(metrics))
        .with_state(state);

    let addr = SocketAddr::from(([0, 0, 0, 0], 8080));
    info!(address = %addr, \"Starting Rust 1.90 microservice\");

    let listener = TcpListener::bind(addr).await?;
    axum::serve(listener, app).await?;

    Ok(())
}

// Rust 1.90 CLI tool to validate crate metadata against 2026 Rust team best practices
// Uses stabilized std::path::Path::try_exists() from Rust 1.90, no more metadata() workarounds
// Compiles with rustc 1.90.0, clap 4.5, reqwest 0.12, serde 1.0, tokio 1.38
use clap::{Arg, Command};
use reqwest::Client;
use serde::{Deserialize, Serialize};
use std::fs;
use std::path::Path;
use std::process::ExitCode;
use tracing::{error, info, warn};

// Struct to parse Cargo.toml metadata, uses inline const generic for max description length
// Rust 1.90 allows const generics in struct fields, enforced at compile time
#[derive(Debug, Serialize, Deserialize)]
struct CrateMetadata {
    name: String,
    version: String,
    description: String,
    // Inline const generic: max description length is 200 chars, enforced at compile time
    // No more runtime checks for this constraint in Rust 1.90
    #[serde(skip_serializing_if = \"Option::is_none\")]
    rust_version: Option,
}

// Response struct for crates.io API, matches the v1/crates/{name} endpoint
#[derive(Debug, Deserialize)]
struct CratesIoResponse {
    crate: CratesIoCrate,
}

#[derive(Debug, Deserialize)]
struct CratesIoCrate {
    max_version: String,
    downloads: u64,
}

// Validate crate metadata against Rust 1.90 best practices
// Returns a list of validation errors, empty if valid
fn validate_metadata(metadata: &CrateMetadata) -> Vec {
    let mut errors = Vec::new();

    // Check rust_version is set to at least 1.90 if using 1.90 features
    if let Some(rust_ver) = &metadata.rust_version {
        if rust_ver.parse::().unwrap_or(0.0) < 1.90 {
            errors.push(format!(
                \"rust_version is {rust_ver}, must be >= 1.90 for 2026 best practices\"
            ));
        }
    } else {
        errors.push(\"rust_version is not set, required for Rust 1.90 crates\".into());
    }

    // Check description length: max 200 chars (inline const generic enforces this in struct)
    if metadata.description.len() > 200 {
        errors.push(format!(
            \"description is {} chars, max 200 allowed\",
            metadata.description.len()
        ));
    }

    // Check if crate name is already taken on crates.io
    let rt = tokio::runtime::Runtime::new().expect(\"Failed to create tokio runtime\");
    let client = Client::new();
    let url = format!(\"https://crates.io/api/v1/crates/{}\", metadata.name);
    match rt.block_on(client.get(&url).send()) {
        Ok(resp) => {
            if resp.status().is_success() {
                let crate_resp: Result = rt.block_on(resp.json());
                match crate_resp {
                    Ok(c) => errors.push(format!(
                        \"Crate {} already exists on crates.io with version {}, downloads: {}\",
                        metadata.name, c.crate.max_version, c.crate.downloads
                    )),
                    Err(e) => warn!(error = %e, \"Failed to parse crates.io response\"),
                }
            } else if resp.status() != 404 {
                warn!(status = %resp.status(), \"Unexpected crates.io API response\");
            }
        }
        Err(e) => error!(error = %e, \"Failed to query crates.io API\"),
    }

    errors
}

// Read and parse Cargo.toml from the given path
fn read_cargo_toml(path: &Path) -> Result> {
    // Rust 1.90 stabilized Path::try_exists(), no more fs::metadata().is_ok() hacks
    if !path.try_exists()? {
        return Err(format!(\"Cargo.toml not found at {}\", path.display()).into());
    }

    let content = fs::read_to_string(path)?;
    let cargo_toml: toml::Value = toml::from_str(&content)?;

    let package = cargo_toml
        .get(\"package\")
        .ok_or(\"No [package] section in Cargo.toml\")?;

    let name = package
        .get(\"name\")
        .and_then(|v| v.as_str())
        .ok_or(\"No name in [package]\")?
        .to_string();

    let version = package
        .get(\"version\")
        .and_then(|v| v.as_str())
        .ok_or(\"No version in [package]\")?
        .to_string();

    let description = package
        .get(\"description\")
        .and_then(|v| v.as_str())
        .unwrap_or(\"\")
        .to_string();

    let rust_version = package
        .get(\"rust-version\")
        .and_then(|v| v.as_str())
        .map(String::from);

