Auto merge of #118881 - matthiaskrgr:rollup-0rl3tir, r=matthiaskrgr

Rollup of 9 pull requests

Successful merges:

 - #116740 (dont ICE when ConstKind::Expr for is_const_evaluatable)
 - #117914 (On borrow return type, suggest borrowing from arg or owned return type)
 - #117927 (Clarify how to choose a FutureIncompatibilityReason variant.)
 - #118855 (Improve an error involving attribute values.)
 - #118856 (rustdoc-search: clean up parser)
 - #118865 (rustc_codegen_llvm: Enforce `rustc::potential_query_instability` lint)
 - #118866 (llvm-wrapper: adapt for LLVM API change)
 - #118868 (Correctly gate the parsing of match arms without body)
 - #118877 (tests: CGU tests require build-pass, not check-pass (remove FIXME))

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

14 files changed, 466 insertions, 53 deletions

diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 190fae95652..5755ae8a8bc 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -676,6 +676,19 @@ impl Pat {
         });
         could_be_never_pattern
     }
+
+    /// Whether this contains a `!` pattern. This in particular means that a feature gate error will
+    /// be raised if the feature is off. Used to avoid gating the feature twice.
+    pub fn contains_never_pattern(&self) -> bool {
+        let mut contains_never_pattern = false;
+        self.walk(&mut |pat| {
+            if matches!(pat.kind, PatKind::Never) {
+                contains_never_pattern = true;
+            }
+            true
+        });
+        contains_never_pattern
+    }
 }
 
 /// A single field in a struct pattern.
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index e9f88d50937..a44b408feec 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -581,8 +581,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         } else {
             // Either `body.is_none()` or `is_never_pattern` here.
             if !is_never_pattern {
-                let suggestion = span.shrink_to_hi();
-                self.tcx.sess.emit_err(MatchArmWithNoBody { span, suggestion });
+                if self.tcx.features().never_patterns {
+                    // If the feature is off we already emitted the error after parsing.
+                    let suggestion = span.shrink_to_hi();
+                    self.tcx.sess.emit_err(MatchArmWithNoBody { span, suggestion });
+                }
             } else if let Some(body) = &arm.body {
                 self.tcx.sess.emit_err(NeverPatternWithBody { span: body.span });
                 guard = None;
diff --git a/compiler/rustc_ast_passes/messages.ftl b/compiler/rustc_ast_passes/messages.ftl
index 876126b02ea..28bd6c2111b 100644
--- a/compiler/rustc_ast_passes/messages.ftl
+++ b/compiler/rustc_ast_passes/messages.ftl
@@ -174,6 +174,10 @@ ast_passes_item_underscore = `{$kind}` items in this context need a name
 ast_passes_keyword_lifetime =
     lifetimes cannot use keyword names
 
+ast_passes_match_arm_with_no_body =
+    `match` arm with no body
+    .suggestion = add a body after the pattern
+
 ast_passes_module_nonascii = trying to load file for module `{$name}` with non-ascii identifier name
     .help = consider using the `#[path]` attribute to specify filesystem path
 
