Unconditionally warn on usage of wasm32-wasi

This commit is a continuation of the work originally proposed in
rust-lang/compiler-team#607 and later amended in
rust-lang/compiler-team#695. The end goal is to rename `wasm32-wasi` to
`wasm32-wasip1` to reflect WASI's development and distinguish the
preexisting target from the `wasm32-wasip2` target that WASI is now
developing. Work for this transition began in #120468 which landed in
Rust 1.78 which became stable on 2024-05-02.

This implements the next phase of the transition plan to warn on usage
of `wasm32-wasi`. This is intended to help alert users that a removal is
pending and all release channels have the replacement available as well.
This will reach stable on 2024-09-05. The next stage of the plan is to
remove the `wasm32-wasi` target some time in October 2024 which means
that the removal will reach stable on 2025-01-09. For reference a full
schedule of this transition is listed [here].

Currently this implementation is a simple unconditional warning whenever
`rustc --target wasm32-wasi` is invoked. As-implemented there's no way
to turn off the warning other than to switch to the `wasm32-wasip1`
target.

[here]: #120468 (comment)

compiler/rustc_session/src/config.rs

@@ -1311,6 +1311,20 @@ pub fn build_target_config(early_dcx: &EarlyDiagCtxt, opts: &Options, sysroot: &
             for warning in warnings.warning_messages() {
                 early_dcx.early_warn(warning)
             }
+
+            // The `wasm32-wasi` target is being renamed to `wasm32-wasip1` as
+            // part of rust-lang/compiler-team#607 and
+            // rust-lang/compiler-team#695. Warn unconditionally on usage to
+            // raise awareness of the renaming. This code will be deleted in
+            // October 2024.
+            if opts.target_triple.triple() == "wasm32-wasi" {
+                early_dcx.early_warn(
+                    "the `wasm32-wasi` target is being renamed to \
+                    `wasm32-wasip1` and the `wasm32-wasi` target will be \
+                    removed from nightly in October 2024 and removed from \
+                    stable Rust in January 2025",
+                )
+            }
             if !matches!(target.pointer_width, 16 | 32 | 64) {
                 early_dcx.early_fatal(format!(
                     "target specification was invalid: unrecognized target-pointer-width {}",