Auto merge of rust-lang#137030 - matthiaskrgr:rollup-267aumr, r=matthiaskrgr

Rollup of 9 pull requests

Successful merges:

 - rust-lang#135778 (account for `c_enum_min_bits` in `multiple-reprs` UI test)
 - rust-lang#136052 (Correct comment for FreeBSD and DragonFly BSD in unix/thread)
 - rust-lang#136886 (Remove the common prelude module)
 - rust-lang#136956 (add vendor directory to .gitignore)
 - rust-lang#136958 (Fix presentation of purely "additive" replacement suggestion parts)
 - rust-lang#136967 (Use `slice::fill` in `io::Repeat` implementation)
 - rust-lang#136976 (alloc boxed: docs: use MaybeUninit::write instead of as_mut_ptr)
 - rust-lang#137007 (Emit MIR for each bit with on `dont_reset_cast_kind_without_updating_operand`)
 - rust-lang#137008 (Move code into `rustc_mir_transform`)

r? `@ghost`
`@rustbot` modify labels: rollup

Author: bors

.gitignore

@@ -54,6 +54,8 @@ no_llvm_build
 /library/target
 /src/bootstrap/target
 /src/tools/x/target
+# Created by `x vendor`
+/vendor
 # Created by default with `src/ci/docker/run.sh`
 /obj/
 # Created by nix dev shell / .envrc

compiler/rustc_errors/src/emitter.rs

@@ -1976,13 +1976,16 @@ impl HumanEmitter {
             Some(Style::HeaderMsg),
         );
 
+        let other_suggestions = suggestions.len().saturating_sub(MAX_SUGGESTIONS);
+
         let mut row_num = 2;
         for (i, (complete, parts, highlights, _)) in
-            suggestions.iter().enumerate().take(MAX_SUGGESTIONS)
+            suggestions.into_iter().enumerate().take(MAX_SUGGESTIONS)
         {
             debug!(?complete, ?parts, ?highlights);
 
-            let has_deletion = parts.iter().any(|p| p.is_deletion(sm) || p.is_replacement(sm));
+            let has_deletion =
+                parts.iter().any(|p| p.is_deletion(sm) || p.is_destructive_replacement(sm));
             let is_multiline = complete.lines().count() > 1;
 
             if i == 0 {
@@ -2167,7 +2170,7 @@ impl HumanEmitter {
                 self.draw_code_line(
                     &mut buffer,
                     &mut row_num,
-                    highlight_parts,
+                    &highlight_parts,
                     line_pos + line_start,
                     line,
                     show_code_change,
@@ -2213,7 +2216,12 @@ impl HumanEmitter {
             if let DisplaySuggestion::Diff | DisplaySuggestion::Underline | DisplaySuggestion::Add =
                 show_code_change
             {
-                for part in parts {
+                for mut part in parts {
+                    // If this is a replacement of, e.g. `"a"` into `"ab"`, adjust the
+                    // suggestion and snippet to look as if we just suggested to add
+                    // `"b"`, which is typically much easier for the user to understand.
+                    part.trim_trivial_replacements(sm);
+
                     let snippet = if let Ok(snippet) = sm.span_to_snippet(part.span) {
                         snippet
                     } else {
@@ -2376,9 +2384,12 @@ impl HumanEmitter {
                 row_num = row + 1;
             }
         }
-        if suggestions.len() > MAX_SUGGESTIONS {
-            let others = suggestions.len() - MAX_SUGGESTIONS;
-            let msg = format!("and {} other candidate{}", others, pluralize!(others));
+        if other_suggestions > 0 {
+            let msg = format!(
+                "and {} other candidate{}",
+                other_suggestions,
+                pluralize!(other_suggestions)
+            );
             buffer.puts(row_num, max_line_num_len + 3, &msg, Style::NoStyle);
         }
 

compiler/rustc_errors/src/lib.rs

@@ -230,10 +230,40 @@ impl SubstitutionPart {
         !self.snippet.is_empty() && self.replaces_meaningful_content(sm)
     }
 
+    /// Whether this is a replacement that overwrites source with a snippet
+    /// in a way that isn't a superset of the original string. For example,
+    /// replacing "abc" with "abcde" is not destructive, but replacing it
+    /// it with "abx" is, since the "c" character is lost.
+    pub fn is_destructive_replacement(&self, sm: &SourceMap) -> bool {
+        self.is_replacement(sm)
+            && !sm.span_to_snippet(self.span).is_ok_and(|snippet| {
+                self.snippet.trim_start().starts_with(snippet.trim_start())
+                    || self.snippet.trim_end().ends_with(snippet.trim_end())
+            })
+    }
+
     fn replaces_meaningful_content(&self, sm: &SourceMap) -> bool {
         sm.span_to_snippet(self.span)
             .map_or(!self.span.is_empty(), |snippet| !snippet.trim().is_empty())
     }
+
+    /// Try to turn a replacement into an addition when the span that is being
+    /// overwritten matches either the prefix or suffix of the replacement.
+    fn trim_trivial_replacements(&mut self, sm: &SourceMap) {
+        if self.snippet.is_empty() {
+            return;
+        }
+        let Ok(snippet) = sm.span_to_snippet(self.span) else {
+            return;
+        };
+        if self.snippet.starts_with(&snippet) {
+            self.span = self.span.shrink_to_hi();
+            self.snippet = self.snippet[snippet.len()..].to_string();
+        } else if self.snippet.ends_with(&snippet) {
+            self.span = self.span.shrink_to_lo();
+            self.snippet = self.snippet[..self.snippet.len() - snippet.len()].to_string();
+        }
+    }
 }
 
