Rollup of 7 pull requests

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -64,8 +64,8 @@ struct AstValidator<'a> {
     /// certain positions.
     is_assoc_ty_bound_banned: bool,
 
-    /// Used to allow `let` expressions in certain syntactic locations.
-    is_let_allowed: bool,
+    /// See [ForbiddenLetReason]
+    forbidden_let_reason: Option<ForbiddenLetReason>,
 
     lint_buffer: &'a mut LintBuffer,
 }
@@ -103,20 +103,28 @@ impl<'a> AstValidator<'a> {
         self.is_tilde_const_allowed = old;
     }
 
-    fn with_let_allowed(&mut self, allowed: bool, f: impl FnOnce(&mut Self, bool)) {
-        let old = mem::replace(&mut self.is_let_allowed, allowed);
+    fn with_let_management(
+        &mut self,
+        forbidden_let_reason: Option<ForbiddenLetReason>,
+        f: impl FnOnce(&mut Self, Option<ForbiddenLetReason>),
+    ) {
+        let old = mem::replace(&mut self.forbidden_let_reason, forbidden_let_reason);
         f(self, old);
-        self.is_let_allowed = old;
+        self.forbidden_let_reason = old;
     }
 
     /// Emits an error banning the `let` expression provided in the given location.
-    fn ban_let_expr(&self, expr: &'a Expr) {
+    fn ban_let_expr(&self, expr: &'a Expr, or_span: Option<Span>) {
         let sess = &self.session;
         if sess.opts.unstable_features.is_nightly_build() {
-            sess.struct_span_err(expr.span, "`let` expressions are not supported here")
-                .note("only supported directly in conditions of `if`- and `while`-expressions")
-                .note("as well as when nested within `&&` and parentheses in those conditions")
-                .emit();
+            let err = "`let` expressions are not supported here";
+            let mut diag = sess.struct_span_err(expr.span, err);
+            diag.note("only supported directly in conditions of `if` and `while` expressions");
+            diag.note("as well as when nested within `&&` and parentheses in those conditions");
+            if let Some(elem) = or_span {
+                diag.span_note(elem, "`||` operators are not allowed in let chain expressions");
+            }
+            diag.emit();
         } else {
             sess.struct_span_err(expr.span, "expected expression, found statement (`let`)")
                 .note("variable declaration using `let` is a statement")
@@ -988,39 +996,52 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
     }
 
