Auto merge of #130249 - compiler-errors:sad-new-solver-coherence, r=lcnr

Revert "Stabilize `-Znext-solver=coherence`"

This is a clean revert of #121848, prepared by running:

```
$ git revert 17b322fa69eed7216dccc9f097eb68237cf62234 -m1
```

Which effectively reverts:
* a138a9261536ac2bdbb7c01a8aa9dc7d43299cae, 69fdd1457d367ce4de044e9784e58a38acf3d847, d93e047c9f1b33892a604273ab3931815f5604a1, 1a893ac648e03732aaa8b5371b602ab683970b0d

see: https://rust-lang.zulipchat.com/#narrow/stream/364551-t-types.2Ftrait-system-refactor/topic/nalgebra.20hang

Closes #130056

r? lcnr

2 files changed, 21 insertions, 15 deletions

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index f3e3b36111c..908d50a041e 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -842,11 +842,6 @@ pub struct NextSolverConfig {
     /// This is only `true` if `coherence` is also enabled.
     pub globally: bool,
 }
-impl Default for NextSolverConfig {
-    fn default() -> Self {
-        NextSolverConfig { coherence: true, globally: false }
-    }
-}
 
 #[derive(Clone)]
 pub enum Input {
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 106628ec8f6..e487a2501e3 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -403,7 +403,7 @@ mod desc {
     pub(crate) const parse_unpretty: &str = "`string` or `string=string`";
     pub(crate) const parse_treat_err_as_bug: &str = "either no value or a non-negative number";
     pub(crate) const parse_next_solver_config: &str =
-        "either `globally` (when used without an argument), `coherence` (default) or `no`";
+        "a comma separated list of solver configurations: `globally` (default), and `coherence`";
     pub(crate) const parse_lto: &str =
         "either a boolean (`yes`, `no`, `on`, `off`, etc), `thin`, `fat`, or omitted";
     pub(crate) const parse_linker_plugin_lto: &str =
@@ -1105,16 +1105,27 @@ mod parse {
         }
     }
 
-    pub(crate) fn parse_next_solver_config(slot: &mut NextSolverConfig, v: Option<&str>) -> bool {
+    pub(crate) fn parse_next_solver_config(
+        slot: &mut Option<NextSolverConfig>,
+        v: Option<&str>,
+    ) -> bool {
         if let Some(config) = v {
-            *slot = match config {
-                "no" => NextSolverConfig { coherence: false, globally: false },
-                "coherence" => NextSolverConfig { coherence: true, globally: false },
-                "globally" => NextSolverConfig { coherence: true, globally: true },
-                _ => return false,
-            };
+            let mut coherence = false;
+            let mut globally = true;
+            for c in config.split(',') {
+                match c {
+                    "globally" => globally = true,
+                    "coherence" => {
+                        globally = false;
+                        coherence = true;
+                    }
+                    _ => return false,
+                }
+            }
+
+            *slot = Some(NextSolverConfig { coherence: coherence || globally, globally });
         } else {
-            *slot = NextSolverConfig { coherence: true, globally: true };
+            *slot = Some(NextSolverConfig { coherence: true, globally: true });
         }
 
         true
@@ -1867,7 +1878,7 @@ options! {
         "the size at which the `large_assignments` lint starts to be emitted"),
     mutable_noalias: bool = (true, parse_bool, [TRACKED],
         "emit noalias metadata for mutable references (default: yes)"),
-    next_solver: NextSolverConfig = (NextSolverConfig::default(), parse_next_solver_config, [TRACKED],
+    next_solver: Option<NextSolverConfig> = (None, parse_next_solver_config, [TRACKED],
         "enable and configure the next generation trait solver used by rustc"),
     nll_facts: bool = (false, parse_bool, [UNTRACKED],
         "dump facts from NLL analysis into side files (default: no)"),