 impl CodeSuggestion {

compiler/rustc_middle/src/mir/mod.rs

@@ -49,7 +49,6 @@ pub mod generic_graphviz;
 pub mod graphviz;
 pub mod interpret;
 pub mod mono;
-pub mod patch;
 pub mod pretty;
 mod query;
 mod statement;

compiler/rustc_mir_dataflow/src/drop_flag_effects.rs

@@ -3,7 +3,26 @@ use rustc_middle::mir::{self, Body, Location, Terminator, TerminatorKind};
 use tracing::debug;
 
 use super::move_paths::{InitKind, LookupResult, MoveData, MovePathIndex};
-use crate::elaborate_drops::DropFlagState;
+
+/// The value of an inserted drop flag.
+#[derive(Debug, PartialEq, Eq, Copy, Clone)]
+pub enum DropFlagState {
+    /// The tracked value is initialized and needs to be dropped when leaving its scope.
+    Present,
+
+    /// The tracked value is uninitialized or was moved out of and does not need to be dropped when
+    /// leaving its scope.
+    Absent,
+}
+
+impl DropFlagState {
+    pub fn value(self) -> bool {
+        match self {
+            DropFlagState::Present => true,
+            DropFlagState::Absent => false,
+        }
+    }
+}
 
 pub fn move_path_children_matching<'tcx, F>(
     move_data: &MoveData<'tcx>,

compiler/rustc_mir_dataflow/src/impls/initialized.rs

@@ -9,7 +9,7 @@ use rustc_middle::ty::util::Discr;
 use rustc_middle::ty::{self, TyCtxt};
 use tracing::{debug, instrument};
 
-use crate::elaborate_drops::DropFlagState;
+use crate::drop_flag_effects::DropFlagState;
 use crate::framework::SwitchIntTarget;
 use crate::move_paths::{HasMoveData, InitIndex, InitKind, LookupResult, MoveData, MovePathIndex};
 use crate::{

compiler/rustc_mir_dataflow/src/lib.rs

@@ -15,7 +15,7 @@ use rustc_middle::ty;
 // Please change the public `use` directives cautiously, as they might be used by external tools.
 // See issue #120130.
 pub use self::drop_flag_effects::{
-    drop_flag_effects_for_function_entry, drop_flag_effects_for_location,
+    DropFlagState, drop_flag_effects_for_function_entry, drop_flag_effects_for_location,
     move_path_children_matching, on_all_children_bits, on_lookup_result_bits,
 };
 pub use self::framework::{
@@ -26,7 +26,6 @@ use self::move_paths::MoveData;
 
 pub mod debuginfo;
 mod drop_flag_effects;
-pub mod elaborate_drops;
 mod errors;
 mod framework;
 pub mod impls;

compiler/rustc_mir_transform/src/add_moves_for_packed_drops.rs

@@ -1,8 +1,8 @@
-use rustc_middle::mir::patch::MirPatch;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, TyCtxt};
 use tracing::debug;
 
+use crate::patch::MirPatch;
 use crate::util;
 
 /// This pass moves values being dropped that are within a packed

compiler/rustc_mir_transform/src/add_subtyping_projections.rs

@@ -1,8 +1,9 @@
-use rustc_middle::mir::patch::MirPatch;
 use rustc_middle::mir::visit::MutVisitor;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 
+use crate::patch::MirPatch;
+
 pub(super) struct Subtyper;
 
 struct SubTypeChecker<'a, 'tcx> {

compiler/rustc_mir_transform/src/coroutine.rs

@@ -1061,9 +1061,8 @@ fn insert_switch<'tcx>(
 }
 
 fn elaborate_coroutine_drops<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
-    use rustc_middle::mir::patch::MirPatch;
-    use rustc_mir_dataflow::elaborate_drops::{Unwind, elaborate_drop};
-
+    use crate::elaborate_drop::{Unwind, elaborate_drop};
+    use crate::patch::MirPatch;
     use crate::shim::DropShimElaborator;
 
     // Note that `elaborate_drops` only drops the upvars of a coroutine, and