Enable rustdoc to document safe wasm intrinsics

This commit fixes an issue not found during rust-lang#84988 where rustdoc is used
to document cross-platform intrinsics but it was requiring that
functions which use `#[target_feature]` are `unsafe` erroneously, even
if they're WebAssembly specific. Rustdoc today, for example, already has
a special case where it enables annotations like
`#[target_feature(enable = "simd128")]` on platforms other than
WebAssembly. The purpose of this commit is to relax the "require all
`#[target_feature]` functions are `unsafe`" requirement for all targets
whenever rustdoc is running, enabling all targets to fully document
other targets, such as WebAssembly, where intrinsics functions aren't
always `unsafe`.

compiler/rustc_typeck/src/collect.rs

@@ -2771,7 +2771,7 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, id: DefId) -> CodegenFnAttrs {
             }
         } else if tcx.sess.check_name(attr, sym::target_feature) {
             if !tcx.is_closure(id) && tcx.fn_sig(id).unsafety() == hir::Unsafety::Normal {
-                if tcx.sess.target.is_like_wasm {
+                if tcx.sess.target.is_like_wasm || tcx.sess.opts.actually_rustdoc {
                     // The `#[target_feature]` attribute is allowed on
                     // WebAssembly targets on all functions, including safe
                     // ones. Other targets require that `#[target_feature]` is
@@ -2785,6 +2785,10 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, id: DefId) -> CodegenFnAttrs {
                     // deterministic trap. There is no undefined behavior when
                     // executing WebAssembly so `#[target_feature]` is allowed
                     // on safe functions (but again, only for WebAssembly)
+                    //
+                    // Note that this is also allowed if `actually_rustdoc` so
+                    // if a target is documenting some wasm-specific code then
+                    // it's not spuriously denied.
                 } else if !tcx.features().target_feature_11 {
                     let mut err = feature_err(
                         &tcx.sess.parse_sess,

src/test/rustdoc-ui/wasm-safe.rs

@@ -0,0 +1,7 @@
+// check-pass
+
+#![feature(wasm_target_feature)]
+
+#[cfg(any(target_arch = "wasm32", doc))]
+#[target_feature(enable = "simd128")]
+pub fn foo() {}