Unrolled build for #148834

Rollup merge of #148834 - Muscraft:fix-doctest-colors, r=fmease

fix(rustdoc): Color doctest errors

`@fmease's` [Deep analysis](#148749 (comment)) on the problem
> Yeah, here's a deep analysis by me from a few weeks back of what's going on ([#148101 (comment)](#148101 (comment))):
>
> > […]
> > However, since said PR ([#147207](#147207): migrating coloring crates), `HumanEmitter::supports_color()` unconditionally(!) returns `false` (in fact, `Emitter::supports_color` is no longer used by anyone else and should be removed), so there's no reason to keep it. Rephrased, since that PR all compiler diagnostics for doctests are uncolored.
> > You could argue that I should keep it and patch `supports_color` in rustc to "work again". However, I'd rather rework our doctest coloring wholesale in a separate PR. At least before that migration PR, our setup was quite busted:
> >
> > 1. First of all, it didn't query+set `supports_color` for syntactically invalid doctests, so syntax errors were always shown without color (contrary to e.g., name resolution errors).
> > 2. Second of all, calling `supports_color()` here was quite frankly wrong: Piping the output of `rustdoc … --test` into a file (or `| cat` or whatever) did **not** suppress colors. I'm not actually sure if we can ever address that nicely (without stripping ANSI codes after the fact) since we pass that diagnostic to `libtest`, right? I might very well be wrong here, maybe it's a non-issue.

<hr>

```rust
/// ```
/// foo
/// ```
fn foo() {}
```
```
rustdoc --test lib.rs
```

Stable:
<img width="377" height="290" alt="stable" src="https://github.com/user-attachments/assets/cd20f947-b58d-42db-8735-797613baa9cc" />

Beta:
<img width="377" height="290" alt="beta" src="https://github.com/user-attachments/assets/f02588fd-41d2-4642-b03a-5554a68671eb" />

Nightly:
<img width="377" height="290" alt="nightly" src="https://github.com/user-attachments/assets/871cb417-f47e-4058-8a76-3bcd538ce141" />

After:
<img width="377" height="290" alt="after" src="https://github.com/user-attachments/assets/5734c01f-3f1c-44bb-9404-628c0c33b440" />

Note: This will need to be backported to `beta`

Fixes: #148749

Author: rust-timer

compiler/rustc_errors/src/emitter.rs

@@ -3466,14 +3466,9 @@ impl Drop for Buffy {
 
 pub fn stderr_destination(color: ColorConfig) -> Destination {
     let buffer_writer = std::io::stderr();
-    let choice = color.to_color_choice();
     // We need to resolve `ColorChoice::Auto` before `Box`ing since
     // `ColorChoice::Auto` on `dyn Write` will always resolve to `Never`
-    let choice = if matches!(choice, ColorChoice::Auto) {
-        AutoStream::choice(&buffer_writer)
-    } else {
-        choice
-    };
+    let choice = get_stderr_color_choice(color, &buffer_writer);
     // On Windows we'll be performing global synchronization on the entire
     // system for emitting rustc errors, so there's no need to buffer
     // anything.
@@ -3488,6 +3483,11 @@ pub fn stderr_destination(color: ColorConfig) -> Destination {
     }
 }
 
+pub fn get_stderr_color_choice(color: ColorConfig, stderr: &std::io::Stderr) -> ColorChoice {
+    let choice = color.to_color_choice();
+    if matches!(choice, ColorChoice::Auto) { AutoStream::choice(stderr) } else { choice }
+}
+
 /// On Windows, BRIGHT_BLUE is hard to read on black. Use cyan instead.
 ///
 /// See #36178.

src/librustdoc/doctest/make.rs

@@ -8,8 +8,8 @@ use std::sync::Arc;
 use rustc_ast::token::{Delimiter, TokenKind};
 use rustc_ast::tokenstream::TokenTree;
 use rustc_ast::{self as ast, AttrStyle, HasAttrs, StmtKind};
-use rustc_errors::emitter::stderr_destination;
-use rustc_errors::{AutoStream, ColorConfig, DiagCtxtHandle};
+use rustc_errors::emitter::get_stderr_color_choice;
+use rustc_errors::{AutoStream, ColorChoice, ColorConfig, DiagCtxtHandle};
 use rustc_parse::lexer::StripTokens;
 use rustc_parse::new_parser_from_source_str;
 use rustc_session::parse::ParseSess;
@@ -446,7 +446,7 @@ fn parse_source(
     span: Span,
 ) -> Result<ParseSourceInfo, ()> {
     use rustc_errors::DiagCtxt;
-    use rustc_errors::emitter::{Emitter, HumanEmitter};
+    use rustc_errors::emitter::HumanEmitter;
     use rustc_span::source_map::FilePathMapping;
 
     let mut info =
@@ -458,9 +458,12 @@ fn parse_source(
 
     let sm = Arc::new(SourceMap::new(FilePathMapping::empty()));
     let translator = rustc_driver::default_translator();
-    info.supports_color =
-        HumanEmitter::new(stderr_destination(ColorConfig::Auto), translator.clone())
-            .supports_color();
+    let supports_color = match get_stderr_color_choice(ColorConfig::Auto, &std::io::stderr()) {
+        ColorChoice::Auto => unreachable!(),
+        ColorChoice::AlwaysAnsi | ColorChoice::Always => true,
+        ColorChoice::Never => false,
+    };
+    info.supports_color = supports_color;
     // Any errors in parsing should also appear when the doctest is compiled for real, so just
     // send all the errors that the parser emits directly into a `Sink` instead of stderr.
     let emitter = HumanEmitter::new(AutoStream::never(Box::new(io::sink())), translator);