Article 13, UCMJ, prohibits: (1) intentional imposition of punishment on an accused before his or her guilt is established at trial; and (2) arrest or pretrial confinement conditions that are more rigorous than necessary to ensure the accused’s presence at trial).

813. ART. 13 PUNISHMENT PROHIBITED BEFORE TRIAL

No person, while being held for trial, may be subjected to punishment or penalty other than arrest or confinement upon the charges pending against him, nor shall the arrest or confinement imposed upon him be any more rigorous than the circumstances required to insure his presence, but he may be subjected to minor punishment during that period for infractions of discipline.

Rule 305(k) of the Rules for Courts-Martial provide for motions for appropriate relief for illegal pretrial confinement (2008 ed.)

Pretrial confinement is only warranted when the evidence shows probable cause (i.e. reasonable grounds) to believe: 1) an offense triable by court-martial was committed; 2) the prisoner committed the offense; 3) confinement is necessary because it is foreseeable the accused will not appear at future hearings or that he will engage in serious criminal misconduct; and, 4) less severe forms of restraint are inadequate. R.C.M. 305(h)(2)(B).

In a 2010 military appeals court ruling finding a prisoner's continued pre-trial confinement illegal, the Judge wrote:

The standard for continued pretrial confinement is that there is probable cause to believe, by the preponderance of the evidence, that: 1) an offense triable by court-martial was committed; 2) the prisoner committed the offense; 3) confinement is necessary because it is foreseeable that the prisoner will not appear at future hearings or will engage in serious criminal misconduct; and 4) less severe forms of restraint are inadequate.

In United States v. Crawford (2006), the court noted:

...[a]rbitrary policies imposing maximum custody upon pretrial prisoners are not condoned; an appellate court will scrutinize closely any claim that maximum custody was imposed solely because of the charges rather than as a result of a reasonable evaluation of all the facts and circumstances of a case; where an appellate court finds that maximum custody was arbitrary and unnecessary to ensure an accused’s presence for trial, or unrelated to the security needs of the institution, it will consider appropriate credit or other relief to remedy this type of violation of Article 13, UCMJ).

A servicemember is entitled, both by statute and the Eighth Amendment, to protection against cruel and unusual punishment. See United States v. Matthews, 16 M.J. 354, 368 (CMA 1983); Art. 55, UCMJ, 10 USC § 855.

The Supreme Court has held that "the Constitution does not mandate comfortable prisons, but neither does it permit inhumane ones." Farmer v. Brennan, 511 U.S. 825, 832.

In United States v. Avila, 53 M.J. 99, 101-102 (C.A.A.F. 2000), the court held that solitary confinement alone doesn't violate the 8th Amendment:

Solitary confinement, per se, has not been held to violate the Cruel and Unusual Punishment Clause. 2 Sostre v. McGinnis, 442 F.2d 178, 192 (2d Cir. 1971), cert. denied, 404 U.S. 1049, (1972). Federal civilian courts have reviewed the specific conditions of solitary confinement to determine whether the confinement involved deprivation of basic needs or unnecessary infliction of pain. See Hutto, supra 437 U.S. at 686-87. They have held that routine conditions associated with punitive or administrative segregation do not rise to the level of a deprivation of life's necessities and violation of the Eighth Amendment. These conditions include restrictions or prohibitions on the opportunity to talk to other prisoners, exercise outside a cell, visitation privileges, telephone privileges, meal choices, and reading material.

...Furthermore, the length of a prisoner's stay in segregation does not, by itself, constitute cruel and unusual punishment, but is simply a factor to be considered along with the other aspects of confinement. Hutto, 437 U.S. at 686.

Later cases, including one in April, 2010, reaffirm that holding. United States v. Adkins, 2010 CCA LEXIS 43 (A.C.C.A. Apr. 13, 2010)

Here are the Navy's regulations on confinement. Manning is in Special Quarters 2.

4205. Special Quarters 4-13

4. Special Quarters. Special Quarters is a group of cells/secure rooms used to house those prisoners who have serious adjustment problems, create anxiety or disruption among other prisoners in the general population, or who need protection from other prisoners. Special quarters is a preventive management tool which shall not be used as punishment, except as allowed under article 5105.3. Programs, movements, and privileges shall be limited only to the minimum degree necessary for maintenance of good order. See article 4205 for further amplification. Habitability and space requirements are identical with other cells.