diff --git a/compiler/rustc_ast_passes/src/errors.rs b/compiler/rustc_ast_passes/src/errors.rs
index 7f6fcb49317..928bf19759a 100644
--- a/compiler/rustc_ast_passes/src/errors.rs
+++ b/compiler/rustc_ast_passes/src/errors.rs
@@ -758,3 +758,12 @@ pub struct AnonStructOrUnionNotAllowed {
     pub span: Span,
     pub struct_or_union: &'static str,
 }
+
+#[derive(Diagnostic)]
+#[diag(ast_passes_match_arm_with_no_body)]
+pub struct MatchArmWithNoBody {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = " => todo!(),", applicability = "has-placeholders")]
+    pub suggestion: Span,
+}
diff --git a/compiler/rustc_ast_passes/src/feature_gate.rs b/compiler/rustc_ast_passes/src/feature_gate.rs
index 897f35a5c4a..ac55c6cabd0 100644
--- a/compiler/rustc_ast_passes/src/feature_gate.rs
+++ b/compiler/rustc_ast_passes/src/feature_gate.rs
@@ -554,7 +554,34 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(explicit_tail_calls, "`become` expression is experimental");
     gate_all!(generic_const_items, "generic const items are experimental");
     gate_all!(unnamed_fields, "unnamed fields are not yet fully implemented");
-    gate_all!(never_patterns, "`!` patterns are experimental");
+
+    if !visitor.features.never_patterns {
+        if let Some(spans) = spans.get(&sym::never_patterns) {
+            for &span in spans {
+                if span.allows_unstable(sym::never_patterns) {
+                    continue;
+                }
+                let sm = sess.source_map();
+                // We gate two types of spans: the span of a `!` pattern, and the span of a
+                // match arm without a body. For the latter we want to give the user a normal
+                // error.
+                if let Ok(snippet) = sm.span_to_snippet(span)
+                    && snippet == "!"
+                {
+                    feature_err(
+                        &sess.parse_sess,
+                        sym::never_patterns,
+                        span,
+                        "`!` patterns are experimental",
+                    )
+                    .emit();
+                } else {
+                    let suggestion = span.shrink_to_hi();
+                    sess.emit_err(errors::MatchArmWithNoBody { span, suggestion });
+                }
+            }
+        }
+    }
 
     if !visitor.features.negative_bounds {
         for &span in spans.get(&sym::negative_bounds).iter().copied().flatten() {
diff --git a/compiler/rustc_codegen_llvm/src/back/lto.rs b/compiler/rustc_codegen_llvm/src/back/lto.rs
index abc33a04598..97444f1a5bd 100644
--- a/compiler/rustc_codegen_llvm/src/back/lto.rs
+++ b/compiler/rustc_codegen_llvm/src/back/lto.rs
@@ -816,6 +816,9 @@ impl ThinLTOKeysMap {
         use std::io::Write;
         let file = File::create(path)?;
         let mut writer = io::BufWriter::new(file);
+        // The entries are loaded back into a hash map in `load_from_file()`, so
+        // the order in which we write them to file here does not matter.
+        #[allow(rustc::potential_query_instability)]
         for (module, key) in &self.keys {
             writeln!(writer, "{module} {key}")?;
         }
diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
index 51df14df644..33bfde03a31 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
@@ -58,6 +58,11 @@ pub fn finalize(cx: &CodegenCx<'_, '_>) {
         return;
     }
 
+    // The entries of the map are only used to get a list of all files with
+    // coverage info. In the end the list of files is passed into
+    // `GlobalFileTable::new()` which internally do `.sort_unstable_by()`, so
+    // the iteration order here does not matter.
+    #[allow(rustc::potential_query_instability)]
     let function_coverage_entries = function_coverage_map
         .into_iter()
         .map(|(instance, function_coverage)| (instance, function_coverage.into_finished()))
diff --git a/compiler/rustc_codegen_llvm/src/lib.rs b/compiler/rustc_codegen_llvm/src/lib.rs
index 33b19ab362a..116108ae5a9 100644
--- a/compiler/rustc_codegen_llvm/src/lib.rs
+++ b/compiler/rustc_codegen_llvm/src/lib.rs
@@ -17,7 +17,6 @@
 #![feature(never_type)]
 #![feature(impl_trait_in_assoc_type)]
 #![recursion_limit = "256"]
-#![allow(rustc::potential_query_instability)]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]
 
diff --git a/compiler/rustc_lint_defs/src/lib.rs b/compiler/rustc_lint_defs/src/lib.rs
index de0abe04611..a25cfe68e0d 100644
--- a/compiler/rustc_lint_defs/src/lib.rs
+++ b/compiler/rustc_lint_defs/src/lib.rs
@@ -345,12 +345,34 @@ pub struct FutureIncompatibleInfo {
 }
 
