Rollup merge of #158516 - kn1g78:codegen-backend-dedupe, r=Kobzol,jieyouxu

 Deduplicate codegen backends in bootstrap config

Deduplicate `rust.codegen-backends` and `target.<triple>.codegen-backends` during bootstrap config parsing while preserving first occurrence order.

This avoids carrying redundant backend entries into later bootstrap planning.

test:
```powershell
$env:CARGO_NET_OFFLINE='false'; $env:CARGO_TARGET_DIR='target-codex-test'; $cargo = Join-Path $env:USERPROFILE '.cargo\bin\cargo.exe'; & $cargo test --manifest-path src/bootstrap/Cargo.toml deduplicates_codegen_backends
```

result:
```text
running 1 test
test core::config::tests::deduplicates_codegen_backends ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 255 filtered out; finished in 0.00s
```

Author: jhpratt

src/bootstrap/src/core/config/tests.rs

@@ -11,6 +11,7 @@ use serde::Deserialize;
 
 use super::flags::Flags;
 use super::toml::change_id::ChangeIdWrapper;
+use super::toml::rust::parse_codegen_backends;
 use super::{Config, RUSTC_IF_UNCHANGED_ALLOWED_PATHS};
 use crate::ChangeId;
 use crate::core::build_steps::clippy::{LintConfig, get_clippy_rules_in_order};
@@ -206,6 +207,15 @@ fn rust_optimize() {
     assert_eq!(parse("rust.optimize = \"s\"").rust_optimize.get_opt_level(), Some("s".to_string()));
 }
 
+#[test]
+#[should_panic(expected = "Duplicate value 'llvm' for 'rust.codegen-backends'")]
+fn rejects_duplicate_codegen_backends() {
+    parse_codegen_backends(
+        vec!["llvm", "llvm", "cranelift"].into_iter().map(str::to_owned).collect(),
+        "rust",
+    );
+}
+
 #[test]
 #[should_panic]
 fn invalid_rust_optimize() {

src/bootstrap/src/core/config/toml/rust.rs

@@ -422,23 +422,33 @@ pub(crate) fn parse_codegen_backends(
                 Please, use '{stripped}' instead."
             )
         }
-        if !BUILTIN_CODEGEN_BACKENDS.contains(&backend.as_str()) {
+        let backend = match backend.as_str() {
+            "llvm" => CodegenBackendKind::Llvm,
+            "cranelift" => CodegenBackendKind::Cranelift,
+            "gcc" => CodegenBackendKind::Gcc,
+            backend => CodegenBackendKind::Custom(backend.to_string()),
+        };
+
+        if found_backends.contains(&backend) {
+            panic!(
+                "Duplicate value '{}' for '{section}.codegen-backends'. \
+                Each codegen backend should only be specified once.",
+                backend.name()
+            );
+        }
+
+        if !BUILTIN_CODEGEN_BACKENDS.contains(&backend.name()) {
             if CiEnv::is_rust_lang_managed_ci_job() {
-                eprintln!("Unknown codegen backend {backend}");
+                eprintln!("Unknown codegen backend {}", backend.name());
                 exit!(1);
             }
 
             println!(
-                "HELP: '{backend}' for '{section}.codegen-backends' might fail. \
-                List of known codegen backends: {BUILTIN_CODEGEN_BACKENDS:?}"
+                "HELP: '{}' for '{section}.codegen-backends' might fail. \
+                List of known codegen backends: {BUILTIN_CODEGEN_BACKENDS:?}",
+                backend.name()
             );
         }
-        let backend = match backend.as_str() {
-            "llvm" => CodegenBackendKind::Llvm,
-            "cranelift" => CodegenBackendKind::Cranelift,
-            "gcc" => CodegenBackendKind::Gcc,
-            backend => CodegenBackendKind::Custom(backend.to_string()),
-        };
         found_backends.push(backend);
     }
     if found_backends.is_empty() {