Simplify CSS when using utilities that use a *-0 or *-1 value (#20196)

## Why?


While inspecting a Tailwind-powered site, I noticed selectors like

```css
.inset-0{inset:calc(var(--spacing) * 0)}
.inset-x-0{inset-inline:calc(var(--spacing) * 0)}
.top-0{top:calc(var(--spacing) * 0)}
```

which seem a bit silly, not to mention more complex for the end-user
device parsing the CSS.

## Summary

This PR adjusts helpers to not generate `calc(... * 0)` expressions.

#19095 does a similar
thing on CSS AST, but that doesn't run during the build proper.

## Test plan

Ran `pnpm build && pnpm test && pnpm test:integration`. (Some unrelated
tests failed on my machine, hopefully less in CI.)

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

Author: akx

CHANGELOG.md

@@ -25,6 +25,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure `@tailwindcss/cli` in `--watch` mode recovers when a tracked dependency is deleted and restored ([#20137](https://github.com/tailwindlabs/tailwindcss/pull/20137))
 - Ensure standalone `@tailwindcss/cli` binaries are ignored when scanning for class candidates ([#20139](https://github.com/tailwindlabs/tailwindcss/pull/20139))
 
+### Changed
+
+- Generate `0` instead of `calc(var(--spacing) * 0)` for spacing utilities like `m-0` and `left-0` ([#20196](https://github.com/tailwindlabs/tailwindcss/pull/20196))
+- Generate `var(--spacing)` instead of `calc(var(--spacing) * 1)` for spacing utilities like `m-1` and `left-1` ([#20196](https://github.com/tailwindlabs/tailwindcss/pull/20196))
+
 ## [4.3.0] - 2026-05-08
 
 ### Added

packages/tailwindcss/src/css-functions.test.ts

@@ -105,6 +105,100 @@ describe('--spacing(…)', () => {
     `)
   })
 
+  describe('optimizations', () => {
+    test('--spacing(…) optimizes the output when the input is `0`', async () => {
+      expect(
+        await compileCss(css`
+          @theme {
+            --spacing: 0.25rem;
+          }
+
+          .foo {
+            margin: --spacing(0);
+            padding: --spacing(0px);
+          }
+        `),
+      ).toMatchInlineSnapshot(`
+        "
+        .foo {
+          margin: 0;
+          padding: 0;
+        }
+        "
+      `)
+    })
+
+    test('--spacing(…) optimizes the output when the input is `0` (with an inlined theme value)', async () => {
+      expect(
+        await compileCss(css`
+          @theme inline {
+            --spacing: 0.25rem;
+          }
+
+          .foo {
+            margin: --spacing(0);
+            padding: --spacing(0px);
+          }
+        `),
+      ).toMatchInlineSnapshot(`
+        "
+        .foo {
+          margin: 0;
+          padding: 0;
+        }
+        "
+      `)
+    })
+
+    test('--spacing(…) optimizes the output when the input is `1`', async () => {
+      expect(
+        await compileCss(css`
+          @theme {
+            --spacing: 0.25rem;
+          }
+
+          .foo {
+            margin: --spacing(1);
+            padding: --spacing(1px);
+          }
+        `),
+      ).toMatchInlineSnapshot(`
+        "
+        :root, :host {
+          --spacing: .25rem;
+        }
+
+        .foo {
+          margin: var(--spacing);
+          padding: var(--spacing);
+        }
+        "
+      `)
+    })
+
+    test('--spacing(…) optimizes the output when the input is `1` (with an inlined theme value)', async () => {
+      expect(
+        await compileCss(css`
+          @theme inline {
+            --spacing: 0.25rem;
+          }
+
+          .foo {
+            margin: --spacing(1);
+            padding: --spacing(1px);
+          }
+        `),
+      ).toMatchInlineSnapshot(`
+        "
+        .foo {
+          margin: .25rem;
+          padding: .25rem;
+        }
+        "
+      `)
+    })
+  })
+
   test('--spacing(…) relies on `--spacing` to be defined', async () => {
     await expect(() =>
       compileCss(css`

packages/tailwindcss/src/css-functions.ts

@@ -2,6 +2,7 @@ import { Features } from '.'
 import { type AstNode } from './ast'
 import type { DesignSystem } from './design-system'
 import { withAlpha } from './utilities'
+import { dimensions } from './utils/dimensions'
 import { segment } from './utils/segment'
 import * as ValueParser from './value-parser'
 import { walk, WalkAction } from './walk'
@@ -62,6 +63,22 @@ function spacing(
     )
   }
 
+  // Optimization:
+  //
+  // - We know that at this point the `--spacing` value must be set.
+  // - We know that `--spacing` must be set to a `<length>` unit, such as `0.25rem`
+  // - We can assume that the `--spacing` value is not set to a `0`-like value.
+  //   Otherwise `p-<anything>` would calculate as `0` which wouldn't make sense.
+  //
+  // - That means that a value of `0` can be replaced by `0`
+  // - That means that a value of `1` can be replaced by `multiplier`
+  let valueDimension = dimensions.get(value)
+  if (valueDimension) {
+    if (valueDimension[0] === 0) return '0'
+    if (valueDimension[0] === 1) return multiplier
+  }
+
+  // No known optimizations available, use full calculation
   return `calc(${multiplier} * ${value})`
 }