 /// The reason for future incompatibility
+///
+/// Future-incompatible lints come in roughly two categories:
+///
+/// 1. There was a mistake in the compiler (such as a soundness issue), and
+///    we're trying to fix it, but it may be a breaking change.
+/// 2. A change across an Edition boundary, typically used for the
+///    introduction of new language features that can't otherwise be
+///    introduced in a backwards-compatible way.
+///
+/// See <https://rustc-dev-guide.rust-lang.org/bug-fix-procedure.html> and
+/// <https://rustc-dev-guide.rust-lang.org/diagnostics.html#future-incompatible-lints>
+/// for more information.
 #[derive(Copy, Clone, Debug)]
 pub enum FutureIncompatibilityReason {
     /// This will be an error in a future release for all editions
     ///
     /// This will *not* show up in cargo's future breakage report.
     /// The warning will hence only be seen in local crates, not in dependencies.
+    ///
+    /// Choose this variant when you are first introducing a "future
+    /// incompatible" warning that is intended to eventually be fixed in the
+    /// future. This allows crate developers an opportunity to fix the warning
+    /// before blasting all dependents with a warning they can't fix
+    /// (dependents have to wait for a new release of the affected crate to be
+    /// published).
+    ///
+    /// After a lint has been in this state for a while, consider graduating
+    /// it to [`FutureIncompatibilityReason::FutureReleaseErrorReportInDeps`].
     FutureReleaseErrorDontReportInDeps,
     /// This will be an error in a future release, and
     /// Cargo should create a report even for dependencies
@@ -358,17 +380,62 @@ pub enum FutureIncompatibilityReason {
     /// This is the *only* reason that will make future incompatibility warnings show up in cargo's
     /// reports. All other future incompatibility warnings are not visible when they occur in a
     /// dependency.
+    ///
+    /// Choose this variant after the lint has been sitting in the
+    /// [`FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps`]
+    /// state for a while, and you feel like it is ready to graduate to
+    /// warning everyone. It is a good signal that it is ready if you can
+    /// determine that all or most affected crates on crates.io have been
+    /// updated.
+    ///
+    /// After some period of time, lints with this variant can be turned into
+    /// hard errors (and the lint removed). Preferably when there is some
+    /// confidence that the number of impacted projects is very small (few
+    /// should have a broken dependency in their dependency tree).
     FutureReleaseErrorReportInDeps,
     /// Code that changes meaning in some way in a
     /// future release.
+    ///
+    /// Choose this variant when the semantics of existing code is changing,
+    /// (as opposed to
+    /// [`FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps`],
+    /// which is for when code is going to be rejected in the future).
     FutureReleaseSemanticsChange,
     /// Previously accepted code that will become an
     /// error in the provided edition
+    ///
+    /// Choose this variant for code that you want to start rejecting across
+    /// an edition boundary. This will automatically include the lint in the
+    /// `rust-20xx-compatibility` lint group, which is used by `cargo fix
+    /// --edition` to do migrations. The lint *should* be auto-fixable with
+    /// [`Applicability::MachineApplicable`].
+    ///
+    /// The lint can either be `Allow` or `Warn` by default. If it is `Allow`,
+    /// users usually won't see this warning unless they are doing an edition
+    /// migration manually or there is a problem during the migration (cargo's
+    /// automatic migrations will force the level to `Warn`). If it is `Warn`
+    /// by default, users on all editions will see this warning (only do this
+    /// if you think it is important for everyone to be aware of the change,
+    /// and to encourage people to update their code on all editions).
+    ///
+    /// See also [`FutureIncompatibilityReason::EditionSemanticsChange`] if
+    /// you have code that is changing semantics across the edition (as
+    /// opposed to being rejected).
     EditionError(Edition),
     /// Code that changes meaning in some way in
     /// the provided edition
+    ///
+    /// This is the same as [`FutureIncompatibilityReason::EditionError`],
+    /// except for situations where the semantics change across an edition. It
+    /// slightly changes the text of the diagnostic, but is otherwise the
+    /// same.
     EditionSemanticsChange(Edition),
     /// A custom reason.
+    ///
+    /// Choose this variant if the built-in text of the diagnostic of the
+    /// other variants doesn't match your situation. This is behaviorally
+    /// equivalent to
+    /// [`FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps`].
     Custom(&'static str),
 }
 
