feat!: migrate workspace to Rust edition 2024 (ADR-T-004)

BREAKING CHANGE: MSRV raised from 1.83 to 1.85; version bumped to
4.0.0-develop.

- Set `edition = "2024"` and `rust-version = "1.85"` workspace-wide
- Promote `rust-2024-compatibility` lint group from warn to deny
- Remove `#[allow(if_let_rescope)]` in download handler (native in 2024)
- Relax located-error doctest assertion for merged-doctest bundling
- Add ADR-T-004 documenting the migration (supersedes ADR-T-003)

Author: da2ce7

Cargo.lock

@@ -23,14 +23,14 @@ authors = ["Nautilus Cyberneering <info@nautilus-cyberneering.de>, Mick van Dijk
 categories = ["network-programming", "web-programming"]
 description = "A BitTorrent Index"
 documentation = "https://docs.rs/crate/torrust-tracker/"
-edition = "2021"
+edition = "2024"
 homepage = "https://torrust.com/"
 keywords = ["bittorrent", "file-sharing", "index", "peer-to-peer", "torrent"]
 license = "AGPL-3.0-only"
 publish = true
 repository = "https://github.com/torrust/torrust-tracker"
-rust-version = "1.83"
-version = "3.1.0-develop"
+rust-version = "1.85"
+version = "4.0.0-develop"
 
 [features]
 default = []
@@ -39,7 +39,7 @@ default = []
 opt-level = 3
 
 [dependencies]
-torrust-index-located-error = { version = "3.1.0-develop", path = "packages/located-error" }
+torrust-index-located-error = { version = "4.0.0-develop", path = "packages/located-error" }
 torrust-index-render-text-as-image = { version = "0.1.0", path = "packages/render-text-as-image" }
 
 anyhow = "1"
@@ -119,7 +119,7 @@ nonstandard-style = { level = "deny", priority = -1 }
 rust-2018-compatibility = { level = "deny", priority = -1 }
 rust-2018-idioms = { level = "deny", priority = -1 }
 rust-2021-compatibility = { level = "deny", priority = -1 }
-rust-2024-compatibility = { level = "warn", priority = -1 }
+rust-2024-compatibility = { level = "deny", priority = -1 }
 unsafe-code = "warn"
 unused = { level = "deny", priority = -2 }
 warnings = { level = "deny", priority = -1 }

Cargo.toml

@@ -0,0 +1,55 @@
+# ADR-T-004: Migrate to Rust Edition 2024
+
+**Status:** Decided
+**Date:** 2026-03-23
+**Supersedes:** [ADR-T-003](./003-edition-2024-preparation.md)
+(preparation / dependency pinning)
+
+## Context
+
+ADR-T-003 raised the MSRV to 1.83 and pinned several transitive
+dependencies to avoid edition-2024-only releases while the workspace
+remained on edition 2021. That was explicitly described as a
+temporary measure — the intention was always to complete the
+migration in a follow-up.
+
+Rust edition 2024 was stabilised in rustc 1.85. `cargo fix --edition`
+reported only one automatic fix (an `if_let_rescope` rewrite in
+`handlers.rs`), confirming that the codebase was already
+well-prepared by the 2024-compatibility lint group enabled in
+ADR-T-003.
+
+## Decision
+
+**Migrate the entire workspace to `edition = "2024"` and raise
+the MSRV to 1.85.**
+
+### Changes
+
+| Setting / File                                 | From            | To              | Reason                                                                 |
+| ---------------------------------------------- | --------------- | --------------- | ---------------------------------------------------------------------- |
+| `workspace.package.version`                    | `3.1.0-develop` | `4.0.0-develop` | Major bump for edition change                                          |
+| `workspace.package.edition`                    | `"2021"`        | `"2024"`        | Edition migration                                                      |
+| `workspace.package.rust-version`               | `1.83`          | `1.85`          | Minimum compiler for edition 2024                                      |
+| `workspace.lints.rust.rust-2024-compatibility` | `warn`          | `deny`          | Now the active edition; violations must not land                       |
+| `handlers.rs` — `#[allow(if_let_rescope)]`     | present         | removed         | Lint no longer needed; behaviour is native to edition 2024             |
+| `located-error` doctest assertion              | `"lib.rs"`      | `".rs"`         | Edition 2024 merged-doctest bundling changes `Location::caller()` path |
+
+### Dependency-pin status
+
+The transitive-dependency pins from ADR-T-003 remain in `Cargo.lock`
+for now. They can be lifted opportunistically: once those upstream
+crates publish releases that no longer conflict with the rest of the
+dependency tree, a routine `cargo update` will pick them up. No
+further manual pinning is required.
+
+## Consequences
+
+- The workspace compiles and passes all tests (`--all-features`,
+  `--no-default-features`, `--release`, `--doc`) on rustc 1.85+.
+- Contributors must use rustc ≥ 1.85.
+- Edition-2024 language features (e.g. `gen` keyword reservation,
+  `unsafe_op_in_unsafe_fn` default, tail-expression temporaries)
+  are now available throughout the codebase.
+- The `rust-2024-compatibility` lint group is promoted to `deny`,
+  preventing future regressions.

adr/004-edition-2024.md

@@ -24,7 +24,7 @@
 //! let l = get_caller_location();
 //!
 //! assert!(b.to_string().contains("Test, "));
-//! assert!(b.to_string().contains("lib.rs"));
+//! assert!(b.to_string().contains(".rs"));
 //! ```
 //!
 //! # Credits

packages/located-error/src/lib.rs

@@ -4,9 +4,9 @@ use std::io::{Cursor, Write};
 use std::str::FromStr;
 use std::sync::Arc;
 
+use axum::Json;
 use axum::extract::{self, Multipart, Path, Query, State};
 use axum::response::{IntoResponse, Redirect, Response};
-use axum::Json;
 use bittorrent_primitives::info_hash::InfoHash;
 use serde::Deserialize;
 use tracing::debug;
@@ -77,7 +77,6 @@ pub async fn download_torrent_handler(
 
     debug!("Downloading torrent: {:?}", info_hash.to_hex_string());
 
-    #[allow(if_let_rescope)]
     if let Some(redirect_response) =
         redirect_to_download_url_using_canonical_info_hash_if_needed(&app_data, &info_hash, maybe_user_id).await
     {