Auto merge of #121926 - tgross35:f16-f128-step3-feature-gate, r=compiler-errors,petrochenkov

`f16` and `f128` step 3: compiler support & feature gate

Continuation of https://github.com/rust-lang/rust/pull/121841, another portion of https://github.com/rust-lang/rust/pull/114607

This PR exposes the new types to the world and adds a feature gate. Marking this as a draft because I need some feedback on where I did the feature gate check. It also does not yet catch type via suffixed literals (so the feature gate test will fail, probably some others too because I haven't belssed).

If there is a better place to check all types after resolution, I can do that. If not, I figure maybe I can add a second gate location in AST when it checks numeric suffixes.

Unfortunately I still don't think there is much testing to be done for correctness (codegen tests or parsed value checks) until we have basic library support. I think that will be the next step.

Tracking issue: https://github.com/rust-lang/rust/issues/116909

r? `@compiler-errors`
cc `@Nilstrieb`
`@rustbot` label +F-f16_and_f128

compiler/rustc_resolve/src/ident.rs

@@ -5,8 +5,10 @@ use rustc_middle::bug;
 use rustc_middle::ty;
 use rustc_session::lint::builtin::PROC_MACRO_DERIVE_RESOLUTION_FALLBACK;
 use rustc_session::lint::BuiltinLintDiag;
+use rustc_session::parse::feature_err;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::hygiene::{ExpnId, ExpnKind, LocalExpnId, MacroKind, SyntaxContext};
+use rustc_span::sym;
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::Span;
 
@@ -598,7 +600,35 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                         result
                     }
                     Scope::BuiltinTypes => match this.builtin_types_bindings.get(&ident.name) {
-                        Some(binding) => Ok((*binding, Flags::empty())),
+                        Some(binding) => {
+                            if matches!(ident.name, sym::f16)
+                                && !this.tcx.features().f16
+                                && !ident.span.allows_unstable(sym::f16)
+                                && finalize.is_some()
+                            {
+                                feature_err(
+                                    this.tcx.sess,
+                                    sym::f16,
+                                    ident.span,
+                                    "the type `f16` is unstable",
+                                )
+                                .emit();
+                            }
+                            if matches!(ident.name, sym::f128)
+                                && !this.tcx.features().f128
+                                && !ident.span.allows_unstable(sym::f128)
+                                && finalize.is_some()
+                            {
+                                feature_err(
+                                    this.tcx.sess,
+                                    sym::f128,
+                                    ident.span,
+                                    "the type `f128` is unstable",
+                                )
+                                .emit();
+                            }
+                            Ok((*binding, Flags::empty()))
+                        }
                         None => Err(Determinacy::Determined),
                     },
                 };

compiler/rustc_resolve/src/late.rs

@@ -27,6 +27,7 @@ use rustc_middle::middle::resolve_bound_vars::Set1;
 use rustc_middle::{bug, span_bug};
 use rustc_session::config::{CrateType, ResolveDocLinks};
 use rustc_session::lint;
+use rustc_session::parse::feature_err;
 use rustc_span::source_map::{respan, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{BytePos, Span, SyntaxContext};
@@ -4138,6 +4139,25 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     && PrimTy::from_name(path[0].ident.name).is_some() =>
             {
                 let prim = PrimTy::from_name(path[0].ident.name).unwrap();
+                let tcx = self.r.tcx();
+
+                let gate_err_sym_msg = match prim {
+                    PrimTy::Float(FloatTy::F16) if !tcx.features().f16 => {
+                        Some((sym::f16, "the type `f16` is unstable"))
+                    }
+                    PrimTy::Float(FloatTy::F128) if !tcx.features().f128 => {
+                        Some((sym::f128, "the type `f128` is unstable"))
+                    }
+                    _ => None,
+                };
+
+                if let Some((sym, msg)) = gate_err_sym_msg {
+                    let span = path[0].ident.span;
+                    if !span.allows_unstable(sym) {
+                        feature_err(tcx.sess, sym, span, msg).emit();
+                    }
+                };
+
                 PartialRes::with_unresolved_segments(Res::PrimTy(prim), path.len() - 1)
             }
             PathResult::Module(ModuleOrUniformRoot::Module(module)) => {