Target option to require explicit cpu

Some targets have many different CPUs and no generic CPU that can be
used as a default. For these targets, the user needs to explicitly
specify a CPU through `-C target-cpu=`.

Add an option for targets and an error message if no CPU is set.

This affects the proposed amdgpu and avr targets.

Author: Flakebi

compiler/rustc_codegen_ssa/messages.ftl

@@ -84,6 +84,8 @@ codegen_ssa_incorrect_cgu_reuse_type =
 
 codegen_ssa_insufficient_vs_code_product = VS Code is a different product, and is not sufficient.
 
+codegen_ssa_cpu_required = target requires to explicitly specify a cpu with `-C target-cpu`
+
 codegen_ssa_invalid_link_ordinal_nargs = incorrect number of arguments to `#[link_ordinal]`
     .note = the attribute requires exactly one argument
 

compiler/rustc_codegen_ssa/src/base.rs

@@ -616,6 +616,11 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
         return ongoing_codegen;
     }
 
+    if tcx.sess.target.need_explicit_cpu && tcx.sess.opts.cg.target_cpu.is_none() {
+        // The target has no default cpu, but none is set explicitly
+        tcx.dcx().emit_fatal(errors::CpuRequired);
+    }
+
     let cgu_name_builder = &mut CodegenUnitNameBuilder::new(tcx);
 
     // Run the monomorphization collector and partition the collected items into

compiler/rustc_codegen_ssa/src/errors.rs

@@ -491,6 +491,10 @@ pub(crate) struct CheckInstalledVisualStudio;
 #[diag(codegen_ssa_insufficient_vs_code_product)]
 pub(crate) struct InsufficientVSCodeProduct;
 
+#[derive(Diagnostic)]
+#[diag(codegen_ssa_cpu_required)]
+pub(crate) struct CpuRequired;
+
 #[derive(Diagnostic)]
 #[diag(codegen_ssa_processing_dymutil_failed)]
 #[note]

compiler/rustc_target/src/spec/mod.rs

@@ -2240,6 +2240,9 @@ pub struct TargetOptions {
     /// Default CPU to pass to LLVM. Corresponds to `llc -mcpu=$cpu`. Defaults
     /// to "generic".
     pub cpu: StaticCow<str>,
+    /// Wether a cpu needs to be explicitly set.
+    /// Set to true if there is no default cpu. Defaults to false.
+    pub need_explicit_cpu: bool,
     /// Default target features to pass to LLVM. These features overwrite
     /// `-Ctarget-cpu` but can be overwritten with `-Ctarget-features`.
     /// Corresponds to `llc -mattr=$features`.
@@ -2676,6 +2679,7 @@ impl Default for TargetOptions {
             link_script: None,
             asm_args: cvs![],
             cpu: "generic".into(),
+            need_explicit_cpu: false,
             features: "".into(),
             direct_access_external_data: None,
             dynamic_linking: false,