Auto merge of #77354 - ecstatic-morse:const-checking-moar-errors, r=oli-obk

Overhaul const-checking diagnostics

The primary purpose of this PR was to remove `NonConstOp::STOPS_CONST_CHECKING`, which causes any additional errors found by the const-checker to be silenced. I used this flag to preserve diagnostic parity with `qualify_min_const_fn.rs`, which has since been removed.

However, simply removing the flag caused a deluge of errors in some cases, since an error would be emitted any time a local or temporary had a wrong type. To remedy this, I added an alternative system (`DiagnosticImportance`) to silence additional error messages that were likely to distract the user from the underlying issue. When an error of the highest importance occurs, all less important errors are silenced. When no error of the highest importance occurs, all less important errors are emitted after checking is complete. Following the suggestions from the important error is usually enough to fix the less important errors, so this should lead to better UX most of the time.

There's also some unrelated diagnostics improvements in this PR isolated in their own commits. Splitting them out would be possible, but a bit of a pain. This isn't as tidy as some of my other PRs, but it should *only* affect diagnostics, never whether or not something passes const-checking. Note that there are a few trivial exceptions to this, like banning `Yield` in all const-contexts, not just `const fn`.

As always, meant to be reviewed commit-by-commit.

r? `@oli-obk`

Author: bors

compiler/rustc_error_codes/src/error_codes.rs

@@ -18,7 +18,6 @@ E0010: include_str!("./error_codes/E0010.md"),
 E0013: include_str!("./error_codes/E0013.md"),
 E0014: include_str!("./error_codes/E0014.md"),
 E0015: include_str!("./error_codes/E0015.md"),
-E0019: include_str!("./error_codes/E0019.md"),
 E0023: include_str!("./error_codes/E0023.md"),
 E0025: include_str!("./error_codes/E0025.md"),
 E0026: include_str!("./error_codes/E0026.md"),
@@ -463,6 +462,7 @@ E0776: include_str!("./error_codes/E0776.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard
+//  E0019, merged into E0015
 //  E0035, merged into E0087/E0089
 //  E0036, merged into E0087/E0089
 //  E0068,

compiler/rustc_error_codes/src/error_codes/E0019.md

@@ -670,7 +670,7 @@ impl Atom for Local {
 }
 
 /// Classifies locals into categories. See `Body::local_kind`.
-#[derive(PartialEq, Eq, Debug, HashStable)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug, HashStable)]
 pub enum LocalKind {
     /// User-declared variable binding.
     Var,

compiler/rustc_middle/src/mir/mod.rs

@@ -57,6 +57,16 @@ impl ConstCx<'mir, 'tcx> {
             && self.tcx.features().staged_api
             && is_const_stable_const_fn(self.tcx, self.def_id.to_def_id())
     }
+
+    /// Returns the function signature of the item being const-checked if it is a `fn` or `const fn`.
+    pub fn fn_sig(&self) -> Option<&'tcx hir::FnSig<'tcx>> {
+        // Get this from the HIR map instead of a query to avoid cycle errors.
+        //
+        // FIXME: Is this still an issue?
+        let hir_map = self.tcx.hir();
+        let hir_id = hir_map.local_def_id_to_hir_id(self.def_id);
+        hir_map.fn_sig_by_hir_id(hir_id)
+    }
 }
 
 /// Returns `true` if this `DefId` points to one of the official `panic` lang items.