     fn visit_expr(&mut self, expr: &'a Expr) {
-        self.with_let_allowed(false, |this, let_allowed| match &expr.kind {
-            ExprKind::If(cond, then, opt_else) => {
-                this.visit_block(then);
-                walk_list!(this, visit_expr, opt_else);
-                this.with_let_allowed(true, |this, _| this.visit_expr(cond));
-                return;
-            }
-            ExprKind::Let(..) if !let_allowed => this.ban_let_expr(expr),
-            ExprKind::Match(expr, arms) => {
-                this.visit_expr(expr);
-                for arm in arms {
-                    this.visit_expr(&arm.body);
-                    this.visit_pat(&arm.pat);
-                    walk_list!(this, visit_attribute, &arm.attrs);
-                    if let Some(ref guard) = arm.guard {
-                        if let ExprKind::Let(_, ref expr, _) = guard.kind {
-                            this.with_let_allowed(true, |this, _| this.visit_expr(expr));
+        self.with_let_management(Some(ForbiddenLetReason::GenericForbidden), |this, forbidden_let_reason| {
+            match &expr.kind {
+                ExprKind::Binary(Spanned { node: BinOpKind::Or, span }, lhs, rhs) => {
+                    let forbidden_let_reason = Some(ForbiddenLetReason::ForbiddenWithOr(*span));
+                    this.with_let_management(forbidden_let_reason, |this, _| this.visit_expr(lhs));
+                    this.with_let_management(forbidden_let_reason, |this, _| this.visit_expr(rhs));
+                }
+                ExprKind::If(cond, then, opt_else) => {
+                    this.visit_block(then);
+                    walk_list!(this, visit_expr, opt_else);
+                    this.with_let_management(None, |this, _| this.visit_expr(cond));
+                    return;
+                }
+                ExprKind::Let(..) if let Some(elem) = forbidden_let_reason => {
+                    let or_span = match elem {
+                        ForbiddenLetReason::ForbiddenWithOr(span) => Some(span),
+                        ForbiddenLetReason::GenericForbidden => None,
+                    };
+                    this.ban_let_expr(expr, or_span);
+                },
+                ExprKind::Match(scrutinee, arms) => {
+                    this.visit_expr(scrutinee);
+                    for arm in arms {
+                        this.visit_expr(&arm.body);
+                        this.visit_pat(&arm.pat);
+                        walk_list!(this, visit_attribute, &arm.attrs);
+                        if let Some(guard) = &arm.guard && let ExprKind::Let(_, guard_expr, _) = &guard.kind {
+                            this.with_let_management(None, |this, _| {
+                                this.visit_expr(guard_expr)
+                            });
                             return;
                         }
                     }
                 }
+                ExprKind::Paren(_) | ExprKind::Binary(Spanned { node: BinOpKind::And, .. }, ..) => {
+                    this.with_let_management(forbidden_let_reason, |this, _| visit::walk_expr(this, expr));
+                    return;
+                }
+                ExprKind::While(cond, then, opt_label) => {
+                    walk_list!(this, visit_label, opt_label);
+                    this.visit_block(then);
+                    this.with_let_management(None, |this, _| this.visit_expr(cond));
+                    return;
+                }
+                _ => visit::walk_expr(this, expr),
             }
-            ExprKind::Paren(_) | ExprKind::Binary(Spanned { node: BinOpKind::And, .. }, ..) => {
-                this.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
-                return;
-            }
-            ExprKind::While(cond, then, opt_label) => {
-                walk_list!(this, visit_label, opt_label);
-                this.visit_block(then);
-                this.with_let_allowed(true, |this, _| this.visit_expr(cond));
-                return;
-            }
-            _ => visit::walk_expr(this, expr),
         });
     }
 
@@ -1772,10 +1793,19 @@ pub fn check_crate(session: &Session, krate: &Crate, lints: &mut LintBuffer) ->
         is_tilde_const_allowed: false,
         is_impl_trait_banned: false,
         is_assoc_ty_bound_banned: false,
-        is_let_allowed: false,
+        forbidden_let_reason: Some(ForbiddenLetReason::GenericForbidden),
         lint_buffer: lints,
     };
     visit::walk_crate(&mut validator, krate);
 
     validator.has_proc_macro_decls
 }
+
+/// Used to forbid `let` expressions in certain syntactic locations.
+#[derive(Clone, Copy)]
+enum ForbiddenLetReason {
+    /// A let chain with the `||` operator
+    ForbiddenWithOr(Span),
+    /// `let` is not valid and the source environment is not important
+    GenericForbidden,
+}

compiler/rustc_ast_passes/src/lib.rs

@@ -4,11 +4,13 @@
 //!
 //! The crate also contains other misc AST visitors, e.g. `node_count` and `show_span`.
 
-#![feature(iter_is_partitioned)]
+#![allow(rustc::potential_query_instability)]
 #![feature(box_patterns)]
+#![feature(if_let_guard)]
+#![feature(iter_is_partitioned)]
+#![feature(let_chains)]
 #![feature(let_else)]
 #![recursion_limit = "256"]
-#![allow(rustc::potential_query_instability)]
 
 pub mod ast_validation;
 pub mod feature_gate;

