Deprecate no-op codegen option -Cinline-threshold=...

This deprecates `-Cinline-threshold` since using it has no effect. This
has been the case since the new LLVM pass manager started being used,
more than 2 years ago.

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -564,9 +564,6 @@ pub(crate) unsafe fn llvm_optimize(
 
     let llvm_plugins = config.llvm_plugins.join(",");
 
-    // FIXME: NewPM doesn't provide a facility to pass custom InlineParams.
-    // We would have to add upstream support for this first, before we can support
-    // config.inline_threshold and our more aggressive default thresholds.
     let result = llvm::LLVMRustOptimize(
         module.module_llvm.llmod(),
         &*module.module_llvm.tm,

compiler/rustc_driver_impl/src/lib.rs

@@ -1136,6 +1136,10 @@ pub fn describe_flag_categories(early_dcx: &EarlyDiagCtxt, matches: &Matches) ->
         early_dcx.early_warn("the -Cno-stack-check flag is deprecated and does nothing");
     }
 
+    if cg_flags.iter().any(|x| x.starts_with("inline-threshold")) {
+        early_dcx.early_warn("the -Cinline-threshold flag is deprecated and does nothing (consider using `-Cllvm-args=--inline-threshold=...`)");
+    }
+
     if cg_flags.iter().any(|x| *x == "passes=list") {
         let backend_name = debug_flags.iter().find_map(|x| x.strip_prefix("codegen-backend="));
 

compiler/rustc_session/src/options.rs

@@ -1485,7 +1485,8 @@ options! {
     incremental: Option<String> = (None, parse_opt_string, [UNTRACKED],
         "enable incremental compilation"),
     inline_threshold: Option<u32> = (None, parse_opt_number, [TRACKED],
-        "set the threshold for inlining a function"),
+        "this option is deprecated and does nothing \
+        (consider using `-Cllvm-args=--inline-threshold=...`)"),
     #[rustc_lint_opt_deny_field_access("use `Session::instrument_coverage` instead of this field")]
     instrument_coverage: InstrumentCoverage = (InstrumentCoverage::No, parse_instrument_coverage, [TRACKED],
         "instrument the generated code to support LLVM source-based code coverage reports \

src/doc/rustc/src/codegen-options/index.md

@@ -184,20 +184,9 @@ incremental files will be stored.
 
 ## inline-threshold
 
-This option lets you set the default threshold for inlining a function. It
-takes an unsigned integer as a value. Inlining is based on a cost model, where
-a higher threshold will allow more inlining.
-
-The default depends on the [opt-level](#opt-level):
-
-| opt-level | Threshold |
-|-----------|-----------|
-| 0         | N/A, only inlines always-inline functions |
-| 1         | N/A, only inlines always-inline functions and LLVM lifetime intrinsics |
-| 2         | 225 |
-| 3         | 275 |
-| s         | 75 |
-| z         | 25 |
+This option is deprecated and does nothing.
+
+Consider using `-Cllvm-args=--inline-threshold=...`.
 
 ## instrument-coverage
 

tests/ui/codegen/issue-82833-slice-miscompile.rs

@@ -1,5 +1,5 @@
 //@ run-pass
-//@ compile-flags: -Ccodegen-units=1 -Cinline-threshold=0 -Clink-dead-code -Copt-level=0 -Cdebuginfo=2
+//@ compile-flags: -Ccodegen-units=1 -Cllvm-args=--inline-threshold=0 -Clink-dead-code -Copt-level=0 -Cdebuginfo=2
 
 // Make sure LLVM does not miscompile this.
 

tests/ui/deprecation/deprecated_inline_threshold.rs

@@ -0,0 +1,4 @@
+//@ check-pass
+//@ compile-flags: -Cinline-threshold=666
+
+fn main() {}

tests/ui/deprecation/deprecated_inline_threshold.stderr

@@ -0,0 +1,2 @@
+warning: the -Cinline-threshold flag is deprecated and does nothing (consider using `-Cllvm-args=--inline-threshold=...`)
+