Remove ParseSess::assume_incomplete_release.

- This field is just a copy of `sess.unstable_opts.assume_incomplete_release`.
- This field has a single use.
- `sess` is also available at that use point

So this commit removes the field and gets the value directly from
`sess`.

Author: nnethercote

compiler/rustc_attr_parsing/src/attributes/cfg.rs

@@ -294,7 +294,7 @@ pub fn eval_config_entry(sess: &Session, cfg_entry: &CfgEntry) -> EvalConfigResu
                 };
             };
             // See https://github.com/rust-lang/rust/issues/64796#issuecomment-640851454 for details
-            let min_version_ok = if sess.psess.assume_incomplete_release {
+            let min_version_ok = if sess.opts.unstable_opts.assume_incomplete_release {
                 RustcVersion::current_overridable() > *min_version
             } else {
                 RustcVersion::current_overridable() >= *min_version

compiler/rustc_session/src/parse.rs

@@ -264,8 +264,6 @@ pub struct ParseSess {
     pub ambiguous_block_expr_parse: Lock<FxIndexMap<Span, Span>>,
     pub gated_spans: GatedSpans,
     pub symbol_gallery: SymbolGallery,
-    /// Whether cfg(version) should treat the current release as incomplete
-    pub assume_incomplete_release: bool,
     /// Spans passed to `proc_macro::quote_span`. Each span has a numerical
     /// identifier represented by its position in the vector.
     proc_macro_quoted_spans: AppendOnlyVec<Span>,
@@ -299,7 +297,6 @@ impl ParseSess {
             ambiguous_block_expr_parse: Lock::new(Default::default()),
             gated_spans: GatedSpans::default(),
             symbol_gallery: SymbolGallery::default(),
-            assume_incomplete_release: false,
             proc_macro_quoted_spans: Default::default(),
             attr_id_generator: AttrIdGenerator::new(),
         }

compiler/rustc_session/src/session.rs

@@ -1045,8 +1045,7 @@ pub fn build_session(
         None
     };
 
-    let mut psess = ParseSess::with_dcx(dcx, source_map);
-    psess.assume_incomplete_release = sopts.unstable_opts.assume_incomplete_release;
+    let psess = ParseSess::with_dcx(dcx, source_map);
 
     let host_triple = config::host_tuple();
     let target_triple = sopts.target_triple.tuple();