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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
#nullable disable
using System;
using System.Composition;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.CSharp.Utilities;
using Microsoft.CodeAnalysis.Editing;
using Microsoft.CodeAnalysis.Formatting;
using Microsoft.CodeAnalysis.Host.Mef;
using Microsoft.CodeAnalysis.PooledObjects;
using Microsoft.CodeAnalysis.ReplaceDiscardDeclarationsWithAssignments;
using Microsoft.CodeAnalysis.Shared.Extensions;
using Roslyn.Utilities;
namespace Microsoft.CodeAnalysis.CSharp.ReplaceDiscardDeclarationsWithAssignments;
using static SyntaxFactory;
[ExportLanguageService(typeof(IReplaceDiscardDeclarationsWithAssignmentsService), LanguageNames.CSharp), Shared]
internal sealed class CSharpReplaceDiscardDeclarationsWithAssignmentsService : IReplaceDiscardDeclarationsWithAssignmentsService
{
private const string DiscardVariableName = "_";
[ImportingConstructor]
[Obsolete(MefConstruction.ImportingConstructorMessage, error: true)]
public CSharpReplaceDiscardDeclarationsWithAssignmentsService()
{
}
public async Task<SyntaxNode> ReplaceAsync(
Document document,
SyntaxNode memberDeclaration,
CancellationToken cancellationToken)
{
var semanticModel = await document.GetRequiredSemanticModelAsync(cancellationToken).ConfigureAwait(false);
var editor = new SyntaxEditor(memberDeclaration, document.Project.Solution.Services);
foreach (var child in memberDeclaration.DescendantNodes())
{
switch (child)
{
case LocalDeclarationStatementSyntax localDeclarationStatement:
if (localDeclarationStatement.Declaration.Variables.Any(IsDiscardDeclaration))
{
RemoveDiscardHelper.ProcessDeclarationStatement(localDeclarationStatement, editor);
}
break;
case CatchDeclarationSyntax catchDeclaration:
if (IsDiscardDeclaration(catchDeclaration))
{
// "catch (Exception _)" => "catch (Exception)"
editor.ReplaceNode(catchDeclaration, catchDeclaration.WithIdentifier(default));
}
break;
case DeclarationExpressionSyntax declarationExpression:
if (declarationExpression.Designation is DiscardDesignationSyntax discardSyntax)
{
// "M(out var _)" => "M(out _)"
// "M(out int _)" => "M(out _)"
var discardToken = Identifier(
leading: declarationExpression.GetLeadingTrivia(),
contextualKind: SyntaxKind.UnderscoreToken,
text: discardSyntax.UnderscoreToken.Text,
valueText: discardSyntax.UnderscoreToken.ValueText,
trailing: declarationExpression.GetTrailingTrivia());
var replacementNode = IdentifierName(discardToken);
// Removing explicit type is possible only if there are no overloads of the method with same parameter.
// For example, if method "M" had overloads with signature "void M(int x)" and "void M(char x)",
// then the replacement "M(out int _)" => "M(out _)" will cause overload resolution error.
// Bail out if replacement changes semantics.
var speculationAnalyzer = new SpeculationAnalyzer(declarationExpression,
replacementNode, semanticModel, cancellationToken);
if (!speculationAnalyzer.ReplacementChangesSemantics())
{
editor.ReplaceNode(declarationExpression, replacementNode);
}
}
break;
case DeclarationPatternSyntax declarationPattern:
if (declarationPattern.Designation is DiscardDesignationSyntax discardDesignationSyntax &&
declarationPattern.Parent is IsPatternExpressionSyntax isPatternExpression)
{
// "x is int _" => "x is int"
var replacementNode = BinaryExpression(
kind: SyntaxKind.IsExpression,
left: isPatternExpression.Expression,
operatorToken: isPatternExpression.IsKeyword,
right: declarationPattern.Type.WithTrailingTrivia(declarationPattern.GetTrailingTrivia()));
editor.ReplaceNode(isPatternExpression, replacementNode);
}
break;
}
}
return editor.GetChangedRoot();
}
private static bool IsDiscardDeclaration(VariableDeclaratorSyntax variable)
=> variable.Identifier.Text == DiscardVariableName;
private static bool IsDiscardDeclaration(CatchDeclarationSyntax catchDeclaration)
=> catchDeclaration.Identifier.Text == DiscardVariableName;
private sealed class RemoveDiscardHelper : IDisposable
{
private readonly LocalDeclarationStatementSyntax _localDeclarationStatement;
private readonly SyntaxEditor _editor;
private readonly ArrayBuilder<StatementSyntax> _statementsBuilder;
private SeparatedSyntaxList<VariableDeclaratorSyntax> _currentNonDiscardVariables = [];
private RemoveDiscardHelper(LocalDeclarationStatementSyntax localDeclarationStatement, SyntaxEditor editor)
{
_localDeclarationStatement = localDeclarationStatement;
_editor = editor;
_statementsBuilder = ArrayBuilder<StatementSyntax>.GetInstance();
}
public static void ProcessDeclarationStatement(
LocalDeclarationStatementSyntax localDeclarationStatement,
SyntaxEditor editor)
{
using var helper = new RemoveDiscardHelper(localDeclarationStatement, editor);
helper.ProcessDeclarationStatement();
}
public void Dispose() => _statementsBuilder.Free();
private void ProcessDeclarationStatement()
{
// We will replace all discard variable declarations in this method with discard assignments,
// For example,
// 1. "int _ = M();" is replaced with "_ = M();"
// 2. "int x = 1, _ = M(), y = 2;" is replaced with following statements:
// int x = 1;
// _ = M();
// int y = 2;
// This is done to prevent compiler errors where the existing method has a discard
// variable declaration at a line following the one we added a discard assignment in our fix.