diff --git a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
index 2601d96b8c8..556dc890a84 100644
--- a/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp
@@ -37,7 +37,9 @@
 #include "llvm/Transforms/Utils/FunctionImportUtils.h"
 #include "llvm/LTO/LTO.h"
 #include "llvm/Bitcode/BitcodeWriter.h"
-
+#if LLVM_VERSION_GE(18, 0)
+#include "llvm/TargetParser/Host.h"
+#endif
 #include "llvm/Transforms/Instrumentation.h"
 #include "llvm/Transforms/Instrumentation/AddressSanitizer.h"
 #include "llvm/Support/TimeProfiler.h"
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 3c0627526be..5b0011e9f70 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -2918,7 +2918,15 @@ impl<'a> Parser<'a> {
             let mut result = if !is_fat_arrow && !is_almost_fat_arrow {
                 // A pattern without a body, allowed for never patterns.
                 arm_body = None;
-                this.expect_one_of(&[token::Comma], &[token::CloseDelim(Delimiter::Brace)])
+                this.expect_one_of(&[token::Comma], &[token::CloseDelim(Delimiter::Brace)]).map(
+                    |x| {
+                        // Don't gate twice
+                        if !pat.contains_never_pattern() {
+                            this.sess.gated_spans.gate(sym::never_patterns, pat.span);
+                        }
+                        x
+                    },
+                )
             } else {
                 if let Err(mut err) = this.expect(&token::FatArrow) {
                     // We might have a `=>` -> `=` or `->` typo (issue #89396).
diff --git a/compiler/rustc_parse/src/validate_attr.rs b/compiler/rustc_parse/src/validate_attr.rs
index cbe75b3dab6..81055431f64 100644
--- a/compiler/rustc_parse/src/validate_attr.rs
+++ b/compiler/rustc_parse/src/validate_attr.rs
@@ -6,9 +6,9 @@ use rustc_ast::token::Delimiter;
 use rustc_ast::tokenstream::DelimSpan;
 use rustc_ast::MetaItemKind;
 use rustc_ast::{self as ast, AttrArgs, AttrArgsEq, Attribute, DelimArgs, MetaItem};
-use rustc_ast_pretty::pprust;
 use rustc_errors::{Applicability, FatalError, PResult};
 use rustc_feature::{AttributeTemplate, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
+use rustc_session::errors::report_lit_error;
 use rustc_session::lint::builtin::ILL_FORMED_ATTRIBUTE_INPUT;
 use rustc_session::parse::ParseSess;
 use rustc_span::{sym, Span, Symbol};
@@ -51,28 +51,44 @@ pub fn parse_meta<'a>(sess: &'a ParseSess, attr: &Attribute) -> PResult<'a, Meta
                 MetaItemKind::List(nmis)
             }
             AttrArgs::Eq(_, AttrArgsEq::Ast(expr)) => {
-                if let ast::ExprKind::Lit(token_lit) = expr.kind
-                    && let Ok(lit) = ast::MetaItemLit::from_token_lit(token_lit, expr.span)
-                {
-                    if token_lit.suffix.is_some() {
-                        let mut err = sess.span_diagnostic.struct_span_err(
-                            expr.span,
-                            "suffixed literals are not allowed in attributes",
-                        );
-                        err.help(
-                            "instead of using a suffixed literal (`1u8`, `1.0f32`, etc.), \
-                            use an unsuffixed version (`1`, `1.0`, etc.)",
-                        );
-                        return Err(err);
-                    } else {
-                        MetaItemKind::NameValue(lit)
-                    }
+                if let ast::ExprKind::Lit(token_lit) = expr.kind {
+                    let res = ast::MetaItemLit::from_token_lit(token_lit, expr.span);
+                    let res = match res {
+                        Ok(lit) => {
+                            if token_lit.suffix.is_some() {
+                                let mut err = sess.span_diagnostic.struct_span_err(
+                                    expr.span,
+                                    "suffixed literals are not allowed in attributes",
+                                );
+                                err.help(
+                                    "instead of using a suffixed literal (`1u8`, `1.0f32`, etc.), \
+                                    use an unsuffixed version (`1`, `1.0`, etc.)",
+                                );
+                                return Err(err);
+                            } else {
+                                MetaItemKind::NameValue(lit)
+                            }
+                        }
+                        Err(err) => {
+                            report_lit_error(sess, err, token_lit, expr.span);
+                            let lit = ast::MetaItemLit {
+                                symbol: token_lit.symbol,
+                                suffix: token_lit.suffix,
+                                kind: ast::LitKind::Err,
+                                span: expr.span,
+                            };
+                            MetaItemKind::NameValue(lit)
+                        }
+                    };
+                    res
                 } else {
-                    // The non-error case can happen with e.g. `#[foo = 1+1]`. The error case can
-                    // happen with e.g. `#[foo = include_str!("nonexistent-file.rs")]`; in that
-                    // case we delay the error because an earlier error will have already been
-                    // reported.
-                    let msg = format!("unexpected expression: `{}`", pprust::expr_to_string(expr));
+                    // Example cases:
+                    // - `#[foo = 1+1]`: results in `ast::ExprKind::BinOp`.
+                    // - `#[foo = include_str!("nonexistent-file.rs")]`:
+                    //   results in `ast::ExprKind::Err`. In that case we delay
+                    //   the error because an earlier error will have already
+                    //   been reported.
+                    let msg = format!("attribute value must be a literal");
                     let mut err = sess.span_diagnostic.struct_span_err(expr.span, msg);
                     if let ast::ExprKind::Err = expr.kind {
                         err.downgrade_to_delayed_bug();
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index df30c185c60..61c7846ea61 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -8,7 +8,7 @@ use crate::{PathResult, PathSource, Segment};
 use rustc_hir::def::Namespace::{self, *};
 