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -7,7 +7,8 @@ use super::{PatCtxt, PatternError};
 use rustc_arena::TypedArena;
 use rustc_ast::Mutability;
 use rustc_errors::{
-    error_code, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
+    error_code, pluralize, struct_span_err, Applicability, Diagnostic, DiagnosticBuilder,
+    ErrorGuaranteed,
 };
 use rustc_hir as hir;
 use rustc_hir::def::*;
@@ -20,7 +21,7 @@ use rustc_session::lint::builtin::{
 };
 use rustc_session::Session;
 use rustc_span::source_map::Spanned;
-use rustc_span::{DesugaringKind, ExpnKind, MultiSpan, Span};
+use rustc_span::{BytePos, DesugaringKind, ExpnKind, MultiSpan, Span};
 
 crate fn check_match(tcx: TyCtxt<'_>, def_id: DefId) {
     let body_id = match def_id.as_local() {
@@ -241,6 +242,9 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
         }
 
         let joined_patterns = joined_uncovered_patterns(&cx, &witnesses);
+
+        let mut bindings = vec![];
+
         let mut err = struct_span_err!(
             self.tcx.sess,
             pat.span,
@@ -257,6 +261,16 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
                 false
             }
             _ => {
+                pat.walk(&mut |pat: &hir::Pat<'_>| {
+                    match pat.kind {
+                        hir::PatKind::Binding(_, _, ident, _) => {
+                            bindings.push(ident);
+                        }
+                        _ => {}
+                    }
+                    true
+                });
+
                 err.span_label(pat.span, pattern_not_covered_label(&witnesses, &joined_patterns));
                 true
             }
@@ -267,13 +281,71 @@ impl<'p, 'tcx> MatchVisitor<'_, 'p, 'tcx> {
                 "`let` bindings require an \"irrefutable pattern\", like a `struct` or \
                  an `enum` with only one variant",
             );
-            if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
-                err.span_suggestion(
-                    span,
-                    "you might want to use `if let` to ignore the variant that isn't matched",
-                    format!("if {} {{ /* */ }}", &snippet[..snippet.len() - 1]),
+            if self.tcx.sess.source_map().span_to_snippet(span).is_ok() {
+                let semi_span = span.shrink_to_hi().with_lo(span.hi() - BytePos(1));
+                let start_span = span.shrink_to_lo();
+                let end_span = semi_span.shrink_to_lo();
+                err.multipart_suggestion(
+                    &format!(
+                        "you might want to use `if let` to ignore the variant{} that {} matched",
+                        pluralize!(witnesses.len()),
+                        match witnesses.len() {
+                            1 => "isn't",
+                            _ => "aren't",
+                        },
+                    ),
+                    vec![
+                        match &bindings[..] {
+                            [] => (start_span, "if ".to_string()),
+                            [binding] => (start_span, format!("let {} = if ", binding)),
+                            bindings => (
+                                start_span,
+                                format!(
+                                    "let ({}) = if ",
+                                    bindings
+                                        .iter()
+                                        .map(|ident| ident.to_string())
+                                        .collect::<Vec<_>>()
+                                        .join(", ")
+                                ),
+                            ),
+                        },
+                        match &bindings[..] {
+                            [] => (semi_span, " { todo!() }".to_string()),
+                            [binding] => {
+                                (end_span, format!(" {{ {} }} else {{ todo!() }}", binding))
+                            }
+                            bindings => (
+                                end_span,
+                                format!(
+                                    " {{ ({}) }} else {{ todo!() }}",
+                                    bindings
+                                        .iter()
+                                        .map(|ident| ident.to_string())
+                                        .collect::<Vec<_>>()
+                                        .join(", ")
+                                ),
+                            ),
+                        },
+                    ],
                     Applicability::HasPlaceholders,
                 );
+                if !bindings.is_empty() && cx.tcx.sess.is_nightly_build() {
+                    err.span_suggestion_verbose(
+                        semi_span.shrink_to_lo(),
+                        &format!(
+                            "alternatively, on nightly, you might want to use \
+                             `#![feature(let_else)]` to handle the variant{} that {} matched",
+                            pluralize!(witnesses.len()),
+                            match witnesses.len() {
+                                1 => "isn't",
+                                _ => "aren't",
+                            },
+                        ),
+                        " else { todo!() }".to_string(),
+                        Applicability::HasPlaceholders,
+                    );
+                }
             }
             err.note(
                 "for more information, visit \