Here's the further amplification:

a. Some prisoners require additional supervision and attention due to personality disorders, behavior abnormalities, risk of suicide or violence, or other character traits. If required to preserve order, the BRIG Os or, in their absence, the brig duty officers/duty brig supervisors may authorize special quarters for such prisoners for purposes of control, prevention of injury to themselves or others, and the orderly and safe administration of the confinement facility. A hearing to determine the need for continued administrative segregation of the prisoner shall be conducted. This hearing may be by board action or by a member of the confinement facility appointed in writing by the BRIG O, and a written recommendation to the BRIG O will be provided within 72 hours of the prisoner's entry into segregation.

b. Special quarters is a group of cells used to house prisoners who have serious adjustment problems or certain medical issues, are highly temperamental or emotional, anti-social, some medical cases, or who cannot get along with other prisoners, or are persistent custodial problems. Special quarters are not a punitive measure and shall not be used as such. Prisoners must be made aware of the reason they are berthed in special quarters. Prisoners are assigned to special quarters by the BRIG O and shall not have normal privileges restricted unless privileges must be withheld for reasons of security or prisoner safety (e.g., suicide risks or aggressive, assaultive or predatory prisoners). For each period of 30 days a prisoner is retained in special quarters, the C&A board shall review and provide a recommendation to the BRIG O, who shall determine and certify the requirement for continuation in special quarters.

What about no mattress or pillow? The rules state:

2204. BERTHING AREA
1. Cells/Secure Rooms
a. Space Requirements
(1) Disciplinary segregation cells shall not be counted against total capacity and shall not normally exceed 5 percent of the confinement facility’s capacity. Cell size shall measure at least 6 feet wide, by 8 feet long, by 8 feet high. For new construction or renovation comply with paragraph 2103.5 of this manual. Cells/secure rooms shall be constructed for single occupancy.

...c. Equipment Requirements
(1) Furniture. All cells/secure rooms in shore confinement facilities will normally be equipped with a chair, locker, a 30" X 78" (minimum dimensions) security type bunk(permanently mounted in segregation cells), mattress, pillow, sheets, blankets, and pillow case for each prisoner. Furniture may be removed from cells only if the prisoner’s conduct warrants it and then only upon specific order of the CO or designee.

Who gets designated to maximum custody:

Maximum Custody (MAX). Prisoners requiring special custodial supervision because of the high probability of escape, are potentially dangerous or violent, and whose escape would cause concern of a threat to life, property, or nationalsecurity. Ordinarily, only a small percentage of prisoners shall be classified as MAX.

Max is not the norm:

3. Ultra-conservative custody classification results in a waste of prisoner and staff manpower. A large number of MAX and MDI prisoners reduce the number of staff available for supervision of the kinds of productive work available to lesser custody classifications (IC, MIN, MDO). Classification system must follow established, but flexible, procedures.

What are the factors:

5. Following are factors, though not all inclusive, to be considered in assessing higher custody classifications (MAX or MDI):

a. Assaultive behavior.
b. Disruptive behavior.
c. Serious drug abuse.
d. Serious civil/military criminal record (convicted or alleged).
e. Low tolerance of frustration.
f. Intensive acting out or dislike of the military.
g. History of previous escape(s).
h. Pending civil charges/detainer filed.
i. Serving a sentence which the individual considers to be unjust or severe.
j. Poor home conditions or family relationships.
k. A mental evaluation indicating serious neurosis or psychosis.
l. Indication of unwillingness to accept responsibility for personal actions past and present.
m. Demonstrated pattern of poor judgment.
n. Length, or potential length, of sentence.

I'm no expert on military criminal justice, but it seems obvious to me Bradley Manning should not be in maximum security or solitary confinement.

To add insult to injury, unlike in the federal criminal system, Manning might not even get credit against his ultimate sentence for the time he spends in maximum security pretrial confinement. United States v. Smith, 56 M.J. 290 (C.A.A.F. 2002). Apparently, 18 USC § 3568, the predecessor to 18 USC § 3585(b), specifically exempted court-martial sentences. 17 M.J. at 127.

the decision whether to extend DODI 1325.7 or RCM 305 to give pretrial confinement credit to persons not sentenced to confinement is a matter of Executive prerogative. To date, neither the President nor the Secretary of Defense has exercised that prerogative. Accordingly, we hold that there is no legal requirement that appellant be given credit for his pretrial confinement.