 use rustc_ast::ptr::P;
-use rustc_ast::visit::{FnCtxt, FnKind, LifetimeCtxt};
+use rustc_ast::visit::{walk_ty, FnCtxt, FnKind, LifetimeCtxt, Visitor};
 use rustc_ast::{
     self as ast, AssocItemKind, Expr, ExprKind, GenericParam, GenericParamKind, Item, ItemKind,
     MethodCall, NodeId, Path, Ty, TyKind, DUMMY_NODE_ID,
@@ -2830,6 +2830,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
             .collect();
         debug!(?in_scope_lifetimes);
 
+        let mut maybe_static = false;
         debug!(?function_param_lifetimes);
         if let Some((param_lifetimes, params)) = &function_param_lifetimes {
             let elided_len = param_lifetimes.len();
@@ -2868,10 +2869,11 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
             if num_params == 0 {
                 err.help(
-                    "this function's return type contains a borrowed value, \
-                 but there is no value for it to be borrowed from",
+                    "this function's return type contains a borrowed value, but there is no value \
+                     for it to be borrowed from",
                 );
                 if in_scope_lifetimes.is_empty() {
+                    maybe_static = true;
                     in_scope_lifetimes = vec![(
                         Ident::with_dummy_span(kw::StaticLifetime),
                         (DUMMY_NODE_ID, LifetimeRes::Static),
@@ -2879,11 +2881,11 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                 }
             } else if elided_len == 0 {
                 err.help(
-                    "this function's return type contains a borrowed value with \
-                 an elided lifetime, but the lifetime cannot be derived from \
-                 the arguments",
+                    "this function's return type contains a borrowed value with an elided \
+                     lifetime, but the lifetime cannot be derived from the arguments",
                 );
                 if in_scope_lifetimes.is_empty() {
+                    maybe_static = true;
                     in_scope_lifetimes = vec![(
                         Ident::with_dummy_span(kw::StaticLifetime),
                         (DUMMY_NODE_ID, LifetimeRes::Static),
@@ -2891,13 +2893,13 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                 }
             } else if num_params == 1 {
                 err.help(format!(
-                    "this function's return type contains a borrowed value, \
-                 but the signature does not say which {m} it is borrowed from"
+                    "this function's return type contains a borrowed value, but the signature does \
+                     not say which {m} it is borrowed from",
                 ));
             } else {
                 err.help(format!(
-                    "this function's return type contains a borrowed value, \
-                 but the signature does not say whether it is borrowed from {m}"
+                    "this function's return type contains a borrowed value, but the signature does \
+                     not say whether it is borrowed from {m}",
                 ));
             }
         }
@@ -2962,11 +2964,238 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                 );
             }
             1 => {
+                let post = if maybe_static {
+                    let owned = if let [lt] = &lifetime_refs[..]
+                        && lt.kind != MissingLifetimeKind::Ampersand
+                    {
+                        ", or if you will only have owned values"
+                    } else {
+                        ""
+                    };
+                    format!(
+                        ", but this is uncommon unless you're returning a borrowed value from a \
+                         `const` or a `static`{owned}",
+                    )
+                } else {
+                    String::new()
+                };
                 err.multipart_suggestion_verbose(
-                    format!("consider using the `{existing_name}` lifetime"),
+                    format!("consider using the `{existing_name}` lifetime{post}"),
                     spans_suggs,
                     Applicability::MaybeIncorrect,
                 );
+                if maybe_static {
+                    // FIXME: what follows are general suggestions, but we'd want to perform some
+                    // minimal flow analysis to provide more accurate suggestions. For example, if
+                    // we identified that the return expression references only one argument, we
+                    // would suggest borrowing only that argument, and we'd skip the prior
+                    // "use `'static`" suggestion entirely.
+                    if let [lt] = &lifetime_refs[..]
+                        && (lt.kind == MissingLifetimeKind::Ampersand
+                            || lt.kind == MissingLifetimeKind::Underscore)
+                    {
+                        let pre = if lt.kind == MissingLifetimeKind::Ampersand
+                            && let Some((kind, _span)) = self.diagnostic_metadata.current_function
+                            && let FnKind::Fn(_, _, sig, _, _, _) = kind
+                            && !sig.decl.inputs.is_empty()
+                            && let sugg = sig
+                                .decl
+                                .inputs
+                                .iter()
+                                .filter_map(|param| {
+                                    if param.ty.span.contains(lt.span) {
+                                        // We don't want to suggest `fn elision(_: &fn() -> &i32)`
+                                        // when we have `fn elision(_: fn() -> &i32)`
+                                        None
