Rollup merge of rust-lang#147873 - RalfJung:deduce_param_attrs, r=tmiasko

comments for deduce_param_attrs

Cc `@saethlin` since IIRC you experimented with codegen doing post-mono MIR ops? That seems to be in conflict with this pass.
Cc `@tmiasko`

r? `@scottmcm`

Author: matthiaskrgr

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -180,6 +180,9 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     let llfn = cx.get_fn(instance);
 
     let mut mir = tcx.instance_mir(instance.def);
+    // Note that the ABI logic has deduced facts about the functions' parameters based on the MIR we
+    // got here (`deduce_param_attrs`). That means we can *not* apply arbitrary further MIR
+    // transforms as that may invalidate those deduced facts!
 
     let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
     debug!("fn_abi: {:?}", fn_abi);
@@ -317,6 +320,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     }
 }
 
+/// Replace `clone` calls that come from `use` statements with direct copies if possible.
 // FIXME: Move this function to mir::transform when post-mono MIR passes land.
 fn optimize_use_clone<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     cx: &'a Bx::CodegenCx,

compiler/rustc_middle/src/middle/deduced_param_attrs.rs

@@ -3,7 +3,7 @@ use rustc_macros::{Decodable, Encodable, HashStable};
 use crate::ty::{Ty, TyCtxt, TypingEnv};
 
 /// Flags that dictate how a parameter is mutated. If the flags are empty, the param is
-/// read-only. If non-empty, it is read-only with conditions.
+/// read-only. If non-empty, it is read-only if *all* flags' conditions are met.
 #[derive(Clone, Copy, PartialEq, Debug, Decodable, Encodable, HashStable)]
 pub struct DeducedReadOnlyParam(u8);
 
@@ -53,6 +53,7 @@ impl DeducedParamAttrs {
         ty: Ty<'tcx>,
     ) -> bool {
         let read_only = self.read_only;
+        // We have to check *all* set bits; only if all checks pass is this truly read-only.
         if read_only.contains(DeducedReadOnlyParam::MUTATED) {
             return false;
         }

compiler/rustc_mir_transform/src/deduce_param_attrs.rs

@@ -4,6 +4,10 @@
 //! body of the function instead of just the signature. These can be useful for optimization
 //! purposes on a best-effort basis. We compute them here and store them into the crate metadata so
 //! dependent crates can use them.
+//!
+//! Note that this *crucially* relies on codegen *not* doing any more MIR-level transformations
+//! after `optimized_mir`! We check for things that are *not* guaranteed to be preserved by MIR
+//! transforms, such as which local variables happen to be mutated.
 
 use rustc_hir::def_id::LocalDefId;
 use rustc_index::IndexVec;