Auto merge of rust-lang#128404 - compiler-errors:revert-dead-code-changes, r=pnkfelix

Revert recent changes to dead code analysis

This is a revert to recent changes to dead code analysis, namely:
* efdf219 Rollup merge of rust-lang#128104 - mu001999-contrib:fix/128053, r=petrochenkov
* a70dc29 Rollup merge of rust-lang#127017 - mu001999-contrib:dead/enhance, r=pnkfelix
* 31fe962 Rollup merge of rust-lang#127107 - mu001999-contrib:dead/enhance-2, r=pnkfelix
* 2724aea Rollup merge of rust-lang#126618 - mu001999-contrib:dead/enhance, r=pnkfelix
* 977c5fd Rollup merge of rust-lang#126315 - mu001999-contrib:fix/126289, r=petrochenkov
* 13314df Rollup merge of rust-lang#125572 - mu001999-contrib:dead/enhance, r=pnkfelix

There is an additional change stacked on top, which suppresses false-negatives that were masked by this work. I believe the functions that are touched in that code are legitimately unused functions and the types are not reachable since this `AnonPipe` type is not publically reachable -- please correct me if I'm wrong cc `@NobodyXu` who added these in #rust-lang#127153.

Some of these reverts (rust-lang#126315 and rust-lang#126618) are only included because it makes the revert apply cleanly, and I think these changes were only done to fix follow-ups from the other PRs?

I apologize for the size of the PR and the churn that it has on the codebase (and for reverting `@mu001999's` work here), but I'm putting this PR up because I am concerned that we're making ad-hoc changes to fix bugs that are fallout of these PRs, and I'd like to see these changes reimplemented in a way that's more separable from the existing dead code pass. I am happy to review any code to reapply these changes in a more separable way.

cc `@mu001999`
r? `@pnkfelix`

Fixes rust-lang#128272
Fixes rust-lang#126169

Author: bors

core/src/default.rs

@@ -103,6 +103,7 @@ use crate::ascii::Char as AsciiChar;
 /// ```
 #[cfg_attr(not(test), rustc_diagnostic_item = "Default")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(bootstrap), rustc_trivial_field_reads)]
 pub trait Default: Sized {
     /// Returns the "default value" for a type.
     ///

std/src/sys/pal/unix/pipe.rs

@@ -47,6 +47,8 @@ pub fn anon_pipe() -> io::Result<(AnonPipe, AnonPipe)> {
 }
 
 impl AnonPipe {
+    #[allow(dead_code)]
+    // FIXME: This function seems legitimately unused.
     pub fn try_clone(&self) -> io::Result<Self> {
         self.0.duplicate().map(Self)
     }
@@ -85,6 +87,8 @@ impl AnonPipe {
         self.0.is_write_vectored()
     }
 
+    #[allow(dead_code)]
+    // FIXME: This function seems legitimately unused.
     pub fn as_file_desc(&self) -> &FileDesc {
         &self.0
     }