Add `Mutability::ref_prefix_str`, order `Mutability`, simplify code

8 files changed, 51 insertions, 97 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 54bd25d6471..0567bb5a6c9 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -775,8 +775,9 @@ pub enum PatKind {
 #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, Copy)]
 #[derive(HashStable_Generic, Encodable, Decodable)]
 pub enum Mutability {
-    Mut,
+    // N.B. Order is deliberate, so that Not < Mut
     Not,
+    Mut,
 }
 
 impl Mutability {
@@ -787,12 +788,21 @@ impl Mutability {
         }
     }
 
+    /// Returns `""` (empty string) or `"mut "` depending on the mutability.
     pub fn prefix_str(&self) -> &'static str {
         match self {
             Mutability::Mut => "mut ",
             Mutability::Not => "",
         }
     }
+
+    /// Returns `"&"` or `"&mut "` depending on the mutability.
+    pub fn ref_prefix_str(&self) -> &'static str {
+        match self {
+            Mutability::Not => "&",
+            Mutability::Mut => "&mut ",
+        }
+    }
 }
 
 /// The kind of borrow in an `AddrOf` expression,
diff --git a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
index 5f99d86b4ea..018f606db64 100644
--- a/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs
@@ -571,11 +571,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 let moved_place = &self.move_data.move_paths[move_out.path].place;
                 let move_spans = self.move_spans(moved_place.as_ref(), move_out.source);
                 let move_span = move_spans.args_or_use();
-                let suggestion = if borrow_level == hir::Mutability::Mut {
-                    "&mut ".to_string()
-                } else {
-                    "&".to_string()
-                };
+                let suggestion = borrow_level.ref_prefix_str().to_owned();
                 (move_span.shrink_to_lo(), suggestion)
             })
             .collect();
diff --git a/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs b/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
index 7457369aa58..5122f9808ed 100644
--- a/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs
@@ -389,13 +389,13 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             // diagnostic: if the span starts with a mutable borrow of
             // a local variable, then just suggest the user remove it.
             PlaceRef { local: _, projection: [] }
-                if {
-                    if let Ok(snippet) = self.infcx.tcx.sess.source_map().span_to_snippet(span) {
-                        snippet.starts_with("&mut ")
-                    } else {
-                        false
-                    }
-                } =>
+                if self
+                    .infcx
+                    .tcx
+                    .sess
+                    .source_map()
+                    .span_to_snippet(span)
+                    .map_or(false, |snippet| snippet.starts_with("&mut ")) =>
             {
                 err.span_label(span, format!("cannot {ACT}", ACT = act));
                 err.span_suggestion(
diff --git a/compiler/rustc_hir_typeck/src/demand.rs b/compiler/rustc_hir_typeck/src/demand.rs
index 934d1240442..13d7ab372a4 100644
--- a/compiler/rustc_hir_typeck/src/demand.rs
+++ b/compiler/rustc_hir_typeck/src/demand.rs
@@ -19,6 +19,7 @@ use rustc_trait_selection::traits::ObligationCause;
 
 use super::method::probe;
 