+                                    } else if let TyKind::CVarArgs = param.ty.kind {
+                                        // Don't suggest `&...` for ffi fn with varargs
+                                        None
+                                    } else if let TyKind::ImplTrait(..) = &param.ty.kind {
+                                        // We handle these in the next `else if` branch.
+                                        None
+                                    } else {
+                                        Some((param.ty.span.shrink_to_lo(), "&".to_string()))
+                                    }
+                                })
+                                .collect::<Vec<_>>()
+                            && !sugg.is_empty()
+                        {
+                            let (the, s) = if sig.decl.inputs.len() == 1 {
+                                ("the", "")
+                            } else {
+                                ("one of the", "s")
+                            };
+                            err.multipart_suggestion_verbose(
+                                format!(
+                                    "instead, you are more likely to want to change {the} \
+                                     argument{s} to be borrowed...",
+                                ),
+                                sugg,
+                                Applicability::MaybeIncorrect,
+                            );
+                            "...or alternatively, you might want"
+                        } else if (lt.kind == MissingLifetimeKind::Ampersand
+                            || lt.kind == MissingLifetimeKind::Underscore)
+                            && let Some((kind, _span)) = self.diagnostic_metadata.current_function
+                            && let FnKind::Fn(_, _, sig, _, _, _) = kind
+                            && let ast::FnRetTy::Ty(ret_ty) = &sig.decl.output
+                            && !sig.decl.inputs.is_empty()
+                            && let arg_refs = sig
+                                .decl
+                                .inputs
+                                .iter()
+                                .filter_map(|param| match &param.ty.kind {
+                                    TyKind::ImplTrait(_, bounds) => Some(bounds),
+                                    _ => None,
+                                })
+                                .flat_map(|bounds| bounds.into_iter())
+                                .collect::<Vec<_>>()
+                            && !arg_refs.is_empty()
+                        {
+                            // We have a situation like
+                            // fn g(mut x: impl Iterator<Item = &()>) -> Option<&()>
+                            // So we look at every ref in the trait bound. If there's any, we
+                            // suggest
+                            // fn g<'a>(mut x: impl Iterator<Item = &'a ()>) -> Option<&'a ()>
+                            let mut lt_finder =
+                                LifetimeFinder { lifetime: lt.span, found: None, seen: vec![] };
+                            for bound in arg_refs {
+                                if let ast::GenericBound::Trait(trait_ref, _) = bound {
+                                    lt_finder.visit_trait_ref(&trait_ref.trait_ref);
+                                }
+                            }
+                            lt_finder.visit_ty(ret_ty);
+                            let spans_suggs: Vec<_> = lt_finder
+                                .seen
+                                .iter()
+                                .filter_map(|ty| match &ty.kind {
+                                    TyKind::Ref(_, mut_ty) => {
+                                        let span = ty.span.with_hi(mut_ty.ty.span.lo());
+                                        Some((span, "&'a ".to_string()))
+                                    }
+                                    _ => None,
+                                })
+                                .collect();
+                            self.suggest_introducing_lifetime(
+                                err,
+                                None,
+                                |err, higher_ranked, span, message, intro_sugg| {
+                                    err.multipart_suggestion_verbose(
+                                        message,
+                                        std::iter::once((span, intro_sugg))
+                                            .chain(spans_suggs.iter().cloned())
+                                            .collect(),
+                                        Applicability::MaybeIncorrect,
+                                    );
+                                    higher_ranked
+                                },
+                            );
+                            "alternatively, you might want"
+                        } else {
+                            "instead, you are more likely to want"
+                        };
+                        let mut owned_sugg = lt.kind == MissingLifetimeKind::Ampersand;
+                        let mut sugg = vec![(lt.span, String::new())];
+                        if let Some((kind, _span)) = self.diagnostic_metadata.current_function
+                            && let FnKind::Fn(_, _, sig, _, _, _) = kind
+                            && let ast::FnRetTy::Ty(ty) = &sig.decl.output
+                        {
+                            let mut lt_finder =
+                                LifetimeFinder { lifetime: lt.span, found: None, seen: vec![] };
+                            lt_finder.visit_ty(&ty);
+
+                            if let [Ty { span, kind: TyKind::Ref(_, mut_ty), .. }] =
+                                &lt_finder.seen[..]
