Rollup merge of #113296 - BoxyUwU:proof_trees_on_error, r=lcnr

add flag for enabling global cache usage for proof trees and printing proof trees on error

This adds a few new things:
- `-Zdump-solver-proof-tree=always/never/on-error`
    - `always`/`never` were previosuly specifiable by whether the flag exists or not, th new flag is `on_error` which reruns obligations of fulfillment and selection errors with proof tree generation enabled and prints them out
- `-Zdump-solver-proof-tree-uses-cache`
    - allows forcing global cache to be used or unused for all generated proof trees, global cache is enabled by default for `always` so that it accurately represents what happend. This flag currently would affect misc uses of `GenerateProofTree::Yes` which will be added in the future for things like diagnostics logic and rustdoc's auto_trait file. We can fix this when we start using proof tree generation for those use cases if it's desirable.

I also changed the output to go straight to stdout instead of going through `debug!` so that `-Zdump-solver-proof-tree` can be adequately used on `nightly` not just a locally built toolchain.

The idea for `on-error` is that it should hopefully make it easier to quickly figure out "why doesnt this code compile"- you just pass in `-Zdump-solver-proof-tree=on-error` and you'll only get proof trees you care about.

---

r? `@lcnr` `@compiler-errors`

2 files changed, 27 insertions, 2 deletions

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 43846e65bca..119eadf6490 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -743,6 +743,14 @@ pub enum TraitSolver {
     NextCoherence,
 }
 
+#[derive(Default, Debug, Copy, Clone, Hash, PartialEq, Eq)]
+pub enum DumpSolverProofTree {
+    Always,
+    OnError,
+    #[default]
+    Never,
+}
+
 pub enum Input {
     /// Load source code from a file.
     File(PathBuf),
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 05d3e71074a..89fd7b4f8df 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -418,6 +418,7 @@ mod desc {
         "a `,` separated combination of `bti`, `b-key`, `pac-ret`, or `leaf`";
     pub const parse_proc_macro_execution_strategy: &str =
         "one of supported execution strategies (`same-thread`, or `cross-thread`)";
+    pub const parse_dump_solver_proof_tree: &str = "one of: `always`, `on-request`, `on-error`";
 }
 
 mod parse {
@@ -1237,6 +1238,19 @@ mod parse {
         };
         true
     }
+
+    pub(crate) fn parse_dump_solver_proof_tree(
+        slot: &mut DumpSolverProofTree,
+        v: Option<&str>,
+    ) -> bool {
+        match v {
+            None | Some("always") => *slot = DumpSolverProofTree::Always,
+            Some("never") => *slot = DumpSolverProofTree::Never,
+            Some("on-error") => *slot = DumpSolverProofTree::OnError,
+            _ => return false,
+        };
+        true
+    }
 }
 
 options! {
@@ -1462,8 +1476,11 @@ options! {
         "output statistics about monomorphization collection"),
     dump_mono_stats_format: DumpMonoStatsFormat = (DumpMonoStatsFormat::Markdown, parse_dump_mono_stats, [UNTRACKED],
         "the format to use for -Z dump-mono-stats (`markdown` (default) or `json`)"),
-    dump_solver_proof_tree: bool = (false, parse_bool, [UNTRACKED],
-        "dump a proof tree for every goal evaluated by the new trait solver"),
+    dump_solver_proof_tree: DumpSolverProofTree = (DumpSolverProofTree::Never, parse_dump_solver_proof_tree, [UNTRACKED],
+        "dump a proof tree for every goal evaluated by the new trait solver. If the flag is specified without any options after it
+        then it defaults to `always`. If the flag is not specified at all it defaults to `on-request`."),
+    dump_solver_proof_tree_use_cache: Option<bool> = (None, parse_opt_bool, [UNTRACKED],
+        "determines whether dumped proof trees use the global cache"),
     dwarf_version: Option<u32> = (None, parse_opt_number, [TRACKED],
         "version of DWARF debug information to emit (default: 2 or 4, depending on platform)"),
     dylib_lto: bool = (false, parse_bool, [UNTRACKED],