Rollup merge of #64698 - Centril:infer-const-with-stash, r=estebank

Recover on `const X = 42;` and infer type + Error Stash API

Here we:

1. Introduce a notion of the "error stash".

   This is a map in the `Handler` to which you can `err.stash(...)` away your diagnostics and then steal them in a later "phase" of the compiler (e.g. stash in parser, steal in typeck) to enrich them with more information that isn't available in the previous "phase".

    I believe I've covered all the bases to make sure these diagnostics are actually emitted eventually even under `#[cfg(FALSE)]` but please check my logic.

2. Recover when parsing `[const | static mut?] $ident = $expr;` which has a missing type.

    Use the "error stash" to stash away the error and later steal the error in typeck where we emit the error as `MachineApplicable` with the actual inferred type. This builds on https://github.com/rust-lang/rust/pull/62804.

cc https://github.com/rust-lang/rfcs/pull/2545

r? @estebank

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libsyntax_ext/format.rs b/src/libsyntax_ext/format.rs
index 26455df17b8..2765346b333 100644
--- a/src/libsyntax_ext/format.rs
+++ b/src/libsyntax_ext/format.rs
@@ -295,7 +295,7 @@ impl<'a, 'b> Context<'a, 'b> {
             .filter(|fmt| fmt.precision_span.is_some())
             .count();
         if self.names.is_empty() && !numbered_position_args && count != self.args.len() {
-            e = self.ecx.mut_span_err(
+            e = self.ecx.struct_span_err(
                 sp,
                 &format!(
                     "{} positional argument{} in format string, but {}",
@@ -336,7 +336,7 @@ impl<'a, 'b> Context<'a, 'b> {
                 sp = MultiSpan::from_span(self.fmtsp);
             }
 
-            e = self.ecx.mut_span_err(sp,
+            e = self.ecx.struct_span_err(sp,
                 &format!("invalid reference to positional {} ({})",
                          arg_list,
                          self.describe_num_args()));