fix(web): expose AppConfig constructor for custom servers

[akasakariko]

PR Type

Bug Fix

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt.
• (Team) Label with affected crates and semver status.

Overview

Custom servers built with actix_server::Server cannot currently construct a supported AppConfig with their actual listener metadata. Falling back to AppConfig::default() can give requests the wrong local address, hostname, and scheme, and produce incorrect generated URLs.

Expose and document the existing AppConfig::new constructor, and update the low-level server example to derive its configuration from the bound listener. This is an additive API change; existing default values and the hidden test constructor are preserved.

Regression tests cover request configuration, scheme/host fallbacks and URL generation, header/URI precedence, and two listeners sharing one worker.

Closes #3180.

Validation

The new external-crate regression tests fail before the change with E0624 because the constructor is private, and pass after the change.

• cargo +stable test -p actix-web --lib --tests --all-features --locked: 479 passed, 1 existing ignored test.
• cargo +stable test -p actix-web --doc --locked: 193 passed, 7 existing ignored doctests.
• Rust 1.88.0: all 3 new regression tests and the minimum-feature build passed.
• Stable minimum-feature build and actix-http's actix-web example build passed.
• Clippy for actix-web and actix-http, rustdoc with warnings denied, Rustfmt, and changelog formatting checks passed.

[robjtede]

I'm interested in addressing the issue that this fixes so I'll check this out once you've undrafted and think it's ready.

[akasakariko]

Closing and reopening as a non-draft PR. I wasn't able to mark this draft ready for review (GitHub returns a permission error for the author on this fork PR), so the replacement PR will be opened ready for review. Same branch and commits.

[akasakariko]

Re-closing to open a replacement non-draft PR (author cannot undraft this one; CLI create also hit a permission error, so opening via the web UI next).

[akasakariko]

Marked ready for review. Earlier undraft attempts failed because this account had already hit the repository open (non-draft) PR limit; temporarily converting #4238 back to draft freed the slot.

[akasakariko]

@robjtede Friendly ping — this is ready for review now and CI is green. Let me know if you'd like any changes.

[akasakariko]

Updated in 72957b7

• Merged main at 15fd3ff3 and resolved the changelog conflict, keeping the constructor entry under Unreleased
• Existing constructor implementation, example changes, and regression tests are unchanged
• GitHub now reports no merge conflicts

Validation

Local checks passed on macOS with stable Rust and MSRV 1.88

Commands and results

Command	Result
cargo test --locked -p actix-web --all-features --test test_app_config	3 passed
cargo test --locked -p actix-web -p actix-http --all-features --lib --tests	816 passed, 2 existing ignored
cargo test --locked -p actix-web -p actix-http --all-features --doc	238 passed, 8 existing ignored
cargo check --locked -p actix-http --example actix-web	Passed
cargo check --locked -p actix-web --no-default-features	Passed
cargo clippy --locked --workspace --all-targets --all-features -- -D warnings	Passed
cargo +1.88.0 test --locked -p actix-web --all-features --test test_app_config	3 passed
cargo +1.88.0 check --locked --workspace --all-targets --all-features	Passed
cargo +1.88.0 check --locked -p actix-web --no-default-features	Passed
cargo +nightly-2026-09-15 fmt --all -- --check	Passed
npx -y prettier --check actix-web/CHANGES.md	Passed
git diff --check and git diff --cached --check	Passed

Cargo reported the existing non-fatal actix-router/Cargo.toml warning for the unused lints.cargo key

Remaining CI blocker

GitHub checks finished with 15 passed, 1 failed, and 1 skipped

• Linux, macOS, and Windows tests passed on stable and MSRV, along with doctests, coverage, Clippy, formatting, and semver checks
• cargo-deny failed on rustls advisories RUSTSEC-2026-0346, RUSTSEC-2026-0347, RUSTSEC-2026-0348, and RUSTSEC-2026-0349
• The affected versions include rustls 0.20.9, 0.21.12, 0.22.4, and 0.23.45; updating only the latest rustls line would not clear the older supported lines
• Cargo.lock, deny.toml, and the lint workflow are identical to main at 15fd3ff3, so this failure is outside the constructor change
• No advisory exceptions or check bypasses were added
• check-external-types was skipped by the workflow