Merge #6769

6769: Add native "remove this semicolon" diagnostics r=matklad a=ivan770

Closes #6739 
![demo2](https://user-images.githubusercontent.com/14003886/101530533-b76c3180-399a-11eb-9d18-5c8457721655.gif)


Co-authored-by: ivan770 <leshenko.ivan770@gmail.com>

crates/hir/src/diagnostics.rs

@@ -5,5 +5,5 @@ pub use hir_expand::diagnostics::{
 };
 pub use hir_ty::diagnostics::{
     IncorrectCase, MismatchedArgCount, MissingFields, MissingMatchArms, MissingOkInTailExpr,
-    NoSuchField,
+    NoSuchField, RemoveThisSemicolon,
 };

crates/hir_ty/src/diagnostics.rs

@@ -216,6 +216,30 @@ impl Diagnostic for MissingOkInTailExpr {
     }
 }
 
+#[derive(Debug)]
+pub struct RemoveThisSemicolon {
+    pub file: HirFileId,
+    pub expr: AstPtr<ast::Expr>,
+}
+
+impl Diagnostic for RemoveThisSemicolon {
+    fn code(&self) -> DiagnosticCode {
+        DiagnosticCode("remove-this-semicolon")
+    }
+
+    fn message(&self) -> String {
+        "Remove this semicolon".to_string()
+    }
+
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile { file_id: self.file, value: self.expr.clone().into() }
+    }
+
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
 // Diagnostic: break-outside-of-loop
 //
 // This diagnostic is triggered if `break` keyword is used outside of a loop.

crates/hir_ty/src/diagnostics/expr.rs

@@ -2,7 +2,7 @@
 
 use std::sync::Arc;
 
-use hir_def::{path::path, resolver::HasResolver, AdtId, DefWithBodyId};
+use hir_def::{expr::Statement, path::path, resolver::HasResolver, AdtId, DefWithBodyId};
 use hir_expand::diagnostics::DiagnosticSink;
 use rustc_hash::FxHashSet;
 use syntax::{ast, AstPtr};
@@ -12,6 +12,7 @@ use crate::{
     diagnostics::{
         match_check::{is_useful, MatchCheckCtx, Matrix, PatStack, Usefulness},
         MismatchedArgCount, MissingFields, MissingMatchArms, MissingOkInTailExpr, MissingPatFields,
+        RemoveThisSemicolon,
     },
     utils::variant_data,
     ApplicationTy, InferenceResult, Ty, TypeCtor,
@@ -76,8 +77,12 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
             }
         }
         let body_expr = &body[body.body_expr];
-        if let Expr::Block { tail: Some(t), .. } = body_expr {
+        if let Expr::Block { statements, tail, .. } = body_expr {
-            self.validate_results_in_tail_expr(body.body_expr, *t, db);
+            if let Some(t) = tail {
+                self.validate_results_in_tail_expr(body.body_expr, *t, db);
+            } else if let Some(Statement::Expr(id)) = statements.last() {
+                self.validate_missing_tail_expr(body.body_expr, *id, db);
+            }
         }
     }
 
@@ -317,6 +322,34 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
             }
         }
     }
+
+    fn validate_missing_tail_expr(
+        &mut self,
+        body_id: ExprId,
+        possible_tail_id: ExprId,
+        db: &dyn HirDatabase,
+    ) {
+        let mismatch = match self.infer.type_mismatch_for_expr(body_id) {
+            Some(m) => m,
+            None => return,
+        };
+
+        let possible_tail_ty = match self.infer.type_of_expr.get(possible_tail_id) {
+            Some(ty) => ty,
+            None => return,
+        };
+
+        if mismatch.actual != Ty::unit() || mismatch.expected != *possible_tail_ty {
+            return;
+        }
+
+        let (_, source_map) = db.body_with_source_map(self.owner.into());
+
+        if let Ok(source_ptr) = source_map.expr_syntax(possible_tail_id) {
+            self.sink
+                .push(RemoveThisSemicolon { file: source_ptr.file_id, expr: source_ptr.value });
+        }
+    }
 }
 
 pub fn record_literal_missing_fields(

crates/ide/src/diagnostics.rs

@@ -131,6 +131,9 @@ pub(crate) fn diagnostics(
         .on::<hir::diagnostics::NoSuchField, _>(|d| {
             res.borrow_mut().push(diagnostic_with_fix(d, &sema));
         })
+        .on::<hir::diagnostics::RemoveThisSemicolon, _>(|d| {
+            res.borrow_mut().push(diagnostic_with_fix(d, &sema));
+        })
         .on::<hir::diagnostics::IncorrectCase, _>(|d| {
             res.borrow_mut().push(warning_with_fix(d, &sema));
         })

crates/ide/src/diagnostics/fixes.rs

@@ -4,7 +4,7 @@ use hir::{
     db::AstDatabase,
     diagnostics::{
         Diagnostic, IncorrectCase, MissingFields, MissingOkInTailExpr, NoSuchField,
-        UnresolvedModule,
+        RemoveThisSemicolon, UnresolvedModule,
     },
     HasSource, HirDisplay, InFile, Semantics, VariantDef,
 };
@@ -105,6 +105,27 @@ impl DiagnosticWithFix for MissingOkInTailExpr {
     }
 }
 
+impl DiagnosticWithFix for RemoveThisSemicolon {
+    fn fix(&self, sema: &Semantics<RootDatabase>) -> Option<Fix> {
+        let root = sema.db.parse_or_expand(self.file)?;
+
+        let semicolon = self
+            .expr
+            .to_node(&root)
+            .syntax()
+            .parent()
+            .and_then(ast::ExprStmt::cast)
+            .and_then(|expr| expr.semicolon_token())?
+            .text_range();
+
+        let edit = TextEdit::delete(semicolon);
+        let source_change =
+            SourceFileEdit { file_id: self.file.original_file(sema.db), edit }.into();
+
+        Some(Fix::new("Remove this semicolon", source_change, semicolon))
+    }
+}
+
 impl DiagnosticWithFix for IncorrectCase {
     fn fix(&self, sema: &Semantics<RootDatabase>) -> Option<Fix> {
         let root = sema.db.parse_or_expand(self.file)?;