// Process all the declared variables in the given local declaration statement,
// tracking the currently encountered non-discard variables.
foreach (var variable in _localDeclarationStatement.Declaration.Variables)
{
if (!IsDiscardDeclaration(variable))
{
// Add to the list of currently encountered non-discard variables
_currentNonDiscardVariables = _currentNonDiscardVariables.Add(variable);
}
else
{
// Process currently encountered non-discard variables to generate
// a local declaration statement with these variables.
GenerateDeclarationStatementForCurrentNonDiscardVariables();
// Process the discard variable declaration to replace it
// with an assignment to discard.
GenerateAssignmentForDiscardVariable(variable);
}
}
// Process all the remaining variable declarators to generate
// a local declaration statement with these variables.
GenerateDeclarationStatementForCurrentNonDiscardVariables();
// Now replace the original local declaration statement with
// the replacement statement list tracked in _statementsBuilder.
if (_statementsBuilder.Count == 0)
{
// Nothing to replace.
return;
}
// Move the leading trivia from original local declaration statement
// to the first statement of the replacement statement list.
var leadingTrivia = _localDeclarationStatement.Declaration.Type.GetLeadingTrivia()
.Concat(_localDeclarationStatement.Declaration.Type.GetTrailingTrivia());
_statementsBuilder[0] = _statementsBuilder[0].WithLeadingTrivia(leadingTrivia);
// Move the trailing trivia from original local declaration statement
// to the last statement of the replacement statement list.
var last = _statementsBuilder.Count - 1;
var trailingTrivia = _localDeclarationStatement.SemicolonToken.GetAllTrivia();
_statementsBuilder[last] = _statementsBuilder[last].WithTrailingTrivia(trailingTrivia);
// Replace the original local declaration statement with new statement list
// from _statementsBuilder.
if (_localDeclarationStatement.Parent is BlockSyntax or SwitchSectionSyntax)
{
if (_statementsBuilder.Count > 1)
{
_editor.InsertAfter(_localDeclarationStatement, _statementsBuilder.Skip(1));
}
_editor.ReplaceNode(_localDeclarationStatement, _statementsBuilder[0]);
}
else
{
_editor.ReplaceNode(_localDeclarationStatement, Block(_statementsBuilder));
}
}
private void GenerateDeclarationStatementForCurrentNonDiscardVariables()
{
// Generate a variable declaration with all the currently tracked non-discard declarators.
// For example, for a declaration "int x = 1, y = 2, _ = M(), z = 3;", we generate two variable declarations:
// 1. "int x = 1, y = 2;" and
// 2. "int z = 3;",
// which are split by a single assignment statement "_ = M();"
if (_currentNonDiscardVariables.Count > 0)
{
var statement = LocalDeclarationStatement(
VariableDeclaration(_localDeclarationStatement.Declaration.Type, _currentNonDiscardVariables))
.WithAdditionalAnnotations(Formatter.Annotation);
_statementsBuilder.Add(statement);
_currentNonDiscardVariables = [];
}
}
private void GenerateAssignmentForDiscardVariable(VariableDeclaratorSyntax variable)
{
Debug.Assert(IsDiscardDeclaration(variable));
// Convert a discard declaration with initializer of the form "int _ = M();" into
// a discard assignment "_ = M();"
if (variable.Initializer != null)
{
_statementsBuilder.Add(
ExpressionStatement(
AssignmentExpression(
kind: SyntaxKind.SimpleAssignmentExpression,
left: IdentifierName(variable.Identifier),
operatorToken: variable.Initializer.EqualsToken,
right: variable.Initializer.Value))
.WithAdditionalAnnotations(Formatter.Annotation));
}
}
}
}
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