+use std::cmp::min;
 use std::iter;
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -937,51 +938,24 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     && let Ok(src) = sm.span_to_snippet(sp)
                 {
                     let derefs = "*".repeat(steps);
-                    if let Some((span, src, applicability)) = match mutbl_b {
-                        hir::Mutability::Mut => {
-                            let new_prefix = "&mut ".to_owned() + &derefs;
-                            match mutbl_a {
-                                hir::Mutability::Mut => {
-                                    replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
-                                        let pos = sp.lo() + BytePos(5);
-                                        let sp = sp.with_lo(pos).with_hi(pos);
-                                        (sp, derefs, Applicability::MachineApplicable)
-                                    })
-                                }
-                                hir::Mutability::Not => {
-                                    replace_prefix(&src, "&", &new_prefix).map(|_| {
-                                        let pos = sp.lo() + BytePos(1);
-                                        let sp = sp.with_lo(pos).with_hi(pos);
-                                        (
-                                            sp,
-                                            format!("mut {derefs}"),
-                                            Applicability::Unspecified,
-                                        )
-                                    })
-                                }
-                            }
-                        }
-                        hir::Mutability::Not => {
-                            let new_prefix = "&".to_owned() + &derefs;
-                            match mutbl_a {
-                                hir::Mutability::Mut => {
-                                    replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
-                                        let lo = sp.lo() + BytePos(1);
-                                        let hi = sp.lo() + BytePos(5);
-                                        let sp = sp.with_lo(lo).with_hi(hi);
-                                        (sp, derefs, Applicability::MachineApplicable)
-                                    })
-                                }
-                                hir::Mutability::Not => {
-                                    replace_prefix(&src, "&", &new_prefix).map(|_| {
-                                        let pos = sp.lo() + BytePos(1);
-                                        let sp = sp.with_lo(pos).with_hi(pos);
-                                        (sp, derefs, Applicability::MachineApplicable)
-                                    })
-                                }
-                            }
-                        }
-                    } {
+                    let old_prefix = mutbl_a.ref_prefix_str();
+                    let new_prefix = mutbl_b.ref_prefix_str().to_owned() + &derefs;
+
+                    let suggestion = replace_prefix(&src, old_prefix, &new_prefix).map(|_| {
+                        // skip `&` or `&mut ` if both mutabilities are mutable
+                        let lo = sp.lo() + BytePos(min(old_prefix.len(), mutbl_b.ref_prefix_str().len()) as _);
+                        // skip `&` or `&mut `
+                        let hi = sp.lo() + BytePos(old_prefix.len() as _);
+                        let sp = sp.with_lo(lo).with_hi(hi);
+
+                        (
+                            sp,
+                            format!("{}{derefs}", if mutbl_a != mutbl_b { mutbl_b.prefix_str() } else { "" }),
+                            if mutbl_b <= mutbl_a { Applicability::MachineApplicable } else { Applicability::MaybeIncorrect }
+                        )
+                    });
+
+                    if let Some((span, src, applicability)) = suggestion {
                         return Some((
                             span,
                             "consider dereferencing".to_string(),
@@ -1005,10 +979,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             // If the expression has `&`, removing it would fix the error
                             prefix_span = prefix_span.with_hi(inner.span.lo());
                             expr = inner;
-                            remove += match mutbl {
-                                hir::Mutability::Not => "&",
-                                hir::Mutability::Mut => "&mut ",
-                            };
+                            remove.push_str(mutbl.ref_prefix_str());
                             steps -= 1;
                         } else {
                             break;
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
index b5aa8cd6e7c..1780226b8e0 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs
@@ -345,8 +345,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
             if annotation {
                 let suggest_annotation = match expr.peel_drop_temps().kind {
-                    hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, _) => "&",
-                    hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, _) => "&mut ",
+                    hir::ExprKind::AddrOf(hir::BorrowKind::Ref, mutbl, _) => mutbl.ref_prefix_str(),
                     _ => return true,
                 };
                 let mut tuple_indexes = Vec::new();
diff --git a/compiler/rustc_hir_typeck/src/method/prelude2021.rs b/compiler/rustc_hir_typeck/src/method/prelude2021.rs
index 89746ce54a6..dea14dd93d6 100644
--- a/compiler/rustc_hir_typeck/src/method/prelude2021.rs
+++ b/compiler/rustc_hir_typeck/src/method/prelude2021.rs
@@ -5,7 +5,6 @@ use crate::{
 use hir::def_id::DefId;
 use hir::HirId;
 use hir::ItemKind;
-use rustc_ast::Mutability;
 use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
@@ -88,14 +87,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     let derefs = "*".repeat(pick.autoderefs);
 