+                            {
+                                // We might have a situation like
+                                // fn g(mut x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()>
+                                // but `lt.span` only points at `'_`, so to suggest `-> Option<()>`
+                                // we need to find a more accurate span to end up with
+                                // fn g<'a>(mut x: impl Iterator<Item = &'_ ()>) -> Option<()>
+                                sugg = vec![(span.with_hi(mut_ty.ty.span.lo()), String::new())];
+                                owned_sugg = true;
+                            }
+                            if let Some(ty) = lt_finder.found {
+                                if let TyKind::Path(None, path) = &ty.kind {
+                                    // Check if the path being borrowed is likely to be owned.
+                                    let path: Vec<_> = Segment::from_path(path);
+                                    match self.resolve_path(&path, Some(TypeNS), None) {
+                                        PathResult::Module(ModuleOrUniformRoot::Module(module)) => {
+                                            match module.res() {
+                                                Some(Res::PrimTy(PrimTy::Str)) => {
+                                                    // Don't suggest `-> str`, suggest `-> String`.
+                                                    sugg = vec![(
+                                                        lt.span.with_hi(ty.span.hi()),
+                                                        "String".to_string(),
+                                                    )];
+                                                }
+                                                Some(Res::PrimTy(..)) => {}
+                                                Some(Res::Def(
+                                                    DefKind::Struct
+                                                    | DefKind::Union
+                                                    | DefKind::Enum
+                                                    | DefKind::ForeignTy
+                                                    | DefKind::AssocTy
+                                                    | DefKind::OpaqueTy
+                                                    | DefKind::TyParam,
+                                                    _,
+                                                )) => {}
+                                                _ => {
+                                                    // Do not suggest in all other cases.
+                                                    owned_sugg = false;
+                                                }
+                                            }
+                                        }
+                                        PathResult::NonModule(res) => {
+                                            match res.base_res() {
+                                                Res::PrimTy(PrimTy::Str) => {
+                                                    // Don't suggest `-> str`, suggest `-> String`.
+                                                    sugg = vec![(
+                                                        lt.span.with_hi(ty.span.hi()),
+                                                        "String".to_string(),
+                                                    )];
+                                                }
+                                                Res::PrimTy(..) => {}
+                                                Res::Def(
+                                                    DefKind::Struct
+                                                    | DefKind::Union
+                                                    | DefKind::Enum
+                                                    | DefKind::ForeignTy
+                                                    | DefKind::AssocTy
+                                                    | DefKind::OpaqueTy
+                                                    | DefKind::TyParam,
+                                                    _,
+                                                ) => {}
+                                                _ => {
+                                                    // Do not suggest in all other cases.
+                                                    owned_sugg = false;
+                                                }
+                                            }
+                                        }
+                                        _ => {
+                                            // Do not suggest in all other cases.
+                                            owned_sugg = false;
+                                        }
+                                    }
+                                }
+                                if let TyKind::Slice(inner_ty) = &ty.kind {
+                                    // Don't suggest `-> [T]`, suggest `-> Vec<T>`.
+                                    sugg = vec![
+                                        (lt.span.with_hi(inner_ty.span.lo()), "Vec<".to_string()),
+                                        (ty.span.with_lo(inner_ty.span.hi()), ">".to_string()),
+                                    ];
+                                }
+                            }
+                        }
+                        if owned_sugg {
+                            err.multipart_suggestion_verbose(
+                                format!("{pre} to return an owned value"),
+                                sugg,
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                    }
+                }
 
