Require sram target feature on AVR

Author: taiki-e

compiler/rustc_target/src/target_features.rs

@@ -902,7 +902,7 @@ static AVR_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("rmw", Unstable(sym::avr_target_feature), &[]),
     ("spm", Unstable(sym::avr_target_feature), &[]),
     ("spmx", Unstable(sym::avr_target_feature), &[]),
-    ("sram", Unstable(sym::avr_target_feature), &[]),
+    ("sram", Forbidden { reason: "minimum requirement for Rust/C/C++" }, &[]),
     ("tinyencoding", Unstable(sym::avr_target_feature), &[]),
     // tidy-alphabetical-end
 ];
@@ -1239,6 +1239,12 @@ impl Target {
                 // because the vector and float registers overlap.
                 FeatureConstraints { required: &[], incompatible: &["soft-float"] }
             }
+            Arch::Avr => {
+                // SRAM is minimum requirement for C/C++ in both avr-gcc and clang,
+                // and backends of them only support assembly for devices have no SRAM.
+                // See the discussion in https://github.com/rust-lang/rust/pull/146900 for more.
+                FeatureConstraints { required: &["sram"], incompatible: &[] }
+            }
             _ => NOTHING,
         }
     }

src/doc/rustc/src/platform-support/avr-none.md

@@ -68,6 +68,8 @@ the possible variants:
 
 https://github.com/llvm/llvm-project/blob/093d4db2f3c874d4683fb01194b00dbb20e5c713/clang/lib/Basic/Targets/AVR.cpp#L32
 
+Note that devices that have no SRAM are not supported, same as when compiling C/C++ programs with avr-gcc or Clang.
+
 ## Testing
 
 You can use [`simavr`](https://github.com/buserror/simavr) to emulate the

tests/ui/abi/avr-sram.no_sram.stderr

@@ -0,0 +1,7 @@
+warning: target feature `sram` must be enabled to ensure that the ABI of the current target can be implemented correctly
+   |
+   = note: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #116344 <https://github.com/rust-lang/rust/issues/116344>
+
+warning: 1 warning emitted
+

tests/ui/abi/avr-sram.rs

@@ -0,0 +1,16 @@
+//@ add-minicore
+//@ revisions: has_sram no_sram
+//@ build-pass
+//@[has_sram] compile-flags: --target avr-none -C target-cpu=atmega328p
+//@[has_sram] needs-llvm-components: avr
+//@[no_sram] compile-flags: --target avr-none -C target-cpu=attiny11
+//@[no_sram] needs-llvm-components: avr
+//@ ignore-backends: gcc
+//[no_sram]~? WARN target feature `sram` must be enabled to ensure that the ABI of the current target can be implemented correctly
+
+#![feature(no_core)]
+#![no_core]
+#![crate_type = "lib"]
+
+extern crate minicore;
+use minicore::*;