                     let autoref = match pick.autoref_or_ptr_adjustment {
-                        Some(probe::AutorefOrPtrAdjustment::Autoref {
-                            mutbl: Mutability::Mut,
-                            ..
-                        }) => "&mut ",
-                        Some(probe::AutorefOrPtrAdjustment::Autoref {
-                            mutbl: Mutability::Not,
-                            ..
-                        }) => "&",
+                        Some(probe::AutorefOrPtrAdjustment::Autoref { mutbl, .. }) => {
+                            mutbl.ref_prefix_str()
+                        }
                         Some(probe::AutorefOrPtrAdjustment::ToConstPtr) | None => "",
                     };
                     if let Ok(self_expr) = self.sess().source_map().span_to_snippet(self_expr.span)
@@ -386,8 +380,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let derefs = "*".repeat(pick.autoderefs);
 
         let autoref = match pick.autoref_or_ptr_adjustment {
-            Some(probe::AutorefOrPtrAdjustment::Autoref { mutbl: Mutability::Mut, .. }) => "&mut ",
-            Some(probe::AutorefOrPtrAdjustment::Autoref { mutbl: Mutability::Not, .. }) => "&",
+            Some(probe::AutorefOrPtrAdjustment::Autoref { mutbl, .. }) => mutbl.ref_prefix_str(),
             Some(probe::AutorefOrPtrAdjustment::ToConstPtr) | None => "",
         };
 
diff --git a/compiler/rustc_hir_typeck/src/method/suggest.rs b/compiler/rustc_hir_typeck/src/method/suggest.rs
index d0ea2b0e664..09101558995 100644
--- a/compiler/rustc_hir_typeck/src/method/suggest.rs
+++ b/compiler/rustc_hir_typeck/src/method/suggest.rs
@@ -1147,19 +1147,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 && assoc.kind == ty::AssocKind::Fn
             {
                 let sig = self.tcx.fn_sig(assoc.def_id);
-                if let Some(first) = sig.inputs().skip_binder().get(0) {
-                    if first.peel_refs() == rcvr_ty.peel_refs() {
-                        None
-                    } else {
-                        Some(if first.is_region_ptr() {
-                            if first.is_mutable_ptr() { "&mut " } else { "&" }
-                        } else {
-                            ""
-                        })
-                    }
-                } else {
+                sig.inputs().skip_binder().get(0).and_then(|first| if first.peel_refs() == rcvr_ty.peel_refs() {
                     None
-                }
+                } else {
+                    Some(first.ref_mutability().map_or("", |mutbl| mutbl.ref_prefix_str()))
+                })
             } else {
                 None
             };
@@ -2625,11 +2617,7 @@ fn print_disambiguation_help<'tcx>(
     let (span, sugg) = if let (ty::AssocKind::Fn, Some((receiver, args))) = (kind, args) {
         let args = format!(
             "({}{})",
-            if rcvr_ty.is_region_ptr() {
-                if rcvr_ty.is_mutable_ptr() { "&mut " } else { "&" }
-            } else {
-                ""
-            },
+            rcvr_ty.ref_mutability().map_or("", |mutbl| mutbl.ref_prefix_str()),
             std::iter::once(receiver)
                 .chain(args.iter())
                 .map(|arg| source_map.span_to_snippet(arg.span).unwrap_or_else(|_| {
diff --git a/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs b/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
index 7b3178e610f..31235ce1918 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
@@ -15,7 +15,7 @@ use rustc_hir::intravisit::{self, Visitor};
 use rustc_hir::{Body, Closure, Expr, ExprKind, FnRetTy, HirId, Local, LocalSource};
 use rustc_middle::hir::nested_filter;
 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
-use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow, AutoBorrowMutability};
+use rustc_middle::ty::adjustment::{Adjust, Adjustment, AutoBorrow};
 use rustc_middle::ty::print::{FmtPrinter, PrettyPrinter, Print, Printer};
 use rustc_middle::ty::{self, DefIdTree, InferConst};
 use rustc_middle::ty::{GenericArg, GenericArgKind, SubstsRef};
@@ -508,10 +508,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     [
                         ..,
                         Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mut_)), target: _ },
-                    ] => match mut_ {
-                        AutoBorrowMutability::Mut { .. } => "&mut ",
-                        AutoBorrowMutability::Not => "&",
-                    },
+                    ] => hir::Mutability::from(*mut_).ref_prefix_str(),
                     _ => "",
                 };