Make From a lang item

Author: beetrees

compiler/rustc_hir/src/lang_items.rs

@@ -433,6 +433,9 @@ language_item_table! {
     // Experimental lang items for implementing contract pre- and post-condition checking.
     ContractBuildCheckEnsures, sym::contract_build_check_ensures, contract_build_check_ensures_fn, Target::Fn, GenericRequirement::None;
     ContractCheckRequires,     sym::contract_check_requires,      contract_check_requires_fn,      Target::Fn, GenericRequirement::None;
+
+    // Used to fallback `{float}` to `f32` when `f32: From<{float}>`
+    From,                    sym::From,                from_trait,                 Target::Trait,          GenericRequirement::Exact(1);
 }
 
 /// The requirement imposed on the generics of a lang item

compiler/rustc_hir_typeck/src/cast.rs

@@ -43,7 +43,7 @@ use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeAndMut, TypeVisitableExt, VariantDef, elaborate};
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint;
-use rustc_span::{DUMMY_SP, Span, sym};
+use rustc_span::{DUMMY_SP, Span};
 use rustc_trait_selection::infer::InferCtxtExt;
 use tracing::{debug, instrument};
 
@@ -487,7 +487,7 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     let mut label = true;
                     // Check `impl From<self.expr_ty> for self.cast_ty {}` for accurate suggestion:
                     if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr_span)
-                        && let Some(from_trait) = fcx.tcx.get_diagnostic_item(sym::From)
+                        && let Some(from_trait) = fcx.tcx.lang_items().from_trait()
                     {
                         let ty = fcx.resolve_vars_if_possible(self.cast_ty);
                         // Erase regions to avoid panic in `prove_value` when calling

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

@@ -2203,7 +2203,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
     /// within `?` desugaring.
     pub fn is_try_conversion(&self, span: Span, trait_def_id: DefId) -> bool {
         span.is_desugaring(DesugaringKind::QuestionMark)
-            && self.tcx.is_diagnostic_item(sym::From, trait_def_id)
+            && self.tcx.is_lang_item(trait_def_id, LangItem::From)
     }
 
     /// Structurally compares two types, modulo any inference variables.

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -1063,7 +1063,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 && self
                     .infcx
                     .type_implements_trait(
-                        self.tcx.get_diagnostic_item(sym::From).unwrap(),
+                        self.tcx.lang_items().from_trait().unwrap(),
                         [self_ty, expr_ty],
                         obligation.param_env,
                     )
@@ -1122,7 +1122,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             if self
                 .infcx
                 .type_implements_trait(
-                    self.tcx.get_diagnostic_item(sym::From).unwrap(),
+                    self.tcx.lang_items().from_trait().unwrap(),
                     [self_ty, err_ty],
                     obligation.param_env,
                 )

library/core/src/convert/mod.rs

@@ -572,7 +572,7 @@ pub trait Into<T>: Sized {
 /// [`String`]: ../../std/string/struct.String.html
 /// [`from`]: From::from
 /// [book]: ../../book/ch09-00-error-handling.html
-#[rustc_diagnostic_item = "From"]
+#[cfg_attr(not(bootstrap), lang = "From")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_on_unimplemented(on(
     all(_Self = "&str", T = "alloc::string::String"),

src/tools/clippy/clippy_lints/src/fallible_impl_from.rs

@@ -56,7 +56,7 @@ impl<'tcx> LateLintPass<'tcx> for FallibleImplFrom {
             && let Some(impl_trait_ref) = cx.tcx.impl_trait_ref(item.owner_id)
             && cx
                 .tcx
-                .is_diagnostic_item(sym::From, impl_trait_ref.skip_binder().def_id)
+                .is_lang_item(impl_trait_ref.skip_binder().def_id, hir::LangItem::From)
         {
             lint_impl_body(cx, item.span, impl_.items);
         }

src/tools/clippy/clippy_lints/src/unconditional_recursion.rs

@@ -406,7 +406,7 @@ fn check_from(cx: &LateContext<'_>, method_span: Span, method_def_id: LocalDefId
         && let (Some(s1), Some(s2)) = (s1.as_type(), s2.as_type())
         && let Some(trait_def_id) = cx.tcx.trait_of_item(fn_def_id)
         && cx.tcx.is_diagnostic_item(sym::Into, trait_def_id)
-        && get_impl_trait_def_id(cx, method_def_id) == cx.tcx.get_diagnostic_item(sym::From)
+        && get_impl_trait_def_id(cx, method_def_id) == cx.tcx.lang_items().from_trait()
         && s1 == sig.inputs()[0]
         && s2 == sig.output()
     {