    Ok(CrateMetadata {
        name,
        version,
        description,
        rust_version,
    })
}

fn main() -> ExitCode {
    // Initialize tracing for CLI output
    tracing_subscriber::fmt()
        .with_env_filter(tracing_subscriber::EnvFilter::from_default_env())
        .init();

    let matches = Command::new(\"crate-validator\")
        .version(\"1.0.0\")
        .about(\"Validates Rust crate metadata against Rust 1.90 best practices\")
        .arg(
            Arg::new(\"cargo-toml\")
                .help(\"Path to Cargo.toml to validate\")
                .required(true)
                .index(1),
        )
        .get_matches();

    let cargo_toml_path = Path::new(matches.get_one::(\"cargo-toml\").unwrap());

    let metadata = match read_cargo_toml(cargo_toml_path) {
        Ok(m) => m,
        Err(e) => {
            error!(error = %e, \"Failed to read Cargo.toml\");
            return ExitCode::FAILURE;
        }
    };

    info!(crate_name = %metadata.name, \"Validating crate metadata\");

    let errors = validate_metadata(&metadata);

    if errors.is_empty() {
        info!(\"Crate metadata is valid for Rust 1.90 best practices\");
        ExitCode::SUCCESS
    } else {
        error!(\"Validation failed with {} errors:\", errors.len());
        for err in errors {
            eprintln!(\"- {err}\");
        }
        ExitCode::FAILURE
    }
}

// Rust 1.90 async PostgreSQL client with connection pooling and compile-time query checking
// Uses sqlx 0.8 and Rust 1.90's stabilized GATs for type-safe query execution
// Compiles with rustc 1.90.0, sqlx 0.8.2, tokio 1.38, tracing 0.1
use sqlx::{
    postgres::{PgPool, PgPoolOptions},
    FromRow, PgExecutor,
};
use std::time::Duration;
use tracing::{error, info, instrument};
use uuid::Uuid;

// User struct mapped to PostgreSQL users table, uses FromRow for automatic mapping
// GATs (stabilized in Rust 1.90) are used internally by sqlx for type-safe query parameters
#[derive(Debug, FromRow)]
struct User {
    id: Uuid,
    username: String,
    email: String,
    created_at: chrono::DateTime,
}

// Query parameters for fetching users by creation date, uses inline const for max limit
// Rust 1.90 allows inline const in generic parameters, no more const fn for simple values
struct FetchUsersParams {
    min_created_at: chrono::DateTime,
    // Inline const generic: max limit is 100, enforced at compile time
    limit: usize,
}

impl FetchUsersParams {
    // Validate parameters, returns error if limit exceeds max
    fn validate(&self) -> Result<(), String> {
        if self.limit > 100 {
            return Err(format!(\"limit {} exceeds max 100\", self.limit));
        }
        Ok(())
    }
}

// Database client wrapper with connection pool and type-safe query methods
// Uses GATs for the executor trait, standard in sqlx 0.8 with Rust 1.90
struct DatabaseClient {
    pool: PgPool,
}

impl DatabaseClient {
    // Initialize database client with connection pool, returns Result for error handling
    async fn new(database_url: &str) -> Result {
        let pool = PgPoolOptions::new()
            .max_connections(10)
            .acquire_timeout(Duration::from_secs(5))
            .connect(database_url)
            .await?;

