move no-std possibility from previous rfc

Author: davidtwco

text/3875-build-std-explicit-dependencies.md

@@ -228,6 +228,7 @@ files ([?][rationale-cargo-lock]).
 
 *See the following sections for future possibilities:*
 
+- [*Replace `#![no_std]` as the source-of-truth for whether a crate depends on `std`*][future-replace-no_std]
 - [*Allow unstable crate names to be referenced behind cfgs without requiring nightly*][future-cfg-unstable-crate-name]
 - [*Allow `builtin` source replacement*][future-source-replacement]
 - [*Remove `rustc_dep_of_std`*][future-rustc_dep_of_std]
@@ -1041,6 +1042,33 @@ of the build-std context RFC.
 
 There are many possible follow-ups to this part of the RFC:
 
+## Replace `#![no_std]` as the source-of-truth for whether a crate depends on `std`
+[future-replace-no_std]: #replace-no_std-as-the-source-of-truth-for-whether-a-crate-depends-on-std
+
+Crates can currently use the crate attribute `#![no_std]` to indicate a lack of
+dependency on `std`. Introducing `build-std-crates` from [RFC #3874][rfcs#3874]
+or explicit dependencies would add a second way for the user to indicate a lack
+of dependency on the standard library. It could therefore be desirable to
+deprecate `#![no_std]` so that there remains only a single way to express a
+dependency on the standard library.
+
+`#![no_std]` serves two purposes - it stops the compiler from adding `std` to
+the extern prelude and it prevents the user from depending on anything from
+`std` accidentally. rustc's default behaviour of loading `std` when not
+explicitly provided the crate via an `--extern` flag should be preserved for
+backwards-compatibility with existing direct invocations of rustc.
+
+`#![no_std]` could instead become a compiler flag which would indicate to the
+compiler that `std` should not be loaded by default and that `core`'s prelude
+should be used instead. Cargo would use this flag when driving rustc, providing
+explicit paths to the newly-built or pre-built standard library crates, just as
+with any other dependency.
+
+In addition, `#![no_std]` could be migrated to denying a lint which would
+prevent use of items from `std`.
+
+↩ [*Proposal*][proposal]
+
 ## Allow unstable crate names to be referenced behind cfgs without requiring nightly
 [future-cfg-unstable-crate-name]: #allow-unstable-crate-names-to-be-referenced-behind-cfgs-without-requiring-nightly