                 // Record as using the suggested resolution.
                 let (_, (_, res)) = in_scope_lifetimes[0];
@@ -2996,7 +3225,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 fn mk_where_bound_predicate(
     path: &Path,
     poly_trait_ref: &ast::PolyTraitRef,
-    ty: &ast::Ty,
+    ty: &Ty,
 ) -> Option<ast::WhereBoundPredicate> {
     use rustc_span::DUMMY_SP;
     let modified_segments = {
@@ -3073,6 +3302,24 @@ pub(super) fn signal_lifetime_shadowing(sess: &Session, orig: Ident, shadower: I
     err.emit();
 }
 
+struct LifetimeFinder<'ast> {
+    lifetime: Span,
+    found: Option<&'ast Ty>,
+    seen: Vec<&'ast Ty>,
+}
+
+impl<'ast> Visitor<'ast> for LifetimeFinder<'ast> {
+    fn visit_ty(&mut self, t: &'ast Ty) {
+        if let TyKind::Ref(_, mut_ty) = &t.kind {
+            self.seen.push(t);
+            if t.span.lo() == self.lifetime.lo() {
+                self.found = Some(&mut_ty.ty);
+            }
+        }
+        walk_ty(self, t)
+    }
+}
+
 /// Shadowing involving a label is only a warning for historical reasons.
 //FIXME: make this a proper lint.
 pub(super) fn signal_label_shadowing(sess: &Session, orig: Span, shadower: Ident) {
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
index 8fa0dceda87..4988222c135 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs
@@ -3525,20 +3525,30 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         }
 
         match obligation.predicate.kind().skip_binder() {
-            ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(ct)) => {
-                let ty::ConstKind::Unevaluated(uv) = ct.kind() else {
+            ty::PredicateKind::Clause(ty::ClauseKind::ConstEvaluatable(ct)) => match ct.kind() {
+                ty::ConstKind::Unevaluated(uv) => {
+                    let mut err =
+                        self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
+                    let const_span = self.tcx.def_span(uv.def);
+                    match self.tcx.sess.source_map().span_to_snippet(const_span) {
+                            Ok(snippet) => err.help(format!(
+                                "try adding a `where` bound using this expression: `where [(); {snippet}]:`"
+                            )),
+                            _ => err.help("consider adding a `where` bound using this expression"),
+                        };
+                    Some(err)
+                }
+                ty::ConstKind::Expr(_) => {
+                    let err = self
+                        .tcx
+                        .sess
+                        .struct_span_err(span, format!("unconstrained generic constant `{ct}`"));
+                    Some(err)
+                }
+                _ => {
                     bug!("const evaluatable failed for non-unevaluated const `{ct:?}`");
-                };
-                let mut err = self.tcx.sess.struct_span_err(span, "unconstrained generic constant");
-                let const_span = self.tcx.def_span(uv.def);
-                match self.tcx.sess.source_map().span_to_snippet(const_span) {
-                    Ok(snippet) => err.help(format!(
-                        "try adding a `where` bound using this expression: `where [(); {snippet}]:`"
-                    )),
-                    _ => err.help("consider adding a `where` bound using this expression"),
-                };
-                Some(err)
-            }
+                }
+            },
             _ => {
                 span_bug!(
                     span,