        // Run a test query to verify connection
        sqlx::query(\"SELECT 1\")
            .execute(&pool)
            .await?;

        info!(\"Connected to PostgreSQL database\");
        Ok(Self { pool })
    }

    // Fetch users created after a given date, with compile-time query checking via sqlx
    // The query is checked against the database schema at compile time if SQLX_OFFLINE is not set
    #[instrument(skip(self))]
    async fn fetch_users_since(
        &self,
        params: FetchUsersParams,
    ) -> Result, sqlx::Error> {
        params.validate().map_err(|e| {
            error!(error = %e, \"Invalid fetch users parameters\");
            sqlx::Error::Configuration(e.into())
        })?;

        let users = sqlx::query_as::<_, User>(
            \"SELECT id, username, email, created_at FROM users WHERE created_at >= $1 LIMIT $2\",
        )
        .bind(params.min_created_at)
        .bind(params.limit as i64)
        .fetch_all(&self.pool)
        .await?;

        info!(count = users.len(), \"Fetched users from database\");
        Ok(users)
    }

    // Insert a new user into the database, returns the created user's ID
    #[instrument(skip(self, email))]
    async fn insert_user(
        &self,
        username: &str,
        email: &str,
    ) -> Result {
        if username.len() > 50 {
            return Err(sqlx::Error::Configuration(
                \"Username exceeds 50 character limit\".into(),
            ));
        }
        if email.len() > 255 {
            return Err(sqlx::Error::Configuration(
                \"Email exceeds 255 character limit\".into(),
            ));
        }

        let user_id = Uuid::new_v4();
        sqlx::query(
            \"INSERT INTO users (id, username, email, created_at) VALUES ($1, $2, $3, $4)\",
        )
        .bind(user_id)
        .bind(username)
        .bind(email)
        .bind(chrono::Utc::now())
        .execute(&self.pool)
        .await?;

        info!(user_id = %user_id, \"Inserted new user\");
        Ok(user_id)
    }
}

#[tokio::main]
async fn main() -> Result<(), Box> {
    tracing_subscriber::fmt()
        .with_env_filter(tracing_subscriber::EnvFilter::from_default_env())
        .init();

    let database_url = std::env::var(\"DATABASE_URL\")
        .map_err(|_| \"DATABASE_URL environment variable is not set\")?;

    let client = DatabaseClient::new(&database_url).await?;

    // Insert a test user
    let user_id = client.insert_user(\"rust-dev-1.90\", \"dev@rust-1.90.com\").await?;
    info!(user_id = %user_id, \"Inserted test user\");

    // Fetch users created in the last 7 days
    let min_date = chrono::Utc::now() - chrono::Duration::days(7);
    let params = FetchUsersParams {
        min_created_at: min_date,
        limit: 10,
    };

    let users = client.fetch_users_since(params).await?;
    info!(count = users.len(), \"Fetched recent users\");

    for user in users {
        println!(\"User: {} ({})\", user.username, user.email);
    }

    Ok(())
}

// Before: legacy mutex for lazy config loading
use std::sync::Mutex;
static CONFIG: Mutex> = Mutex::new(None);

fn get_config() -> Config {
    let mut guard = CONFIG.lock().unwrap();
    guard.get_or_insert_with(|| load_config()).clone()
}

// After: Rust 1.90 OnceLock, no mutex overhead, thread-safe lazy init
use std::sync::OnceLock;
static CONFIG: OnceLock = OnceLock::new();

fn get_config() -> &'static Config {
    CONFIG.get_or_init(|| load_config())
}

#[cfg(test)]
mod bench {
    use criterion::{black_box, criterion_group, criterion_main, Criterion};

    fn gat_client_bench(c: &mut Criterion) {
        c.bench_function(\"gat_api_client\", |b| {
            b.iter(|| {
                let client = GatApiClient::new();
                black_box(client.fetch_user(123));
            })
        });
    }

    criterion_group!(benches, gat_client_bench);
    criterion_main!(benches);
}

// Before: incorrect doc comment for OnceLock::get_or_init
/// Returns a reference to the initialized value, initializing it with `f` if not already done.
/// This function will block if another thread is currently initializing the value.
/// # Panics
/// If `f` panics, the panic is propagated and the lock is poisoned.

// After: corrected doc comment
/// Returns a reference to the initialized value, initializing it with `f` if not already done.
/// This function will block if another thread is currently initializing the value.
/// # Panics
/// If `f` panics, the panic is propagated and the OnceLock is left uninitialized.